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www.americanradiohistory.comAmericanRadioHistory.Com Sitting before the sound -photo machine in the New York Daily Mirror radio car, John Reidy, the Mirror's chief photographer, advises the Mirror office (extreme left) that a photo is coming throyrgh. The heart of the sound-photo receiving equipment at the office is Sylvania's Glow Modulator Tube, Type R1130B. A pin -point of light emitted by this tube is focused on a sheet of photographic paper attached to a revolving cylinder. As the cylinder revolves, the photograph is faithfully reproduced as it is being broadcast!

LINK FM SETS SYLVAN IA- tubed, too - transmit on- the -spot photos to paper's home office!

Pioneer in the radio car field, the New York Daily Mirror has made excellent use of its sound -photo apparatus. Equipped with Link Radio Communication units, the Mirror car has been in a position to scoop other newspapers by radioing pictures taken on SYLVA the spot as soon as they are developed in the mobile NIA photo -lab. In these units, 36 highest quality Sylvania tubes, ranging from Lock -Ins to standard glass and GT tubes, ELEC RIC help insure trouble -free operation of this ultra -modern RADIO TUBES; CATHODE RAY TUBES; ELECTRONIC DEVICES; FLUORES- method of photo -news reporting! For information on CENT LAMPS, FIXTURES, WIRING DEVICES; LIGHT BULBS; P- OTOLAVPS Sylvania tubes, see Sylvania Distributors, or write Radio Tube Division, Emporium, Pa.

AUDIO ENGINEERING NOVEMBER, 1948

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2 AUDIO ENGINEERING NOVEMBER, 1948

www.americanradiohistory.comAmericanRadioHistory.Com John H. Potts, Editor Sanford R. Cowan, Publisher

C. G. McProud, Managing Editor S. L. Cahn, Ads. Director Lawrence LeKashman, Asst. Editor H. N. Reizes, Acv. Mgr. Louisa B. Dresser, Edit. Prod. Mgr. D. Reissman, Circ. Mgr. ENGINEERING Editorial Advisory Board Representatives Howard A. Chinn James C Galloway John D. Colvin 816 W. 5th St., Lc s Angeles 13, Calif. C. J. LeBel Dale Internationa Publications, Ltd. 105 Boisover St. J. P. Maxfield NrNiia London W I, England UOIT George M. Nixon UREAU Successor to RADIO Harris & Floyd Off Winston Wells RcuLAn 297 Swanston St., Melbourne S. Young White C. I, Victo ia, Australia

Established 1917 CONTENTS NOVEMBER, 1948 Vol. 32, No. I I

Editor's Report 11

Letters 8

Audio System Design Fundamentals-Howard A. Chinn 11

Audio Engineering Society, San Francisco Section 14

Design of a Continuously Variable Audio Signal Generator -Brunton Bauer 15

Characteristics of Amplifying Crystals -S. Young White 18

Elements of Residence Radio Systems, Part III -C. G. McFroud 21)

Sound on Film -John A. Maurer 24

Record Revue-Edward Tatnall Canby 28

Two -Way Speaker from Commercial Components-John Winslow 29

New Products 32

Professional Directory 32

Advertising Index .18

COVER

Photomicrograph of the surface of a treated germanium crystal amplifier unit. Photo- graphed by Winston Wells especially for AUDIO ENGINEERING.

AUDIO ENGINEERING (title registered U. S. Pat. Off.) is published monthly at New York, N.Y., by Radio Magazines, Inc., J. H. Potts, President: S. R. Cowan, Sec'y- Treas. Executive and Editorial Offices at 342 Madison Avenue. New York 17, N. Y. Subscription rates -United States, U. S. for 35.00 years 54.00 per year. Single copies Possessions and Canada, 33.00 1 year, for 2 ; elsewhere 35c. Printed in U. S. A. All rights reserved. Entire contents copyright 1948 by Radio Magazines, Inc. Entered as Second Class Matter July 29, 1948 at the Post Office, New York, N.Y., under the Act of March 3, 1879.

AUDIO ENGINEERING NOVEMBER, 1948 3

www.americanradiohistory.comAmericanRadioHistory.Com The 1304 is TOPS!

REPRODUCTION QUALITY? The Western Electric and an adjustable spring loaded idler pulley prevent 1304 Set combines the 109 Type Reproducer belt slippage problems. Croup with its extremely low intermodulation dis- OPERATING CONVENIENCE? Speed change -over at tortion and a unique new driving mechanism (shown the throw ofa switch. Acceleration to 33-1/3 rpm in 1/9 in Fig. 1) that cuts flutter to a value lower than many revolution -to 78 rpm in less than 1/2 revolution. Rapid standard recording equipments. slowdown - no overdrive - convenient flange on Even the small amount of flutter originating in the platter for quick stopping. mechanism's simple gearing is damped in the novel And playing time variation is less than ± 2 seconds filter of Fig. 2. Result: a flutter level, including wow, in 15 minutes! of less than 1/10 of 1% at both 78 and 33 -1/3 rpm. Scientific placement of elements facilitates operation. The platter has been isolated from the sources of An annular groove in the platter makes it easy to grasp rumble by means of the drive isolation coupling (Fig. edge of 10- or 12 -inch records. 706A Guard provides 4), the fabric belt, and by mounting the entire drive automatic arm rest, keeps stylus from dropping on mechanism on rubber vibration mounts (Fig. 3). The panel, catching in turntable felt, or striking edge of large drive pulleys, the use of large belt wrap around, revolving platter.

d FIG. 2 FIG. 1 As shown in cut -away view, a coupling (e) allows each pinion and A single helical ring gear (a), is permanently meshed with two associated shaft to move a short distance along its axis. The pinion gears (b), each driven by an overriding clutch (c). Revers- bottom of each pinion shaft projects into an oil- filled chamber (d) ing direction of motor rotation disengages one overriding clutch. for damping axial motion. Because of the helical gearing and the engages the other to change platter speed. Permanently meshed high inertia of the turntable platter, irregularities in the drive gears eliminate possibility of flutter caused by wear of engaging tending to cause flutter are taken up and damped in axial motion and disengaging. of the driving pinion.

FIG. 3 FIG. 4 The entire mechanism, including motor, floats separately from Drive isolation coupling (h), provides the only connection between frame and platter shaft on three large rubber mountings (f). driving gear (i),platter shaft (D and platter support (k),completing Motor, in turn, is isolated from the gear system by smaller rubber the separation of drive mechanism from platter. This coupling - mountings (g) and the use of belt drive. very rigid in rotational plane, highly flexible in all others -transmits the driving motion, but isolates the rumble -causing motion. Platter and support ride on a hardened single ball thrust bearing (m).

4 AUDIO ENGINEERING NOVEMBER, 1948

www.americanradiohistory.comAmericanRadioHistory.Com THE WESTERN ELECTRIC 1304 Type Repro- ments. It is available in a variety of cabinet . ducer Set is a single compact unit, readily arrangements to permit the greatest possible adaptable to a wide range of installation require- flexibility in installation.

FIG. 5 FIG. 6 The 1304 Reproducer Set. includes a floor type cabinet with or with. mounting standard amplifying equipment (Fig. 6). In either case, out a removable door. The 701A Shelf is available which provides additional space for equipment is available at the rear of the record storage space (Fig. 5), or the cabinet may be arranged for cabinet.

FIG. 7 FIG. 8 If you want the superb reproduction and the operating convenience You can also use the drive mechanism of the 1304 with your own of the 1304-but prefer to use an existing table or a specially built reproducer group. The 305A Panel is drilled to take the 109 Type cabinet just specify the 304 Type Reproducer Panel. - This is a Group, and is furnished with 706A Guard, equalizer knob and the complete panel unit, all ready to install, with exactly the same required hardware for mounting the 109 Type Group. The 3056 drive mechanism used in the 1304. The 109 Group with 706A Guard, Panel can be drilled in the field to mount reproducer groups other on -off and speed-change switches and platter are all included. than the 109.1706A Guard and equalizer knob not included.)

For complete information on the 1304 Reproducer or Reproducer Group - or on the 304, 305A or 3 05B Panels - call your nearest Graybar Broadcast Representative. Or write Graybar Electric Corn - pany, 420 Lexington Avenue, New York 17, N. Y.

DISTRIBUTORS: IN THIE U.s.A.- Graybar Electric Company. IN CANADA AND NEW- FOUNDLAND- Northern Electric Co., Ltd. - QUALITY COUNTS -

AUDIO ENGINEERING NOVEMBER, 1948 5

www.americanradiohistory.comAmericanRadioHistory.Com EDITOR'S REPORT

COVER PIX speaker. The separation between the waves is thus of the order of four inches. The signal level used was only PERHAPS you may have wondered why the remark- a few milliwatts and the same technique can be applied able cover photographs by Winston Wells which we in determining the slip stream pattern of an airplane have used in our last three issues have been pre- propeller, in plotting standing wave patterns in a room, sented with so little editorial comment. Here's the story : leakage patterns around shielding, either at r -f, micro- We had planned on running a cover picture showing a wave, or audio frequencies, etc. So many things can be sound wave traveling out from a loudspeaker and, be- done by this method that Mr. Wells has applied for a cause this is strictly a job for a top -flight audio engineer basic patent, and will present a complete story about it rather a than commercial photographer, Winston Wells in an early issue. volunteered to undertake it. Two methods were available to produce sound waves in a form which could be The cover picture for this month, also by Winston photographed, the schlieren (spark) method, and one Wells, likewise required a great deal of work. It shows described by Bergmann, by which a smoke pattern is a germanium crystal which has been chemically treated formed in air at high relative humidity. to improve its characteristics as an amplifier, and should be of extraordinary interest to crystallographers, as well After considerable thought, Wells decided to use the as to audio engineers, for whom this new development latter method because, while difficult to set up extremely holds great potentialities. Recent investigations show with the facilities available, it promised far more spec- that it presents great advantages in transconductance and tacular results. Two days were spent in preparation, ',power efficiency over vacuum tubes now used. From the scale shown, the degree of magnification can readily be calculated. In photographs of this type, crystallographers prefer flat lighting for maximum detail, but for spectac- ular effect, sharp contrast and brilliant highlights are desired. Note that Wells has produced both.

Our next scheduled cover is another presentation of a sound pattern, which undoubtedly will hereafter be called a "Wells" pattern. showing a unique application of this technique.

PERSONAL MENTION Just a few days ago Dr. W. W. Wetzel of the Minnesota Mining and Manufacturing Company joined me for lunch and discussed, among other things, some of the new developments in tape recording. His company, as you know, is producing excellent magnetic tape as well as the sticky kind. Dr. Wetzel has promised us more articles on magnetic recording but when everything was ready it was found that the in the near future. One which deals with an extremely important point is sound power required to produce a pattern was so great now awaiting company clearance. that other occupants of the building complained and it had to be abandoned. By this time we were perched on Mr. W. S. Barrell, managing director of the recording our deadline, minus the desired cover photo. But the subsidiary of E.M.I. in England arrived a few days ago. resourceful Mr. Wells cooked up another method, one His organization produces HMV records, famous through- never before used so far as we have been able to discover, out the world. They record flat, at the outside turn, up yet so simple that a mere explanation of the method was to 20,000 cycles as a matter of course and their pressings sufficient to convince your editor that it was unquestion- lose very little. Mr. Barrel! addressed a meeting of the ably feasible. But, because the photograph had not yet Audio Engineering Society in New York and told of the been made at the time the contents page describing it work of the British Sound Recording Society, of which had to go to press, our description was pretty vague. The he is president. An arrangement for an interchange of photograph, which is again reproduced, on this page, rep- papers between the two Societies has been made. resents a 3300 -cycle note traveling out from a small

6 AUDIO ENGINEERING NOVEMBER, 1948

www.americanradiohistory.comAmericanRadioHistory.Com

PERMANENT MAGNETS

As electrical constituents go, permanent magnets are relatively new. They made tremendous advances within the past decade, especially in the communications and aviation industries, and in the general fields of instruments, controls, meters and mechanical holding devices. Many of these uses were problems that just couldn't be solved until permanent magnet materials were developed to do the job -a work of pioneering to which Arnold con- tributed a heavy share. Many other applications were those where permanent magnets supplanted older materials because of their inherent ability to save weight, size and production time, as well as greatly improve the performance of the equipment. To these advantages, Arnold Permanent Magnets add another very important value- standards of quality and uniformity that are unmatched within the industry. Arnold Products are 100% quality -controlled at every step of manufacture. What's more, they're available in all Alnico grades and other types of magnetic materials, in cast or sintered forms, and in any shape, size or degree of finish you need. Let's get our engineers together on your magnet applications or problems.

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AUDIO ENGINEERING NOVEMBER, 1948 7

www.americanradiohistory.comAmericanRadioHistory.Com cAttivi.ß Sir: Mr. Redman's recent letter prnposrs a solution to your desire to include in AUDIO SHORE "900MG" ENGINEERING material on television without conflicting with present policies. Of course, any journal exists because of the readers it attracts; and if broadcasting engineers predominate among yours, then the journal should certainly be on an everything- except- r.f. sound broadcasting basis, thus including, audio engineering, television, facsimile, ' wired music and wired television. For my part, I believe that you can at- tract enough readers among the "informed laity," as musicians, public address engineers and recordists, to keep AUDIO ENGINEER- ING expanding with present editorial policies. Vincent Salmon, 6220 S. Moody Ave. Chicago 38, III. We agree. Ed.

Sir: I just wanted to let you know that I agree with the first two paragraphs of Mr. James F. Redm.am`s letter (September issue, page 4) that AUDIO ENGINEERING is doing a most outstanding job. One never gets the impression that certain articles are used just to fill out the book. Each presentation is informative, helpful, and authoritative. As far as publication is concerned, the audio field- surely the electronic field of greatest interest among the largest number of people -has finally come into its own. Your September editorial especially ap- pealed to me. I can probably push a slide Gives Maximum Reproduction of rule about as fast as average and I keep a good supply of pencils and paper on hand, Micro -Groove Record Fidelity but when I read a radio magazine I want to keep a cigarette in my hand, not a log table. When the time comes to apply the design information the formula will be The Shure "900MG" Pickup is an ideal instru- used, but before that, an article must be ment for tracking on the new micro - groove readable. Of course, when the time comes to apply the data, RADIOFILE will also be records. It tracks at 6 grams ... uses a special useful in finding it (advt.). I want to compliment you, in addition, on offset osmium - tipped needle with a point radius the makeup of the magazine. The choice of type faces is excellent and the use of of only .001" ... and has an output of 1 volt! color and bleeds extremely tasteful. The Audio Design Notes are very useful (though The Shure lever system has been adapted in the sometimes, as im September, the audio application seems to get lost) and Canby's development of this new pickup - providing a column is always entertaining, though not compliance. will be necessarily particularly informative. high needle Listen to it -you I might as well get in on the TV contro- versy, too. It might be true, theoretically, thrilled with the results ! that adding a TV section to the magazine would give the reader more information for his money. But I believe it would detract Model "900MG" Code: RUZUZ from the flavor of the magazine. We now feel that AUDIO ENGINEERING is the audio Shure Patents Issued and Pending. Licensed under the Patents of the Brush Development Co. oracle. AUDIO ENGINEERING would, in other words, lose its distinctiveness as a special- FtwaA.1:4 aW ..-;a ANd ,W-4_4z;:. ist's publication. In short, you have an excellent publication which is doing a fine job. Please don't dilute it with other material. SHURE BROTHERS, Inc. Best wishes for continued success. Microphones and Acoustic Devices Richard H. Dorf, 255 W. 84th St. 225 W. HURON ST., CHICAGO 10, ILL. CABLE ADDRESS: SHUREMICRO New York City.

8 AUDIO ENGINEERING NOVEMBER, 1948

www.americanradiohistory.comAmericanRadioHistory.Com /41.11.1

C AC MI 1

4 Customode is the answer to the ever expanding requirements of audio -

video equipment. Today, you may install a 15" speaker, communications. receiver, tuner, and a record changer. Tomorrow, you can add a television set, a pick -up for Micro -groove records, and a record cabinet. The illustrations show only a few of hundreds of cabinet variations for Home Entertainment Centers. Designed by leading furniture stylists and electronics engineers, Customode's "building block" versatility enables you to create your own layouts as you wish, when you wish. Write today for literature and scale cut -up illustrations. Jensen Manufacturing Company, 6633 S. Laramie Ave., Chicago 38, Illinois.

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10 AUDIO ENGINEERING NOVEMBER, 1948

www.americanradiohistory.comAmericanRadioHistory.Com Audio System Design Fundamentals HOWARD A. CHINN*

First of a series for audio engineers.

THE DESIGN of audio facilities for proper impedance matches and a sys- lifiers). As a matter of fact, the out- broadcasting, recording, or for tematic method of portraying the basic put levels of present -day wide -band sound reinforcing (public -address) circuit design. From a physical layout microphones and pickups are often so applications divides itself naturally in- viewpoint, consideration must be given low that, where involved, they may be to two parts; (a), the design of the to convenience and ease of operation, the limiting factor in achieving a high individual components and (b), the de- visibility and legibility of controls, signal -to -noise ratio. Under circum- sign of the complete system utilizing lines of vision into adjacent areas and stances such as these, it is obviously these components. Many of the essen- similar considerations. This paper necessary to avoid any degradation of tial components, such as microphones, concerns itself only with the first of the signal -to -noise ratio because of poor amplifiers, volume controls, loudspeak- these two broad categories, namely, the system design. ers, volume indicators, etc., are readily circuit fundamentals. System Noise Levels available in several grades and in var- Program Levels Audio system components, when pro- ious forms and sizes. Thus, except The prime consideration in the de- perly designed and carefully installed, for certain specialized applications. sign of any high -quality audio system need not contribute any appreciable such as are found in the more elabor- is the establishment of proper signal noise to a system other than that ate and meticulous audio installations, levels at all points in the system. This caused by thermal agitation. In other it is usually possible either to use com- words, using the components that are mercially available components that available today, no problems need be fulfill the requirements or to design faced because of noise caused by hum, them readily by employing conventional microphonics, or poor contacts. methods. Under these circumstances, On the other hand, thermal noise is the design of audio systems themselves a limitation that is always present. It can be approached on the assumption AMPLIFIER establishes the limit below which the that most of the components required circuit noise cannot be reduced. By are readily available. the same token, it automatically estab- The most complex audio installation lishes the signal level that must be can generally be broken down into maintained in order to achieve a spe- smaller units. For example, in a cified signal -to -noise ratio. Similarly, broadcasting plant the audio facilities it determines the lower limit of useful may be segregated into studio, portable, Fig. I. In this diagram the amplifier output from microphones and pickups. the load program distribution, building and output load is 150 ohms, but As is well known, the thermal agita- is only 86 ohms. office monitoring, sound reinforcement, facing each section tion voltage, e, of a circuit element is recording and allied systems. The ba- stems from the need (a), for obtaining given by the following equation:1,2 sic principles governing the design of the required audio levels for the pur- -20 audio systems apply equally well to e= V1.605 x 10 x of x R (volts) pose in hand, (b), for avoiding over- broadcasting, to sound recording (pho- where AP= bandwidth in cps loading in so doing, and (c), for main- nograph records, transcriptions, mo- R =resistance of element in ohms taining the desired signal -to-noise ra- tion picture), to public -address, wired - tio. Knowing the signal intensities re- Audio system levels are usually ex- music and other applications. quired, overloading can be averted pressed in terms of either "dbm'' or The most important considerations in simply by the proper choice of compo- "vu ", depending upon which term is any system are the operating require- nents. The successful achievement of applicable.3 It is therefore convenient ments that must be fulfilled. Estab- the required signal -to-noise ratios, on also to express noise on a similar basis. lishment of these determines the ela- the other hand, hinges upon the basic Assuming a band -width of 15 kc and borateness of the installation, the grade system design and the skill with which carrying through the calculation, the of equipment and workmanship re- its actual realization is undertaken. thermal noise level in a circuit having quired, the physical layout, and, of The problem becomes more and more matched, pure resistance source and course, the over -all cost. The most acute as higher and higher quality sys- load impedances is found to be equiva- extensive installation can be success- tems are demanded. High -quality sys- fully formulated, however, by faithfully tems imply, among other things, wider 1. J. B. Johnson, "Thermal Agitation of Elec- following certain fundamentals. From frequency bands and higher signal -to- tricity in Conductors" Physical Review, an electrical viewpoint these include Vol. 32, No. I, pg. 97 (1928) noise ratios. On the other hand, wide- establishment of adequate signal levels, 2. H. Nyquist, "Thermal Agitation of Elec- band electronic devices usually entail tric Charge in Conductors" Physical Re- avoidance of overloading, observance of lower output level (microphones), lower view, Vol. 32, No. 1, pg. 110 (1928) sensitivity (microphones, loudspeakers 3. H. A. Chinn, "VU vs. DBM", Auolo EN- *Chief Audio -Video Engineer, Columbia GINEERING, Vol. 32, No. 3, p. 28 (March Broadcasting System. Inc., N.Y.C. and phono pickups) or lower gain (amp- 1948)

AUDIO ENGINEERING NOVEMBER, 1948 II

www.americanradiohistory.comAmericanRadioHistory.Com AMPLIFIER m PRIVATE LINE TELEPHONE OUTLET GROUND ON CENTER TAP

AUTOMATIC GAIN -ADJUSTING AMPLIFIER PRIVATE LINE EXTENSION OUTLET y-} CIRCUIT CONNECTION

BOOSTER AMPLIFIER POWER SUPPLY CIRCUIT NOT CONNECTED m I BRIDGING LINE AMPLIFIER m RELAY AUDIO CIRCUIT

MIC. PRELIMINARY AMPLIFIER RDO RADIO RECEIVER --' --- CONTROL CIRCUIT Ap PROGRAM AMPLIFIER LINE, BRIDGING, OR IMPEDANCE MATCHING TRANSFORMER - - - -- POWER CIRCUIT RECORDING AMPLIFIER m TURNTABLE MECHANICAL COUPLING

L S MONITORING AMPLIFIER m VOLUME CONTROL fly PATCH CORD ATTENUATOR m VOLUME INDICATOR EQUALIZER 1:3 WALL RECEPTACLE Mil 111E1 LOW -PASS FILTER LATERAL RECORDER

SINGLE JACK DOUBLE JACK 1E1 HIGH -PASS FILTER m VERTICAL RECORDER

BAND -PASS FILTER AMMETER

EI KEY SWITCH O VOLTMETER SINGLE JACKS NORMALED DOUBLE JACKS NORMALES LOUDSPEAKER O POSITIVE BATTERY SUPPLY MICROPHONE o NEGATIVE BATTERY SUPPLY 0 PAD -/V\/- SERIES RESISTOR

m HEADPHONES SHUNT OR TERMINATING RESISTOR SINGLE JACKS NORMALED DOUBLE JACKS NORMALED PH-0 HEADPHONE CUE OUTLET 1 GROUND ON ONE SIDE WITH MULTIPLE WITH MULTIPLE

Fig. 2. Standard symbols used in audio engineering drawings. lent to -129 vu. This is the theoretic- permissible minimum, amplification amplifiers are usually given prime con- ally lowest noise level that can be must first be provided to avoid degra- sideration. It is equally important, achieved in any circuit having a 15 kc dation of the signal -to -noise ratio. however, to recognize that other units, band -width. In practice, with careful Once the signal -to -noise ratio of an particularly equalizers, filters, match- design, this figure can often be ap- audio system suffers, it cannot usually ing and bridging transformers, loud- proached within a few decibels. It can be reclaimed since both the signal and speakers, and recording heads also have never be equalled, however, since there the noise are always amplified equally. their limitations. The response, im- is always some residual noise present That point in the system where the pedance and /or distortion as a func- ill an actual system. In any event, signal -to -noise ratio is the lowest is tion of frequency may change markedly knowing the lowest possible noise level, the controlling one-it is the weakest with level in some of these units. the minimum signal level required to link in the chain and a stronger link Their characteristics at both high and achieve any desired signal -to -noise ra- further on can do nothing to overcome low operating levels should always be tio is immediately known. the basic weakness. investigated. For example, assume a 60 -db signal - Overloading Impedance Matching to -noise ratio is required. Under these The avoidance of overloading in au- ßigorously speaking, except for sim- circumstances the signal level can nev- dio system design simply requires the ple elements such as a fixed resistance er be permitted to fall below -69 vu. proper choice of components. As has attenuation pad or a terminating re- Since, in practice, the noise level will been detailed elsewhere3, a 10 -db safety sistor, few audio components exhibit always be somewhat higher than the factor is usually allowed between the ahosolutely constant input or output theoretical minimum, some safety fac- sine -wave level that corresponds to the impedances at all operating levels and tor should be applied. A typical good maximum permissible distortion and throughout the audio spectrum. Fur- engineering practice would be never to the safe level for program material. thermore, the impedance of some de- allow the signal level to fall below --60 That is, if a given component is cap- vices may be a function of the adjust- vu. some services In the signal -to- able of handling a maximum sine -wave ment of controls (e.g., ladder attenua- noise requirements may not be so strin- level of +20 dbm, without exceeding tors, variable equalizers, and adjustable gent as the example cited. In any case, specified distortion limits, it may be wave filters). the basic principle outlined will apply. safely used with program levels of +10 Fortunately, except for filters and A simple rule is therefore established. vu. To be sure, greater factors of safe- some types of equalizers, the impedances If, at any point in a system, the loss ty may be used but they are not gener- of many components are reasonable, entailed by the use of some device ally economically justifiable. constant and uniform over at least a (e.g., a mixer control, a matching not - In any discussion of overloading in fair portion of operating range. It is work, an equalizer, filter, a or a bridg- an audio system, the capabilities of the the impedance values that obtain in ing coil or pad) will cause the level these regions (where, incidentally, the of the desired signal to fall below the 3Loc. cit. phase angle is usually small) that are

12 AUDIO ENGINEERING NOVEMBER, 1948

www.americanradiohistory.comAmericanRadioHistory.Com generally specified as the nominal im- consider an amplifier that is to feed networks are again indicated if impedance of the device in question. four identical loads, each of which is pedance mismatches are to be avoided. Most audio components are designed intended to operate from a source im- Block Diagrams to operate from a source impedance pedance equal to its input impedance A very effective way to practice the and into a load impedance that is a (Fig. 1). Since a numerical example four principles that have been expound- pure resistance of finite value. As a will quickly illustrate the point, assume ed is to make use of a systematic me- matter of fact, the performance of au- impedance of each load has an input thod for portraying the circuit that is dio equipment is usually s ^ecified and will 600 ohms. The four in parallel being designed. Many years ago4 a measured on this basis. It is therefore result in a net load impedanee for the method was evolved which has with- good engineering practice to provide source of 150 ohms. If, now, the gener- stood the test of time and is worth the the proper resistive source and load this ator has an output impedance of repeating. Basically, it is nothing impedances to insure that the complete- value and is designed to accommodate more than the well -known block dia- ly assembled system will perform as a like load impedance, all seems well. gram. It is the embellishments on predicted from the characteristics of Con- This is not the case, however. this diagram, however, that make it so the individual components. The aver- sider the impedance from the viewpoint useful. age audio system lends itself readily to load. Looking back from of any given First, a series of standard symbols this treatment since volume controls -ohm any given load, one sees the 150 are used (Fig. 2.) For simplicity and (other than ladder networks and simple in output impedance of the amplifier in all the potentiometers), differential networks, speed drafting practically parallel with which there are the other symbols are rectangles of uniform size. attenuation pads, terminations and three 600 -ohm loads. In other words, The letters within the boxes are chosen other constant resistance devices are each load faces the net impedance that so as to suggest, insofar as possible, almost universally interposed between from the is just under 86 ohms -far the item being identified. The conno- the other elements of an audio system. conditions of 600 -ohm value that the tation of each symbol is almost so ob- Under these circumstances the prob- to the example specified. The solution vious that once reviewed they require lem of providing the proper source and is the use of a the problem, of course, almost no further identification. load impedance for each coml;'onent network differential matching -but Next, the conventional block diagram simply requires the observance of two that is a subject in itself and will not is adorned with those features shown rules: be treated here. in Fig. :3 that are applicable to the a. Interpose constant resistance elements The principle cited must also be ob- between components whose input and case in hand. It is this supplementary output impedances do not provide the served in the reverse situation -that is, information that really establishes the be com- proper load and source impedances. when several sources are to value of method of presentation being b. At a junction point where more than two A very bined into a single channel. described. Taking up the items in the components meet, scrutinize the imped- of kind is in ance relations from the viewpoint of each common example this branch of the circuit. mixer circuits where a number of pro- 4H. A. Chinn, "Broadcast Studio Audio Fre- It is this latter requirement that gram sources are brought together in- quency System Design," Proc. I.R.E., Vol. often trips the novice. For example, to a common channel. Differential 27, No. 2 (February 1939)

Fig. 3. Application of symbols in a typical block diagram showing signal levels.

As +30VUlAl D 13 1 INDICATES CONTROL CIRCUIT r4 DB- SKn } 150-rt 150.n.

Ap +16 VU © 0 +I.O^VU LINE I (+51 DB) (-6 061 15011. 7Cr 150A

I QUOTATION MARKS INDICATE LEVEL SHOWN IS "EFFECTIVE" VALUE. 2 INDICATES PROGRAM LEVEL AT DESIGNATED POINT IN CIRCUIT. 3 INDICATES GAIN OR LOSS IN ASSOCIATED UNIT OF EQUIPMENT. 4 INDICATES CIRCUIT IMPEDANCE IN DIRECTION INDICATED. 5 INDICATES TERMINATING SHORT CIRCUIT ON INPUT WHEN KEY IS IN OFF POSITION. 6 INDICATES TERMINATING RESISTOR ON OUTPUT WHEN KEY IS IN OFF POSITION. 7 INDICATES COMMON SIDE GROUNDED. S INDICATES ONE SIDE OF OUTPUT WINDING GROUNDED. 9 INDICATES CENTER TAP GROUNDED. 10 INDICATES RELAY COIL ENERGIZED IN THIS POSITION. I I INDICATES RELAY COIL OPEN IN THIS POSITION. 12 INDICATES CABLE CONNECTOR. 13 INDICATES DIRECTION OF FLOW.

AUDIO ENGINEERING NOVEMBER, 1948 13

www.americanradiohistory.comAmericanRadioHistory.Com order in which they are illustrated : Items 8 and 9: In some cases one side or Item 1: Microphones are intended to work the center -tap of an input or an LEGEND into what is essentially an open output circuit is grounded. This CODE DESCRIPTION MFR. is indicated by showing a ground circuit (although their output im- Au MIC. PRELIM. AMP. LANGEVIN 116-A pedance is of finite value), conse- connected to the proper side of the quently their output levels cannot block symbol. When the center -tap AP PROGRAM AMP. RCA BA 3A is be rigorously stated in terms of vu grounded, the letters "CT" are As L.S. MONITORING AMP. RCA BA 4A design pur- added. When unaccompanied by (or dbm). For system Ki KEY SWITCH WE 2ARN poses, however, a microphone may any notation the ground is assumed be considered to have an "effective" connected to the appropriate side of LS LOUD SPEAKER RCA LC -IA the circuit. By output level that is stated in the showing all grounds M MICROPHONE WE 639.8 conventional terms for designating on a block diagram, the need for an Pi FIXED'H°PAD,6De,150 :150n DAVEN 154 audio levels. isolation coil automatically becomes Item 2: Along the circuit, each time there evident as, for example, when a RS RELAY WE U -1017 one -side is a change in program level, the grounded component is to VC VOLUME CONTROL DAVEN T-330 new level is shown. This practice be connected to a center-tap ground- keeps the designer constantly in- ed element. WR WALL RECEPTACLE CANNON UA3 -13 formed as to conditions that might Items 10 and 11: When relays are used to Q JACK WE 239 -E lead to overloading on one hand and transfer a circuit from one point degradation of the signal -to-noise to another, it is important to know ratio on the other. The levels shown which path prevails when the coil are generally the peak program is energized and which when it is Fig. 4. Legend used with block diagram. levels that may be expected under open. A good engineering practice normal transmission conditions. is to have all relays at rest (coil Item 3: The loss or the gain, for normal circuit open) when `on the air" or for battery control circuits, when setting of the controls, of each cir- when in normal service. When it shown. cuit element is indicated. These is possible to follow this practice, data are useful in calculating pro- a failure of the relay power supply Legends gram levels along the circuit and in will not interrupt the program Even though standard symbols are selecting the particular component transmission. The letter "O" in- used, each block diagram should be ac- that will fulfill the requirements. dicates the circuit when the coil companied by a legend (Fig. 4). The Item 4: The internal impedance of the com- is open; the letter "E" indicates ponent in the direction of the asso- the circuit when the relay coil is legend serves the purpose of identify- ciated arrow head is indicated. In energized. ing each symbol for those who have only those instances where applicable, Item 12: When a plug is used to connect casual contact with them. Equally im- directions show- is arrows face in both one component to another, it a de- ing that the impedances are shown by an arrow head. portant, it also provides complete matched. In others, an arrow faces Item 13: Tracing the progress of a signal scription, including the manufacturer in one direction only. For example, through a complex system is great- and type number, of every item. the microphone has a finite output ly facilitated by the use of direc- impedance, but the input of the tion -of -flow arrow. Another helpful Conclusion associated preliminary amplifier is convention is to arrange the block The basic design of even the most very high. diagram so that the flow is always complex and elaborate audio system can controlled Items 5 and 6: Circuits that are from left to right unless otherwise to by key switches are often terminated indicated. When, for one reason or be greatly simplified by adhering or shorted when the key is in the another, this direction of flow can- certain fundamentals, the more im- "off" position. Appropriate symbols not be adhered to, a flow -arrow portant of which have been described. for both conditions are shown. Un- should be shown. These include the establishment and less otherwise noted, the value of the terminating resistor corresponds Special attention is called to the con- maintenance of adequate signal levels to the iterative impedance of the vention where key switches or relays as determined by the required signal - circuit. having multiple inputs or outputs are to -noise ratios, the avoidance of over- Item 7: Often one side of a circuit element is grounded. In fact, it is involved. The connection to the switch loading and the observance of proper usually mandatory in the case of T arm is shown on one side of the block impedance matches. and pi- section attenuators, pads, symbol while all connections to the Perhaps most important of all, from equalizers and filters. These often switch points are shown on the oppo- the viewpoint of simplifying the work, have one circuit that is common to both the input and output circuit. site side of the block. Audio circuit is the employment of a block diagram When this is the case and this com- connections are never shown to the top containing all the essential facts that mon leg is grounded, the connection or bottom edge of the block, these being influence the design. In some estab- is shown attached to the lower side resistors [Continued 41] of the block symbol. reserved for terminating or on page

audio Eriqine,2liiHg Society, San .3A.ancii.cv 5s2ctir n

Newly elected officers of the recently formed San Francisco S ?ction of the Audio Engineering Society. Left to right: Myron Stolaroff, Ampex Electric Corp.; Jack Mullen, Bing C-osby En- terprises; Harold Lindsay, Ampex; Frank Lennart, Ampex; Don E. Lincoln, Audiophone; I. R. Ganic, Audiophone, (Chairman); Walter T; Selsied, Pacific B, oadcasting; Dick Beck, C. C. B. own, and Ross H. Snyder, KJBS -FM.

www.americanradiohistory.comAmericanRadioHistory.Com cD 4 a CDflfiì UJcL6IQ AUDIO SIGNAL GENERATOR

BRUNTON BAUER*

This instrument meets the need for a stable, accu;aie unit for testing high- quality audio apparatus.

VER THE LAST FEW YEARS ception of the receiving -section indi- Application of the carrier null method audio engineers have tightened cator. Because of the attenuator ar- of measuring percentage modulation of up on fidelity requirements so rangement, this needs only to be the an f -m transmitter, for instance, re- strenuously0 that one day's laboratory simplest indicating instrument or vtvm quires this continuous variability. In equipment is the next day's commercial capable of revealing a change in level. amplifier. This state of affairs calls for Both this instrument and the meter in 206A H -P EQUIPMENT RECEIVING SECTION AUDIO UNDER INDICATOR higher- caliber measuring equipment the signal generator are eliminated SIGNAL GENERATOR TEST (NO SPECIAL ACCURA- than, for example, the time -honored from the accuracy of measurement be- CY REQUIRED) audio oscillator. A unit has been need- cause they are kept at a fixed reading ed which could emit a known value of by adjustment of the attenuator. Typi- Fig. 2. Typical test set -up. signal at a known impedance level with cal equipment -test setup is shown in extremely low distortion. Continuous Fig. 2. this measuring technique an unmodu- variability and self- contained design Output frequency is selected by the lated 200 -kc i -f signal containing the are additional desirable features. combined use of the frequency control full f -m swing of the transmitter is Use of the term signal generator for monitored on a communications -type SELECTIVE receiver with a several-hundred -cycle the piece of apparatus illustrated here- OSCILLATOR OUTPUT AMPLIFIER AMPLIFIER note while sinewave is in implies that a high degree of con- beat modulation applied to the transmitter in increasing + , i trol and measurement can be exercised IMPEDANCE MATCHING ATTENUATOR VTVM amplitude. The beat note will pass over the output signals, and major TRANSFORMER effort in the over -all design has been through successive nulls at a known applied in that direction. In this series of combinations of modulating Fig. I. Block diagram of generator frequency and percentage modulation generator it has been possible to sections. as the carrier disappears in accordance keep distortion below 0.1 per cent with Bessel- function relationships. from 50 to 20,000 cps and be- and the range switch. The total range Smooth frequency variation is low 0.25 per cent between 20 and 50 essential of 20 cps to 20 kc is covered in three in the detection of these nulls. cps. This has been accomplished with ranges: Times 1 -20 to 200 cps; Times a circuit combining a resistance -tuned 10 -200 to 2000 cps; and Times 100- Oscillator oscillator with a tuned amplifier that 1000 to 20,000 cps. This control gives The complete signal of the audio sig- is tracked with the oscillator in such complete coverage of the audio spec- nal generator is shown in Fig. 3. The a way as to cause rejection of harmonic trum and makes it possible to study oscillator section consists of tubes Vi voltages while passing the fundamental any part of it in extreme detail for and V2. This section is in the form along to the output amplifier. Arrange- detection of peaks which might pass of a conventional resistance -tuned os- ment of these sections is shown in Fig. unnoticed if the analysis were limited cillator. The two stages are resistance 1, the block diagram. to discrete steps of frequency. coupled and positive feedback through The output amplifier has a negative feedback loop for distortion reduction. The output of this amplifier goes to the The audio signal gener- in output system consisting of a vacuum - ator (left) operation tube voltmeter, a three- section atten- on test bench. uator, and a line- matching transform er-a combination permitting the transmission of a signal at known level from a known impedance source. An electronic voltage regulator is included in the power supply to provide stead] \ plate voltages throughout. All equipment necessary to making complete gain measurements is thus consolidated in one unit-with the ex-

*Chip l Engineer. Hewlett- Packard Co., Palo Alto, Calif.

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16' AUDIO ENGINEERING NOVEMBER, 1948

www.americanradiohistory.comAmericanRadioHistory.Com the resistance -capacitance network associated with the A section of switch Si and the four -section capacitor gang Ci sustains oscillation and determine; oscillation frequency. These circuit low coeffi- elements have temperature 4,'ë:úi+ cients for over -all stability. Frequent: calibration is accurate within 1 per cent. The oscillator has a negative -feedback loop Ris, R17, and R13. Resistor R13 is a non -linear element consisting of a 3 -watt, 120 -v lamp which acts as an automatic amplitude limiter and maintains the system at its optithum operating point. This lamp is standard except that it is subject to the following selection: Voltage from the high side of R24, measured to ground with a high -impedance voltmeter, must read 28 volts + 1 volt. It can be adjusted by potentiometer Ras with a lamp of the proper characteristics in the socket. An appreciable percentage of production lamps fall outside these limits and cannot be used. Tuned Amplifier Fig. 4. Top view of signal generator from behind panel. Oscillator- section is Electron tubes Vs, V4, Vs, Vs, and to the right tuned amplifier to left. VT-together with their related circuit elements-comprise the tuned -amplifier ing section in more detail, the rear deck loads with a minimum of distortion. visibi!- loop from section of the signal generator. Tubes having been removed for better Negative-feedback Cie Rej The two of the tuning the plate V9 to the cathode of Vs V8 and V4 are conventional voltage ity. sections of factory amplifiers. Cathode follower Vs has, capacitor are tracked at the further minimizes distortion. flexible in its cathode circuit, a Wien bridge and driven by a stainless -steel Vacuum Tube Voltmeter copper tuned to a null at the fundamental fre- cable sheathed with beryllium The voltmeter, M1 is an 0- to -1-ma, resistance. The oscillator quency of the output. It is tuned by for abrasion 4- in.- square d -C instrument connected is the in another resistance-capacitance network section of the gang to right into a full-wave bridge rectifier with is on the identical with that in the oscillator sec- both views. The amplifier four 1N34 germanium crystals as recti- tion described previously. Capacitance opposite side. fier elements. This meter is calibrated is varied by another four -section unit Output Amplifier both in volts and dbm (zero dbm: 1 of the tuning capacitor ganged to the Tubes Vs and Ve are the major parts milliwatt into a 600 -ohm load). Series first. The second section, B, of switch of the output amplifier, Vs operating as resistance is provided by Res and Rea Si, ganged to section A, switches the a driver for output tube Vo. Variable -the latter providing adjustment for three sets of resistors for range deter- resistor R57 in the grid circuit of Vs calibration. mination. serves as an amplitude control. Pen- There are three sections in the atten- Tuning this bridge to a null elimin- tode V9 is connected here as a triode uator which gives a range of 111 db in ates the fundamental from the follow- for working into very low- impedance [Contiiued on page 41] ing stages without affecting the har- removed. Capacitor -gang monies present. These are amplified Fig. 5. Rear view of unit with power- supply deck turns range -selector switch, center. in Vs -a conventional voltage amplifier drive is at center top. Bevel gearing connected across the center of the bridge -and fed to V7, a cathode follower, whose cathode circuit is returned to that of Va. Because the cathode of V7 is in phase with that of Vs, negative feedback is established at all frequencies except the one to which the bridge is balanced-the output frequency. Out- put from this section of the generator is taken from the top of the bridge circuit -directly from the cathode of Vs-and has the selective characteristic necessary for tracking with the oscillator and rejecting harmonics to the extent of 10 db or more of distortion reduction. The tuning capacitor can be seen in Fig. 4, a view directly down on top of the open unit. Figure 5 shows the tun-

www.americanradiohistory.comAmericanRadioHistory.Com Characteristics of Amplifying Crystals

S. YOUNG WHITE*

Performance characteristics of experimental germanium crystal amplifier and oscillator units.

IN ORDER to make efficiently o large number of characteristic curves of amplifying crystals, it is necessary first to design a mounting so that the catwhiskers can be adjusted to an accuracy of about .0001 inch in the X and Y planes. The Z plane, which is the compression of the spring catwhisker, is not so critical. If a com- Fig. I. Experi- pression .0025 of inch is desired, a to'er- mental mounting ance of half a thousandth is adequate. The design shown in Fig. 1 meets for germanium the requirements rather nicely. As in crystal amplifier the previous few articles, we use a mounting that can plug into a minia- unit. ture tube socket. The crystal, mounted on its .073 diameter rod, which is a quarter -inch long, is held in a brass cylinder, and can be locked with a set- screw. The catwhiskers are procure 1 from IN21A silicon crystal units, as these are longer and straighter than those that come with the IN34, as a rule. The crystal face is forced firmly In assembly the crystal is retracted Bakelite barrier, we have a new spot. against two Bakelite blocks, A -A, and 0025 inch by a spacer. A catwhisker is Incidentally, there are so many spots the height of these blocks is held to loosely laid in a slot in the pin of the on such a crystal it is impossible to .0001 inch, and thus establishes the X miniature tube base, and forms a burn them all out. axis. Between the blocks is a mica compound angle with the crystal We have one trouble with this assem- sheet about a thousandth thick, which face, which causes it to fall in the bly-it works very well with crystals forms an insulating barrier between the pocket formed by the barriers and the whose ends are truly squared off, but not catwhiskers, thus establishing the Y crystal face. You might be able to vis- axis. so well with those crystals lapped- at an ualize it as jamming a broomstick in a angle. The early crystals we bought corner of a room -it is firmly guided were about 90% true on the ends, but *Consulting Engineer, 52 -12 Van Horn St., into position by the two baseboards and Elmhurst, L.!., N.Y. later batches show as much as 50% off the floor. By watching an ohmmeter we true. the crystal establish light contact with This assembly will take ordinary The assémbly is then soldered. face. untreated crystals from an IN34, of The spacer is then removed, and the course. Some of them make good oscil- crystal forced firmly into the brass lators, but they are very non -uniform, cylinder. The Bakelite barrier stops as even partial treatment of the cry- it, and the contacts are forced stal makes it much more uniform in back 21/2 thousandths, and held apart useful performance. by the mica spacer. With a one -thou- Test Gear sandth spacer, separation of points is A test set up was developed as shown about 11/2 thousandths. in the photograph. The two meters show 60001 By using thinner mica, closer T grid and plate currents, and the two location of the two points is obtained voltage controls are set to the desired 450C but the tendency toward cross -leakage point by an adjustable series resistor. from the plate to the grid is enhanced. 3000. Grid voltage is furnished by a built -in After all, we may operate with over 60 battery, or can be obtained from ex- Z or 80 volts between these points, and it tenbal source. Plate voltage is secured is a miracle that the leakage is so small. from a 250 -volt supply. A built -in The whole assembly is made eccen- plate loading transformer with tapped tric, so by rotating the crystal after primary and six secondary loads pro- pulling it out by any distance in excess vides all plates impedances from 100 Fig. 2. Static characteristics of "soft" of 3 thousandths, and then firmly ohms to 100,000 ohms at 1,000 cycles. crystal. pressing it back in place against the High- frequency oscillator assemblies

18 AUDIO ENGINEERING NOVEMBER, 1948

www.americanradiohistory.comAmericanRadioHistory.Com can also be plugged in, and all leads are accessible for inserting meters or 40 4 loads. Static Characteristics 30 IMPEDANCE There are three types of crystals to (ohms) discuss. We start with the untreated PLATE GRID =.10 20 20,000 200 AC volts crystal of high plate impedance, about otross 50,000 ohms, and of otherwise highly 2p primary variable characteristics. The flashover lo 10,000 100 2q point is medium, say 50 to 60 volts, AC volts but it will not withstand repeated flash- on orle o 0 overs, which rapidly lower the flashover o 10 20 30 40 50 o 10 20 30 40 points to the 5 -10 volt region. PLATE VOLTAGE (Negotive) PLATE VOLTAGE A partly treated, or "soft," crystal is interesting. The flashover is slightly Fig. 3 (left). Dynamic characteristics of "soft" crystal. higher, but again it will have its flash- Fig. 4 (right). Oscillator performance curve. over point lowered by repeated use. Its usable stable transconductance Note that the grid current starts at either when an output transformer is reaches high values, and the number 1 ma and steadily increases to about used for the feedback element. of active spots is about 50 %. The 5 ma. Some of this is due to leakage R2 is the bank of resistors across input impedance is unfortunately very from the plate. the secondary of the Stancor 10 watt low and plate impedances may be from Dynamic Curves universal output transformer built in 5,000 to 20,000 ohms. Using the same soft crystal, the plate the test equipment. These resistors The fully treated, or "hard," crystal, impedance was taken by inserting a tuned the oscillator frequency through has a quite high flashover, often above variable resistive load in the plate and one octave-S00 cycles to 1600 cycles- 100 volts, which is seldom lowered with a change of resistance from 200 below about 80 volts by repeated ohms to 2000 ohms, without affecting flashovers. Its transconductance is often the voltage output. When fed to an somewhat lower than that of a soft amplifier and speaker, it sounded like crystal. It has the very fortunate char- a rather pleasing electronic organ, and acteristic that these flashovers weld the arious "tones'' could be had by varying point of the catwhiskers to the crystal the plate voltage to limits shown in the face, so that unchanging current is wave form figure. Optimum turns ratio drawn. There is no pressure that will for smooth oscillation was 4 to 1 (8000 really give equally steady conditions on ohms to 500 ohms). At 40 volts, plate, an untreated or "soft" crystal. Plate im- the oscillator maintained an output pedance is about 10,000 ohms, and the signal of ten volts for four hours with- input resistance is higher than that of out changing in intensity more than a soft crystal. 2 %. Fig. 6. Improved oscillator circuit. A cathode resistor R3 was inserted and stopped oscillation with a plate Fig. 5. Exper, adjusting it until the voltage drop voltage of 30 volts when set at a value of 400 ohms. mental oscilla r across the 100,000 ohm plate resistor is halved. It will be noted that the The 400 -ohm cathode resistor was circuit plate resistance is constant from 25 also inserted when we were investigat- volts up. The gain values are normal, ing stage gain. It raised the input 0.5v. the highest gain we ever obtained into impedance from 100 to 1000 ohms. The static curves of Fig. 2 are from 100,000 ohms being about 50. Unfortunately, the gain fell to about a soft crystal, which we will show in An interesting point is that the zero. The input signal was 0.1 volt, dynamic and also oscillating condition change in input impedance due to plate across cathode resistor 0.1 volt also, (Figs. 3 and 4). There is one odd fact voltage variation is small, and if the and across plate load 0.5 volts. about these static curves that makes plate be optimum loaded the reflection When amplifying a sine wave signal, them doubtful for calculation purposes. of this dynamic load causes not more the output signal distortion is unnot- If we slowly run the curves by rotating than 2 percent change in input resist- iceable on an oscilloscope. a grid bias potentiometer, for instance, ance. the curves are correct. But if we switch Oscillator Use the voltage on suddenly, there is a This same crystal oscillated quite sluggish response -that is the meters nicely. The first circuit (Fig. 5) used will jump to a new value, and then direct feedback through the output drift for about two seconds, the drift transformer. This gave interrupted accounting for about ten per cent of oscillations, so a grid condenser the total change. and leak (Fig. 6) was inserted. This The curves show high values of trans - circuit gave much smoother oscillations. conductance, 6,000 at 30 volts. This Oscillation started at 8 volts on the is a stable condition. Note the change plate, and reached ten volts rms at 40 in grid volts is only 0.5. The mu is volts, plate. The wave form was pecul- around ten, and varies with plate iar, as is shown in Fig. 7, but remember voltage. a vacuum tube does not give a sine wave Fig. 7. Oscillator output waveforms.

AUDIO ENGINEERING NOVEMBER, 1948 19

www.americanradiohistory.comAmericanRadioHistory.Com Fig. I. External view of the control amplifier, showing the unique arrangement of the components. Operating controls are on the opposite side of the chassis. sknunfiL RESIDENCE RADIO SYSTEMS

C. G. McPROUD* PART III

A description of the control unit, with design data on the phonograph equalizer and noise suppressor circuits

N THE FIRST TWO ARTICLES The phonograph preamplifier and 600 -ohm line feeding the power ampli- of this series, both AM and FM the noise suppressor are grouped into fier, which has a transformer input. tuners were discussed as suitable one unit known as the control ampli- This practice has the advantage of input sources for a specific type of fier, so called because it is the only separating the various sections into Residence Radio system. These units point in the system where any control units which may be changed indivi- were designed with two primary con- over volume or frequency response is dually if a desire for modification siderations- quality of reproduction exercised. It includes a number of cir- should arise or if new developments and convenience in use. Tuners alone cuit sections, but is constructed as a make it advisable, thus avoiding scrap- Flo not make a radio receiver, and this single unit. Throughout this entire ping a complete system as would be article covers the next stage in the system, professional practices are fol- necessary if it were built in one unit. system. lowed to a large extent, and the output It is not economical from the stand- of the control amplifier appears as a point of tubes or chassis or plugs and *Managing Editor, AUDIO ENGINEERING. cables, but it does provide a high degree of flexibility. The control amplifier consists of a phonograph equalizer with two turnover points, a preamplifier, switching facilities for selecting various inputs, a Scott -type noise suppressor, and a compensated volume control. Since the suppressor can serve as a low -pass filter, and since the volume control furnishes almost complete compensation for the Fletcher -Munson hearing curve, no tone controls are employed. If they are ever considered necessary, they may be installed in the 600 -ohm line to the power amplifier without disturbing either chassis, and constant-

Fig. 2. Internal view of the chassis prior to completion. The input transformer is at the upper left corner, and resistors and capacitors will be mounted on the Bakelite strips. Note use of copper tubing for shielding long leads.

www.americanradiohistory.comAmericanRadioHistory.Com impedance equalizers can be used in Phonograph Equalizer (A) in Fig. 5. The easiest method: of accordance with professional practice. A more accurate low- frequency adjusting Ci and R2 is to reduce the So far, no such controls are deemed equalization curve, with lower losses, latter to about 2,000 ohms, and then necessary. can be obtained from a magnetic pick- to vary Ci to obtain a dip at the turn- The physical arrangement is shown up equalizer by the use of low- imped- over frequency. Then increase Rs clearly in Fig. 1. The chassis is a ance elements. The unit used here gradually until the curve is flat above standard 7x15x3 radio chassis, but all consists of the elements to the left of the turnover. This will result in a the tubes and some other components Ti in Fig. 3, the over -all schematic. smooth but fairly sharp turnover. The are mounted on one of the 3x15 sides, over -all schematic shows two capacitors, with the controls on the opposite one. CI R2 Ci and 02, together with two resistors, This makes parts accessible and pro- R2 and R3. These provide two turn- vides a desirable chassis arrangement. over frequencies, one at 800 cps and The plate and filament power are ob- one at 400 cps, a compromise between tained from the main power supply the common 300 -and and 500 -cps points. which furnishes 225 volts, regulated, The resistor R4 adjusts the slope of for the plates, 12 volts d.c. for six of the curve below the turnover, as shown the heaters, and 6.3 volts a c. for the at (B) of Fig. 5. In order to make seventh. Figure 2 shows the inside of measurements on this type of circuit, the unit before completion, with some the oscillator should be connected with of the resistors and capacitors mounted Fig. 4. Simplified circuit of the phono- a series inductance equivalent to that on the strips. Note the use of solid graph equalizer. of the pickup to be used. A simpler copper tubing for shielding leads that procedure is to connect the actual pick- must be run any great distance. These To adjust such an equalizer requires up between the oscillator and the in- tubes are shaped and soldered in place, the use of an audio `oscillator, but. once put terminals, as at (C). The input and the wire pulled through afterward. the principles are understood, an ideal transformer provides a voltage step -up Tubing provides better shielding and equalization curve can be obtained. to the grid of the first tube. Any good is self -supporting. The basic equalizer circuit is shown quality, well shielded input transform- Electrically, the control and ampli- in Fig. 4. The series inductance Li er should suffice, with the primary in- fier consists of a phono-preamplifier and is tunhd by the capacitor Cl to the tended to work from a 250 to 600 -ohm equalizer, an input stage, two high - turnover frequency and with R2, serves source. Resistor Ri across the input frequency gate tubes, one low- frequency to adjust the response in that region. terminals is used to flatten the response gate tube, and the output stage, with The shunt elements, R4 and Cs, control over the higher end of the spectrum, the necessary side amplifier and recti- the low- frequency rise. The value of and a switch on the phonograph motor fiers to actuate the suppressor gates. R2 is somewhat critical, as shown at board shunts an additional 1800 -ohm

Fig. 3. Complete schematic of the control amplifier and noise suppressor.

CT c+ .006 s J n II o o:T

sG o CIO .1

SW2 0,-00

O 025 R27 1000 u .001

R28 120 C28 C29 Xi R46 49 .001 470 lief 1N34 1.0MEG 1.0MEG

R2g 120 HEATER 0 o 32 033 V7 R30 I50 0 o AI .006 006 R31 ISO X2 RSI R52 1N34 rc0 1.0 MEG IO MEG R 32 IS0 CHASSIS 0 R33 150 034 035 036 37

SOO .01 TI Ti01 R34 120 eef

AUDIO ENGINEERING NOVEMBER, 1948 21

www.americanradiohistory.comAmericanRadioHistory.Com resistor across the pickup to provide a can accommodate these variations roll -off almost equivalent to that re- (A) easily. quired for NAB transcriptions, and Filter circuits which employ high - also necessary with the new Columbia Q coils are capable of extremely sharp LP records. TOO HIGH cut -off, and as a result, it is not un- This type of equalizer is more effi- common for the response to rise 2 to cient than RC types, and has a total \ I' 3 db above normal just before the loss of only 20 db, not counting the gain \V4-R2 TOO LOW steep droop. This peak usually results in the transformer. Actual measure- \ TOO LOW \rR4 in a poor -sounding output, and its ments on this unit show a voltage of R4 CORRECT presence is readily recognized by a .023 on the grid of Vi for an input of \ - 6db /Octave R4 TOO trained ear. By way of digression, the .016 volts at 1,000 cps. The transform- filter circuits used in this type of noise er has an impedance ratio of 1:120, or (B) suppressor are actually of the band - a voltage step -up of approximately 11, elimination type, and only the yet the equalization realizes the total lower portion is normally measured. the of 20 db. Typical RC equalizers lose If (G) measurements were carried up up to 40 db for complete correction, to 40 or 50 kc, the signal voltage would rise and in addition, the curve is consider- 00 A-F OSC again, assuming the ably more rounded at the turnover other circuits would pass the higher frequencies. point. The response curve of this Such a filter is normally designed to equalizer is shown in Fig. 6. The pre- work between specific impedances. amplifier tube, V1, provides a voltage Fig. 5. Phonograph equalization curves at and B As the suppressor operates, the various gain of approximately 31 as used in A show the effect of resistor this circuit. values. C shows method of connecting tuned circuits must be altered in order oscillator to obtain correct results. to remain correct for a given source The Noise Suppressor impedance. However, in the suppres- The input to the second stage, V2, high -impedance line from the plate of sor, the capacitance of the shunt cir- is preceded by a selector switch. Two V2 to the volume control. Cii isolates cuits -that of the reactance tubes -is positions are used for recorde, the the plate voltage from the stator of the the only element that is varied. Also, ganged section, SW1., adjusting the trimmer C12 so the latter will snot be the coil L2 in the series leg and its turnover frequency. One other posi- "hot" to chassis. Cs and C14 perform tuning capacitors are not changed. tion is used for radio input, and the similar functions. According to filter design theory, fourth is a spare. Rs and R9 serve as The coils used in this particular a voltage divider to balance the output sup- practically any combination of coils pressor will probably not be generally and of the preamplifier to the radio input. capacitors can be made to serve available. They have a Q of around in a band -elimination The noise suppressor section is simi- circuit, but the 60 at 1000 cps -too high a value-and impedance changes. However, the im- lar to that previously described by the L2 is 1.0 H; L3 and L4 are both 0.45 H pedance of the writers. However, some modifications source and load in the units. The exact values of these coils suppressor does not have been made, and the reasons there- change appreciably, are not important, provided the tuning and the filter is not perfectly matched for will be discussed. Ce across Rn capacitors are chosen to tune the cir- to its terminations. -Q coils, is used to equalize the frequency re- With high cuits to the required frequencies. Any writer.1 However, some modifications the peak becomes objectionable. This coils having an inductance ratio of effect can be eliminated by loading in the high frequencies to compensate the approximately 2:1, and with the larger circuit by a resistor, which is the func- for all the circuits "hanging" on the being from 0.8 to 2.0 H should be satis- tion of R22. Its value should be adjus-

1 "General Purpose 6AS7G Amplifier," C. G. factory. The Q should range from 10 ted to eliminate the peak just prior to McProud, AUDIO ENGINEERING, June, 1948. to 20. The final adjusting operations the cutoff.

Fig. 6 (below). Final response cuw's of phonograph equalizer with two positions of the selector switch. Fig. 8 (right). Over -all response curves of noise suppressor for !,!111- 111 11_1111 nine positions of control switch. I1111Il111111111A u1111111 111111111111 111111 I IC:11 11111= 111111 111111 = i111Ì1 11!111 ® Ì11ÌÌÌ iii1111111111111l11111C111111111 ..11OVER I!i;iiill.1111111111!111011111 uÌ1 ; IIIIIIA .11111 !.iime . ii ro111 rirmig 1111 as 111URVEÌ !oIÏ la ! .!i'i111ni1111NA11,,111\ 111Ií7 ® !.1111=L1111111 \11111:1\\1 ÌÌÌÌÌI Ì11Ì111111 IiII!!nN1111111i \11111\1 111111 111111 WITH 1800 -OHM SHUNT ACROSS INPUT íi1;111 11111 11íK!;1111 CURVES SHIFTED VERTICALLY FOR CLARITY IIIIIII, II 111 111101111111111111101111111 24 , , , , r , 111111 11111 100 1000 AlII 1ro FREQUENCY IN CYCLES PER SECOND 10300 FREQUENCY IN CYCLES' PER SECOND

22 AUDIO ENGINEERING NOVEMBER, 1948

www.americanradiohistory.comAmericanRadioHistory.Com Another characteristic which is also objectionable is occasionally observed. That is the return of the response curve after the resonant point of the filter. For example, a filter of this type will have a response similar to that of the solid curve of Fig. 7, where fr is the frequency of the dip due to the shunt circuits, Ls -Va and L4 -V4, and f co is the frequency of the dip due to L2 -Cii, contours. C12. This curve also shows, at A, the Fig. 9. Schematic of the volume control compensated for loudness rise before the dropoff. The return, at B, often rises as much as 10 db above than does that of V4, and the second The Compensated Volume Control the dip at fr, and this too does not dip in the curve remains approximately Following the suppressor section is sound well. Therefore, in this suppres- midway between fr and f oo, as fr is in- the volume control. This unit is com- sor, L4 -V4 is tuned to one frequency creased by a shift of grid bias. pletely compensated for the Fletcher - and L3 -V3 is tuned to a frequency The side amplifier and the rectifiers Munson curve, and was originally de- midway between fr and fop, giving a are conventional, electrically, but 1N34 scribed in these pages2. Since it is curve like that of the dotted line in germanium crystal diodes are used in- the only volume control in this system, Fig. 7. Now, while the dip at fr is not stead of tubes. Turning to the side - it has more steps than the original as great as before by about 6 db, the amplifier input, C25 couples to the sig- model, but the operation is similar. response does not rise appreciably above nal circuit at the plate of V2. This This unit was assembled on an IRC capacitor is small to reduce low -fre- commutator -type attenuator, with the quency response. C26 across R41 serves capacitors mounted externally with to reduce high -frequency response. Kovar bead seals leading through the Thus, the gates are controlled prin- shield case. The complete schematic cipally by the mid -range signal, rather of the volume control is shown in Fig. than by either highs or lows. The cir- 9. cuits between V7 and the two rec- Until a listener becomes familiar tifiers are essentially a dividing` net- with this type of volume control, he is work, and each rectifier is followed certain to be somewhat amazed by its by an RC filter. characteristics. It is necessary to ad- The switch controlling the suppres- just the over -all gain of the system so sion has nine position. At 1, the sup- that the control is operated about five pressor is inoperative because of a high steps below maximum for normal room grid bias, and another pair of contacts volume. This gives a slight boost to shorts out L2, giving a curve which is the low frequencies, but it is nearly essentially flat. At 2, the reactance correct for normal room volume when tubes are still cut off, but the short compared to the original sound source. Fig. 7. Curves resulting from noise across L2 is removed, providing a roll - As the control is turned, however, suppressor. Solid line represents initial the off down 1 db at 5,000 cps and down apparent balance between low and measurement before correction, show- high 13 db at 10 kc. This circuit is reson- frequencies remains so nearly constant ing peak at A and return at B. Dotted ated at approximately 16 kc. is line curve after applying correc- In the that it sometimes difficult to deter- shows remaining positions, mine tive measures described. seven the bias on whether or not the level has been the reactance tubes is reduced gradu- changed. It is this control that relieves ally, selecting a number of about equal- fr, and results are more pleasing to the [Continued on page 46] ly spaced frequencies for fr. The mea- ear. The problem now is to keep the 2 "Loudness Reproducing sured curves of the completed suppres- Control for Sys- dip at higher frequency midway be- tems," David C. Bomberger, AUDIO ENGIN- sor are shown in Fig. 8. EEatxc, May, 1948. tween fr and f oo during the shift of fr by the action of the gate control circuits. This can be accomplished readily by proper choice of the operating charac- N o Imams teristics of the two reactance tubes. - I Fig. 10. -5 V4 is tuned to 4,500 cps at maximum 0 --,- suppression, and its screen voltage is Curves -Io -3 supplied from a voltage divider, B18 -R19. obtained -15 i f This tube is connected to the side amp- r with various -2 lifier-rectifier circuit to operate more w settings of V3, so - ;t -25 - quickly than does the lowest J ' - compen- -13 frequency dip moves upward quickly é -30 -IS in the presence of high frequencies in sated -35 -17 the signal. V3, however, has its screen volume -40 supplied by a series dropping resistor, control. thus changing the IL bias characteristics -45 of the tube. With this connection, the 19 >a 10000 effective capacitance of V3 varies less 100 . om 10000 rREOUENCY IN CYCLES PER SECOND slowly with a given change in bias

AUDIO ENGINEERING NOVEMBER, 1948 23

www.americanradiohistory.comAmericanRadioHistory.Com Sound on Film

JOHN A. MAURER*

The author discusses in detail the methods and technique cf modern sound -on -film recording.

Fig. I. The operating side of the film - the files of the Journal of the Society of major importance. What is im- moving mechanism of a typical sound - of Motion. Picture Engineers. This lit- portant is that the photographic pro- on -film recorder. The recording optical erature covers a period of more than cesses may profoundly affect the qual- system is at the right -hand side. twenty years. As might be expected, ity of the record, and, if improperly many of the articles are not applicable handled, may ruin it altogether. Fur- to present practice. Two valuable sum- thermore, all of the steps in the photo- OPTICAL methods of recording marizing articles exist,8,4, one of graphic procedure are interrelated, and sound on photographic film were which is available in reprint form. are strongly influenced by one another. developed for the motion picture The present article and those which For example, correct original exposure industry, which needed a type of sound are to follow will attempt to set forth of the sound track is essential, but this record that could be . carried on the those facts which are of practical im- must be established by co- operative same support with the pictorial record portance to engineers who must learn tests between the recording studio and and thus maintained in synchronism to use sound -on -film recording and the film laboratory that will develop with it. Because photographic film is reproducing equipment or who will be and print the records. The recording more costly than other readily avail- making decisions regarding its use. A lamp current that is correct for the able record media, sound -on -film rec- critical bibliography will be attached development given in one laboratory ords have found very few applications to each article. Since the emphasis may be seriously wrong if the film is apart from motion pictures, and most will be on what is of practical value sent to a different laboratory. Since audio engineers have had no occasion today, no mention will be made of de- an error by either the recording per- to concern themselves with them. To- vices and methods that are no longer sonnel or the film laboratory may lead day, however, these recording processes used, even though some of them are to bad quality, it is difficult for the are of general interest because they extremely interesting. beginner to determine what is at fault will be needed for the recording of when results are unsatisfactory. If, Photographic Processing television programs and in preparing however, proper checking procedures The most important practical dif- programs on film to be televised. are maintained, the results of sound - ference between photo -optical sound The engineer who seeks to educate on -film recording can be made as con- recording and the other methods in himself on the subject of sound -on -film sistent as those of any other high - recording will find that with the ex- common use is that the record on film quality recording method. ception of two books,1,2, one of which is not complete when the recording is is in the German language and both of finished. It does not even exist in a Basic Outline which are somewhat out of date, al- usable form until it has been developed The fundamental steps in photo- most all of the useful literature is in and printed. This means, of course, optical recording and reproduction are that immediate playback for checking as follows: is not possible. This is an incon- *President, I. A. Maurer, Inc., 37 -01 31st St., 1. The electrical signal corresponding to Long Island City, N. Y. venience, but is not in itself a matter the sound wave is caused to modulate

Fig. 2. A variable area negative (bottom), a variable area print (center), and a variable density print (top) of the same spoken syllables.

24 AL!!O ENGINEERING NOVEMBER, 1948

www.americanradiohistory.comAmericanRadioHistory.Com a beam of light, which is focused on the Film Speed and Frequency Range moving film in a mechanism such as is - MIRROR shown in Fig. 1. GALVANOMETER Ti4p standard linear speed of the 2. The exposed record is developed to film in 35 -mm recording is 18 inches per produce (in most cases) a negative second; in 16 -mm recording, 7.2 inches sound track. TRIANGULAR per second. Thus at 7200 cycles, which 3. The sound track negative is printed on MASK another strip of film, on which, in most is near the upper limit of the fre- cases, the picture image has been 'or quency range normally reproduced by will be printed. theatre L3 equipment, the wave length on 4. The print is developed to produce the 35 -mm film is .0025 inch, and on 16- positive sound track. mm film .001 inch. The latter value 5. The sound track on the print is used is commonly believed to modulate a second beam of light i to be near the which is transmitted to a photo -electric SLIT upper limit of the resolving power of the cell. IMAGE film. That this is incorrect is 6. The electrical signal generated in the OF clearly shown in Fig. 5. The peaks of photo cell is amplified. MASK the wave on this record are .001 inch Step 6 is not a photo- optical pro- apart, and it is obvious that much finer cess, but is included in the outline be- detail could be reproduced. cause it presents special problems *-___LINE IMAGE OF SLIT The upper limit of the frequency encountered in which are unlike those range in sound film recording is usually reproducing sound from discs or mag- Fig. 3. Diagram of galvanometer and fixed in practice by the tuning point of netic tape. . optical system for variable area the galvanometer or light valve. This is Methods of Recording on Film recording. usually about 9,000 cycles in 35 -mm practice. processes are able to Substantially the same modu- Photographic which is essentially a low -power micro- the shapes of objects with lating devices are used on 16 -mm re- reproduce scope objective, forms a reduced image accuracy; they are also corders, though in the equipment built considerable of the slit on the moving film. This within limits, of reproducing by the writer the tuning point of the capable, line image is usually about .00025 inch galvanometer differences of light intensity. Cor- has been moved out to wide in 35 -mm film recording, and not 12,000 responding to these two aspects of cycles in order to remove the much more than .0001 inch wide in 16- mechanical photography we have two distinct sys- resonance peak from the mm film recording. The maximum working of sound recording. In the frequency range. Low pass tems illuminated length of the line is .076 area system the light modula- filters cutting off at frequencies slight- variable inch in standard 35 -mm recording, and on the film what is -essen- ly lower than the tuning points of the tor produces .060 inch in 16 -mm recording. picture of the sound light modulating devices are included tially a graphical The corresponding light modulator Films and processes are chosen in the audio inputs of most film re- wave. and optical system for variable density cording to reproduce the shape of the systems. so as recording are shown in Fig. 4. The as accurately as possible. recorded wave "light valve" generally used for this the track which are ex- Effect of Recording Image Width Those parts of type of recording consists of a pair of all receive the same exposed to light tiny stretched duralumin ribbons in a It is obvious that in order to obtain the exposed posure, but the width of strong magnetic field. These ribbons are clean records of high frequencies it is density area varies. In the variable arranged in the same plane, with their necessary to make the width of the re- hand, the entire system, on the other edges about .001 inch apart, forming a cording light beam considerably less is available width of the sound track slit about .200 inch long. This than the wave length of the highest varies slit is always exposed, but the exposure illuminated by the lamp and lens Li, frequency to be recorded. In variable instant. Films and from instant to while the lenses Ls and L3 form a sys- area recording too wide a recording are chosen so as photographic processes tem that images the slit on the film at image introduces both wave form dis- a linear to obtain as nearly as possible a reduction of about four to one in the tortion and attenuation of high fre- of relation between the variations direction of its width and two to one quencies. A ratio of one to five be- sound transparency in the positive in the direction of its length. Thus the tween image width and wave length is of exposure track and the variations final image, in 35 -mm recording, meas- about the minimum for good results. Typi- in the recording of the negative. ures about .00025 inch by .100 inch. In most of the equipment now used in cal variable area and variable density The audio signal currents are passed practice this ratio. is about one to eight sound tracks are shown in Fig. 2. through the ribbons, which move in at a frequency of 9000 cycles. Light Modulating System such a way as to widen and narrow In variable density recording, a wide the slit, recording A typical light modulating system thus varying the amount of image causes high -frequency light for variable area recording is shown which reaches the film. attenuation but does not introduce diagrammatically in Fig. 3. The galvanometer in this system is usually a moving- iron -armature device carrying a mirror about 1/8 inch square. When Fig. 4. Light an electrical signal is applied, this mir- valve and opti- ror swings about a central axis in such cal system for a way that the triangular image which falls on the slit is moved at right variable den- angles to the slit. Obviously. this varies sity recording. the length of the slit that is illuminated, but the light intensity remains at a constant value. The lens La,

AUDIO ENGINEERING NOVEMBER, 1948 25

www.americanradiohistory.comAmericanRadioHistory.Com by variable area recording, but really high quality is not obtained in either case when recording on films designed for picture photography. Variable Area Processing In developing and printing variable area sound tracks we are principally interested in getting a high signal to noise ratio and in minimizing a form

uo 40. of distortion known as cross modulo,- lion. Both high signal level and low noise r are assured if the black areas in the Fig. 5. Enlargement a I of portion of a 6-mm variable area negative sound sound track print are sufficiently track of 7000 -cycle sine wave. opaque and if at the same time the wave -form distortion. Since the high - widely used of these are DuPont type transparent areas are free from fog frequency attenuation is not serious 602 and Eastman types 1372 and 5372, and are clean. In good commercial and can easily be compensated in the which are the 35 -mm and 16 -mm sizes practice the blacks transmit only three electrical input to the system, record- of the same basic type of film. to five per cent of the incident light, ing images about twice as wide as For variable density recording, a and the clear areas absorb about the those mentioned above are used in film stock of low contrast is desirable same amount. The reproduction level some variable density recording sys- because the requirements of overall of such a track, if fully modulated, is tems. linear brightness reproduction are best within one decibel of the maximum Choice of Film Types satisfied when the contrast that results possible. Cleanliness is the most im- In motion picture sound recording from the combined action of the neg- portant factor in obtaining a low level it must always be remembered that the ative and positive films is about the of background noise. film used for the positive print also same as the contrast that was present Cross- modulation distortion arises carries the pictorial image, and its in the original. (In photographic because the emulsion of an undevel- characteristics and the treatment it is terminology, the overall gamma should oped photographic film is opalescent, given in development are determined be not much greater than unity.) Un- and scatters the incident light. This primarily by the requirements of the til recently no films of high resolving scattering causes any image impressed picture. Therefore in both variable power, fine grain, and low contrast on the film to spread somewhat beyond area and variable density recording it were available, and it was necessary the limits of the area exposed. The result, as is necessary to produce a sound nega- to obtain the low contrast by care- it affects the recording of a tive that has the proper characteristics sine wave, is shown in Eig. 6. The for printing on this predetermined negative image has both harmonic distortion, second final type of film. The factors avail- CHANGE principally harmonic, SPREADING OF AVERAGE and a rectification component which able for control of the final result are OF IMAGE TRANSMISSION causes groups the choice of the negative film, the ex- of high frequency waves in posure in recording, the development the record to generate lower frequency noises of the negative, and the exposure used 7 corresponding to their in printing the positive. envelopes. Speech is affected more noticeably than music. In particular, Fine grain motion picture positive ORIGINALLY EXPOSED AREA uncompensated film is a high contrast material, though cross modulation produces an extremely unpleasant distor- its contrast is not as high as that of the Fig. 6. Diagram of a portion of one tion of "ess" sounds which is as though so called "Contrast Process" materials side of a variable area negative sound two pieces of sandpaper were brushed used in photo engraving. As ordinarily track, showing how spreading of the together at the each sibilant is developed gives very fog; image causes cross -modulation. instant it little that reproduced. is, any part of the film which receives Cross modulation can be eliminated no exposure comes out almost perfectly fully controlled development of inher- in properly controlled printing because transparent after processing. This is ently high contrast films. The contrast the exposure of the is through the extremely important in variable area increases with the degree of develop- print areas that are clear in the negative, records, since any clouding or graying ment. This situation has been much and thus the spreading of the image of what should be the clear areas of improved by the introduction of East- is in the opposite direction. In order the positive sound track reduces the man film types 1373 and 5373, which to obtain accurate cancellation it is level of the reproduced sound and at have inherently low contrast. necessary to determine the proper the same time increases background It is important to note that the printing exposure in relation to the noise. higher speed films designed for picture exposure and development used in The properties that are desirable in taking, although most of them have producing the negative. film for the variable area sound nega- inherently low contrast, are not suit- tive are high resolving power, high able for variable density sound re- Sound Track Printing contrast, fine grain structure, and suf- cording. The grain patterns of these Good cross- modulation cancellation ficient speed to yield dense images films are a great deal coarser than can only be obtained with accurate with the relatively small amount of those of the sound recording films men- printing equipment. Most sound track light that can be transmitted through tioned above, and in variable density printing is still done on printers of an optical system of the type shown in recording this results in an intoler- type that has been in use for many Fig. 3. Special types of film have been ably high level of background noise. years in the motion picture industry. developed for the purpose. The most Somewhat better results are obtained The process is shown diagrammatically

26 AUDIO ENGINEERING NOVEMBER, 1948

www.americanradiohistory.comAmericanRadioHistory.Com in Fig. 7. The negative film and the used to determine optimum processing is considered a permissible maximum print "raw stock" are passed, with conditions for variable density rec- of distortion, whereas in variable area their emulsion surfaces in contact, ords. Values of intermodulation as recording the overload point is per- around part of the periphery of a large low as four per cent are obtainable in fectly definite. As long as the ampli- sprocket. At about the middle of the good commercial practice. The output tude of the recorded wave is less than arc of engagement of the two films level of the prints, however, is at best the width allotted to the track no sig- with the sprocket there is an exposure about six decibels lower than the out- nificant distortion occurs. As soon as aperture at which a light shines put of fully modulated variable area this width is exceeded the peaks of the through the negative sound track and prints. When the modulation level of waves are clipped and the distortion thus exposes the positive. The excel- may be immediately noticeable. For lence of the result depends on uniform this reason unusual care must be ex- motion of the two films and on close ercised in monitoring the signal for contact between them. Unfortunately, variable area recording. both motion and contact are periodi- cally disturbed by the action of the Noise sprocket teeth in entering and leav- The background noise in sound -film ing the perforations of the film. reproduction is due to dirt and dust Another type of sound track printer on the film, abrasions caused by uses two film driving mechanisms simi- handling of the film, the grain structure of lar to the one shown in Fig. 1 to drive the photographic image itself, and the negative and positive films separ- hiss generated in the photo cell and its ately at constant speeds. An illumi- coupling circuit when the cell is ex- nating system is placed back of the posed to light. All these sources of negative sound track and an optical noise are reduced in importance if the system between the two ,films produces light transmission of the sound track an image of the negative track on the is reduced. This may be done in either positive film. This image travels in the variable area or variable density re- same direction as the positive film and cording by applying to the light mod- at the same speed. Such printers have ulator a biasing current, superimposed been used principally to produce 16- Fig. 7. Diagram of sprocket -type on the audio signal, in such a way as mm sound track prints from 35 -mm mechanism used for printing sound to reduce the amount of light trans- negatives, but the writer has recently track. In practice the sprocket has 40 mitted to the sound negative film dur- shown5 that optical printing leads to to 64 teeth instead of 8 as shown here. ing periods of silence or low signal superior results even when the nega- level. This biasing current is removed tive and print are of the same size. the negative is not too high it is pos- in proportion to the peak amplitude sible to obtain volume control over a of the incoming signal, so that when Processing Variable Density Records range of about 12 db by varying the the signal reaches approximately In determining the photographic exposure in printing without increas- ninety per cent of full modulation the treatment of variable density records ing the intermodulation distortion biasing current is removed completely. we have the problem of combining two enough to make it noticeable. This technique is known as "noise re- non -linear characteristics so as to pro- A major advantage of variable den- duction." The sound tracks shown in duce an overall characteristic that is sity records is that they exhibit no Fig. 2 were made with noise reduc- linear over as great a range as pos- cross-modulation distortion; therefore tion, and the operation of the system sible. The light-transmission vs. ex- the reproduction of the sibilants in can readily be observed in them. posure characteristics of the negative speech is pleasing even when the rec- It is obvious that in "noiseless" re- and positive films individually have ords have not received optimum pro- cording the actual background noise about the same amount of curvature as cessing. On the other hand, improperly level fluctuates continually with the the plate current vs. grid voltage char- processed variable density records. signal level, but the masking effect of acteristic of an output triode when it especially of music, exhibit objection- the signal is such that the noise is is driven from near plate current cut- able distortion of the familiar "over- heard only during periods of low sig- off to the grid current point. The ex- load" type. nal level. The time constants of the posure in recording or printing deter- Overload Characteristics biasing amplifier must be adjusted mines the "operating point ", and the An important practical difference correctly. Too rapid a change of bias time of development and the constitu- between variable area and variable current produces thumping sounds; too tion of the developer influence the density records is that in the latter, as slow an "opening" action causes ex- shape and steepness of the curve. When in disc recording and magnetic re- cessive distortion by clipping of the properly combined, the two curves can cording, the overload point is some- peaks of the waves each time the be made to yield an overall result that what arbitrary and depends upon what [Continued on page 43] is linear up to about 90 per cent modulation of the recording light valve. +5 Since three independent variables are Fig. 8. Standard available (negative exposure, negative theater repro- o d development, and printing exposure) it ducing charac- -5 F is not surprising to find that even with teristic J'w . curve of -10 one selection of negative film the prob- the Academy of o lem has many fairly satisfactory solu- Motion Picture -15 tions. The now familiar intermodula- Arts and Scien- -20 tion type of test, which was invented ces. 40 100 500 1000 5000 10,000 for this specific purpose, is generally Frequency - C.P.S.

AUDIO ENGINEERING NOVEMBER, 1948 27

www.americanradiohistory.comAmericanRadioHistory.Com ideally speaking, I T1 -lE wining of the present articles' on the one that works, and, control systems for the dynamic noise EDWARD TATNALL CANBY* find that this control is the most useful of suppressor has been interesting and all. (Remember, it appears in some form perhaps a bit delicate in more ways in most suppressor models.) than one. Although my entire discussion Let us look at the possible usefulness of concerns the outward control functions in the floor control vs. the ceiling control with relation to actual musical results to be had up and from radio to phono. But, given an this logic, my own: and therefore is well within my official pro- adjusted input level, there are other variables The variation we most need in noise sup- vince- nevertheless it is difficult not to too, not only in the proportion of noise pre- pression is the degree of noise reduction. become more involved in the technical aspects sent in relation to the music but more Let us say, then, the noisier the record, the we of design than is good for one of my important. the type of music and the noise - more filtering (noise reduction) need. training. masking ability it has. A vital point is the This, note well, is exactly as in any fixed - Fortunately, though I do now have certain tremendous variation in this respect found filter situation. well- backed opinions as to how I should like in many types of recorded music, and some , Next, note that the basic principle of most to see the controls of a suppressor further gate Sensitivity adjustment at least the dynamic suppressor is that noise is most arranged on my own amplifier, i can say is clearly required in order to meet such objectionable with a low signal level, there- instantly that there is no question of black greatly varying conditions. fore, filter most with the least signal. It and white -if one grants that the dynamic But the question is, how much? And more, follows nicely that our main adjustable fil- suppressor is a good idea. There are not shall gate sensitivity be the most important tering action. to meet differing conditions, only numerous ways of using the circuit adjustment to be made? must be at the low -level end of the dynamic with satisfaction, but good results can cer- Here I have found in actual experience range -the gates -closed position, or floor. tainly be achieved with any of the three that, once input levels are set, the needed In other words, we can treat the dynamic leading commercial exemplars of the design, variation in gate sensitivity, even to cover filter as we use the fixed filter and vary its those of Messrs. H. H. Scott, Avery Fisher the widest variations in noise level and type maximum action (i.e., at low signal, gates and John Goodell, as well as with several of music, is surprisingly small. This is a closed) as we would vary a corresponding seem to variants of the one -control, simplified type fundamental point. As above said, T do not fixed filter. This argument would now appearing. Therefore I will continue think sensitivity can be fixed (not counting me to justify trying a type of design that by giving my own preferences were I act- input level' adjustment) for all conditions. would make the variable floor control more ually to be consulted on the panel control When it has been so fixed, I have resorted prominent than the others. requirements of some hypothetical new to making quite frequent slight adjustments It is quite possible, I admit, to get similar noise suppressor amplifier. of the input control to achieve what is in results by varying the gate sensitivity -in a Let us begin with the premise that the effect a change in gate sensitivity. But I sense this varies the effective amount of sup- with less supposed machine is a wide range type for feel that to provide more than this slight pression, because, sensitivity, the use with high quality equipment, and will amount is a bit risky, in that it may be gates are closed a greater proportion of the is difficult to visualize have "everything." ( Some suppressors are asking for trouble for the inexperienced time. In fact, it quite designed specifically for average home con- user-which is almost everyone. Too great the exact relation between these two ways of ditions, with a low -cut -off system.) And sensitivity leads to the unpleasant swish achieving roughly the same thing. that, in line with this, it is desirable to have that means noise is coming through above the Look at it this way: When the gate sensi- the more complex, more flexible laboratory music in the louder passages; too little, tivity alone is varied, the floor remains fixed, type controls, involving, as illustrated last means a generally dull tone and a near -fixed- no matter how far you turn the knob; what September, at least two principal control filter action. is varied is the proportion of floor to ceiling, centers, a "main" control, and a secondary I would be inclined to reason, next, that so to speak-how much of the music is filtered fixed -position range control, the general since gate sensitivity is to be varied in a toward the floor and how much comes form that all the better suppressor models rather limited way, it might be wiser not to through at the ceiling limit. The gate - have taken. These controls cover the func- it as the principal control function assum- sensitivity control, then, does not adjust use not provide tions of three basic operations (again ing that you accept my argument. The the filtering depth at all; it does deeper filtering action for noisier situations. ignoring the low- frequency gate) : Al Gate "Main" control is on most suppressors rather Sensitivity, B) H. F. Ceiling, and Cl H. F. forcibly brought to the user's attention by Thus, whatever system my theoretical am- Floor. Their relative importance, as reflected some such designation as "Suppressor plifier would use, floor control would have to in the type and positioning of the controls, Action" or the like-implying that here is be provided. It may be in a series of fixed has been a matter of disagreement, as where listening attention should be centered. steps, as some manufacturers have arranged over already outlined. Tt would seem advisable somehow to subord- it, or it may be continuously variable I would suggest first that the real fanatic inate the gate sensitivity control a bit-either the desired range-which clearly must be a - fixed -up and most of us are that -would appreciate by making another control more obvious or wide one, just as a filter set covers an amplifier with at least three controls, by merely labelling it in a more neutral way. a wide range. one specifically for each of these functions. On all the suppressor amplifiers I have tried. Because. the variable floor seems to me to be The biggest question to be decided somehow I have been able to accomplish this to my the most useful of all the functions, I would is the relative importance of the three own satisfaction in one way or another, if tend to place it in the most prominent posi- aspects. The table in the September issue only by restricting the variable sensitivity tion, or otherwise (by labels, for instance) will give an idea of differing opinion in knob to a small arc of its total movement. draw the user's attention to it. ( Similarly, actual production models. My recommendation would be to make this in any "automatic" compound -function, one - T to see a A. Gate Sensitivity. As I see it, there is no function continously variable, but through a knob type of control, would prefer willy into question at all about the necessity for some narrow range, and give it a neutral or sub- variable floor incorporated -nilly user is to make variation in gate sensitivity on any type ordinate positioning in the control panel, or whatever adjustments the with his single knob.) dynamic suppressor, so that increasing noise a neutral lebal. being passed is covered by increasingly loud B. Floor Control. Conceivably, if a "main" C. Ceiling Control. There remains one more music. We may dispense quickly with one control is wanted (and human nature seems big consideration, the third possible control obvious need, and to adjust the input level, to want one) either of the remaining func- function, the h.f. ceiling (again. see AUDIO which is bound to vary from pickup to pick- tions might be used -kf. ceiling (B), or ENGINEERING for September). There are two h.f. floor (C), technical problems of -course ways in which a ceiling control here may be 1) *279 W. 4th St., New York 14, N. Y. entirely aside. Of the two there is no ques- had: via the regular attenuation treble 1See AUDIO ENGINEERING, Sept., 1948. tion, in practice, that the floor control is [Continued on page 35]

28 AUDIO ENGINEERING NOVEMBER, 1948

www.americanradiohistory.comAmericanRadioHistory.Com TWO -WAY SPEAKERS Jiwm Çn»tmcùd CDmpwwth

JOHN WINSLOW

Assembly directions for adding tweeters to cone speakers to give better reproduction.

HILE the two -way speaker sys- taut-resistance dividing network is of the h -f unit is necessary, and while this may be tem may well be considered the parallel configuration. done by the use of a series capacitor, the This is an ideal unit when used with any network also removes the high frequencies ideal for sound reproduction, it good low- frequency cone as a "woofer." from the woofer, preventing cone breakup is a costly device if purchased as a fac- Users should be cautioned about the correct and avoiding the possibility of phase can- tory -built item, or it requires consider- balance between the two speakers, since the cellation at the higher frequencies. If the user desires, the h -f horn may be able work if entirely home built. The removed from the cabinet and mounted in- principal problem centers around the D side a speaker baffle, since the horn is flat construction of the high -frequency along the front surface, and is equipped flanges by which it may be mounted horn. Once this is solved, the assembly G with on a flat surface. This begins to require of a good quality speaker system can be some constructional work, however, which accomplished easily. Fortunately, sev- is completely avoided if the unit is used in eral manufacturers have taken steps to its own cabinet. solve this very problem, and a two -way University Models speaker can now be a working reality Another equally simple method is to add a Model 4404 University Dual Tweeter to without any sheet metal work, and with an existing low- frequency speaker and baffle. a minimum of measurement and adjust- This unit, somewhat smaller than the Atlas, ment. Fig. 2. Curves resulting from various also comes with its own cabinet, and with components of speaker system. (A) is Atlas High- Frequency Speaker output of woofer; (B) is of (hie of the simplest methods of assembling output a two -way speaker system is to employ an tweeter; (C) combined output of system when properly adjusted; (D) output when tweeter gets too much signal. Dotted line indicates action of normal tone control.

action of the control is not similar to that of a tone control. 'Figure 2 shows the output resulting from adjustment of the control. The entire band above the cross -over frequency is raised or lowered, putting a step in the response curve. The dotted line Fig. shows the action of a normal tome control. I. Atlas HF-I high- frequency With the tone control of the amplifier or speaker assembly, which includes its receiver in the "flat" position, the balance own dividing network in the cabinet adjustment should be set for normal repro- which is designed to be placed on top duction. Fig. 4. Experimental assembly of 12" One advantage of the Atlas unit is radio or that cone of speaker cabinet. it is supplied with the dividing network al- and University Model 4402, with ready built in. While these networks are dividing network and impedance - Atlas HF -1 speaker with an existing cone not difficult to construct if suitable measur- matching input transformer. speaker in its normal baffle. This model of ing equipment is available, it is at least "tweeter" is made in the form of an attrac- more convenient to have it already made. a high -pass filter and balancing potentiomet- tive cabinet, shown in Fig. 1, which is in- Some means for preventing the low frequen- er built in. Two small horns are used, each tended to rest on top of a radio or a speaker cies from reaching the diaphragm of the with its own driver, and they reinforce the enclosure. The dividing network is includ- woofer above about 2,000 cps. Since the ed in the HF -1 cabinet, and all that is neces- filter consists solely of a series capacitor, sary is to remove the present leads from the low frequencies are kept off the h -f unit, loudspeaker, connect them to the dividing and it is adequately protected. network chassis, and run another pair of Another University unit, Model 4407, is leads from the chassis to the loudspeaker. shown in Fig. 3. It consists of a 12" adapter That is all there is to it. Phasing adjustments ring which is used as a spacer between a 12" may be made by moving the entire cabinet cone speaker and the baffle, setting the cone backward or forward, and the balance be- back far enough to accommodate a single tween lows and highs is made by a control small horn unit which mounts on a metal mounted on the dividing network chassis. strap across the back surface of the adapter This unit is designed for an impedance ring. To install this unit, it is only neces- of 8 ohms, which is a common value in better sary to remove the cone from its baffle, grade cone speakers, and works at a cross- assemble the h -f unit and horn to the strap, over frequency of 1,900 cps. The h -f horn and attach the cone to the adapter ring. is made of cast aluminum to avoid reson- The enti -e assembly is then remounted on Fig. 3. ances, and a moulded phenolic diaphragm University Model 4407 tweeter the baffle, and a two-way system results. is used; the magnet is Alnico V. The con- assembly, with adapter ring. The same type of high -pass filter may be

AUDIO ENGINEERING NOVEMBER, 1948 29

www.americanradiohistory.comAmericanRadioHistory.Com Fig. 5 (left). Schematic of speaker shown in Fig. 4. 10 0 pf oN B 2 8 0 00 J TABLE I FF o DIVIDING NETWORK COMPONENTS o.°

INPUT TWEETERS Li L2 CI C2 Ni 16.0 pf é Z TYPE Z Cps mh mh of of ohms

8 1 -UNIV 12 21300 .636 .398 9.95 15.9 10 QQ3 2 -UNIV 6° 2,000 .636 398 9.95 15.9 15 0 64 MH RACON 15 1,200 1.06 .662 162 26.6 10

10 I -UNIV 12 2,000 .795 ,497 7.95 12.7 15 2 -UNIV 24 2,000 .795 .497 7.95 12.7 10 250 0 O 0 BACON 15 1,200 1.32 .825 13.2 21.1 15 C of 500 15 B 2 UTO 12 I -UNIV 12 2,000 .955 .596 6.64 10.6 20 VAR S -I4 4 3 2 -UNIV 24 2,000 255 396 6.64 10.6 15 OUTPUT o C1 o TRANS RACON 15 1,200 1.59 .995 11.0 17.6 IS

15 -16 I -UNIV 12 2,000 125 .782 5.07 8.11 30

2 -UNIV 24 2000 1.25 .782 5.07 8.11 20 RACON 15 1,200 2.08 1.30 8.45 13.5 25

UNITS IN PARALLEL used with this arrangement, or a separate p]oyed because of the fact that the The values for the balancing potentio- dividing network may be built. The single unit and horn, Model 4401, sound then comes from either the woof- meter, R1, were determined for the may be installed directly on a speaker er or the tweeter (except right in the condition where the h -f speaker is 6 db baffle; or the dual unit, Model 4402, con- crossover range) rather than from the more efficient than the cone, which is sisting of two similar horns made as a sin- woofer only in the lower ranges and about normal for average cone speakers gle casting and set at an angle of approximately 30° may be used, the latter giving from both together in the upper range. and the nearest standard value is listed somewhat better spatial distribution of the It is definitely recommended that the in the table. A 10 -watt resistor with an high frequencies. This type of mounting extra effort be expended to construct adjustable tap is ideal for this applica- entails more work, but the final results are generally more satisfactory. The impedance the network because of the improved tion, since it should be set when the of each driver unit is 12 ohms, and they performance. system is assembled and then left at may be connected either in parallel or in Table I shows the components to be the optimum position. Impedance series, depending upon the impedance of used with single or dual University changes resulting from this method the woofer with which they are to be used. of Figure 4 shows an experimental two -way CI adjustment will not affect the perform- system using a 12" Stromberg- Carlson cone ance noticeably. and the dual tweeter, together with a divid- As to making the inductances, close ing network operating at a 2,000 -cps crossover. This box, although too small for good approximations can be obtained if a bass response, also incorporates a tranform- certain size of wooden spool is used er and switch, and provides input impedances and specific winding directions are fol- ranging from 8 to 10,000 ohms, making it a lowed for a given wire size. For these general purpose test speaker. The schematic is shown in Fig. 5. values of inductance, a suitable spool The Racon Two -Cell Horn size has a core 1 -/" in diameter and Still another model is available commer- L1 offers a 3" winding space. With No. 16 cially to provide a simple assembly of a good Fig. 6. Crossover network configura- enameled wire, laid thirteen turns per system. This is the Racon two -cell horn and layer, the total number of turns re- the unit designed to work with it. This com- tion for which values are given in bination operates at a cross -over frequency Table I. quired for the listed inductances are of 1,200 cps, and requires a dividing net- shown in the curve of Fig. 7. No iron work for best results, although a series tweeters, or with the Bacon two -cell [Continued on page 34] will suffi- capacitor protect the driver unit horn and unit, with 8 -, 10 -, 12 -, and 15- 3.0 ciently. If this model is employed, it is 16 necessary to provide a suitable means for or -ohm woofers. Since the cross -over mounting, such as cutting an opening in the network is an integral part of the Atlas 2.0 baffle or woofer cabinet. unit, it is not necessary to build one if Any one of these three methods will give this model is used. added brilliance and definition to reproduction from a high quality amplifier or radio The configuration for the network receiver. It must be remembered that addi- is shown in Fig. 61. There are a number tional h-f output will tend to show up any of usable configurations, but the one distortion in the amplifier, so it is important to reduce distortion to a minimum when shown is best suited for this purpose adding a separate high-frequency speaker. since the phasing for the two speakers In this connection, it is well to mention is most nearly correct at crossover when one fault common to many two -way speaker the horns are mounted on the .4 system users. Having a means for reproduc- same ing highs well, it is not unusual for systems baffle as the woofer. The component .3 to be operated with an unbalance between values were calculated from the form the two units, on the apparent theory that ulas: "mow we have a tweeter so you're going to R. henries 80 100 120 140 160 I80 200 220 hear highs!" Good reproduction should be TOTAL NUMBER OF TURNS based on the proper balance between treble 2,rf, Fig. 7. Chart for determining number and bass, with the principal benefit being L1 henries gained in a truer response in the upper mid- 1.6 of turns to give inductance values dle range -1,500 to 4,000 cps. necessary for various networks. This 1 farads Dividing Networks 2r f .R. chart applies to coils wound on 114" The most satisfactory operation of core of wooden spool to a width of C2 = 1.6C1 farads any two -way speaker system will be where R. is the impedance of the network, 3/14", using 16 enameled wire with 13 obtained if a dividing network is em- and f. is the cross -over frequency. turns per layer.

30 AUDIO ENGINEERING NOVEMBER, 1948

www.americanradiohistory.comAmericanRadioHistory.Com ENGINEERED TO THE HIGHEST FM and AM BROADCAST STANDARDS

NSW Nigh fidelity Dynamic

>I 0AflCAST Microphones

Model 650 (Output-46 db)

\ FEATURES LIKE THESE WIN TOP RATING Model 645 (Output -50 db) dy .Staud t awe 'Íteewavck &.cgeoevtil

Flat out to 15 kc! Extremely high output! Impedance selector! Dual- type shock -mount! Remarkably rugged! Individually calibrated! Developed in cooperation with station and network engineers, the new "650" and "645" meet exacting requirements of modern high fidelity FM and AM broadcast service. Proved in studio and remote use. Polar pattern is non- directional at low frequencies, becoming directional at high frequencies. Recessed switch gives instant selection of 50 or 250 ohms impedance. Exclu- sive Acoustalloy diaphragm withstands toughest use. Many other important features assure the ultimate in broadcast quality. Satin chromium finish. Fully guaranteed. Model 650. Output level -46 db. List $150.00 Model 645. Output level -50 db. List $100.00 Broadcast Engineers: Put the "650 "or "645" to the test in your station. Know the thrill of using the newest and finest. Write for full details.

ELECTRO- VOICE, INC. BUCHANAN, MICHIGAN Export: 13 East 40th St., New York 16, U. S. A. Cables: Arlab

NO FINER CHOICE THA gLerertracz

AUDIO ENGINEERING NOVEMBER, 1948 31

www.americanradiohistory.comAmericanRadioHistory.Com NEW PRODUCTS

NEW TONE GENERATOR greater than most records on the market. oersteds at 7% inches per second, the oper at 40 kc is 1.7 ma and the A small, portable tone generator, de- A specially matched pickup is included for sting bias level recording signed primarily for use in broadcasting wide range performance and low needle- audio signal current for standard studios, is now in production, the RCA En- talk; and a single control connected with gineering Products Department tells us. a two -wire cable enables great flexibility The new instrument (Type WA -26A), in mounting and operation. combining a high -quality audio oscillator Only one connection is necessary to in- and sensitive meter, supplies a suitable tone stall the Dynamic Noise Suppressor 110 -A for use in equalizing remote telephone lines. in its position between the pickup and the amplifier. Being small and compact, it may be located in any convenient spot within the phonograph cabinet. For further data, write Hermon Hosmer Scott, Inc., Dept. AE, 385 Putnam Ave., Cambridge, Mass. HIGH QUALITY AMPLIFIER A new 10-watt model of the Brook level (as defined by RMA Standards) is High Quality Amplifiers which makes avail- 0.15 ma. Recorded in such manner, the able the advantages of all -triode audio per- signal output from the high impedance formance to the moderate price field, has playback winding is 5 millivolts. Imped- recently been announced by Brook Electron- ance at 1000 cps is 1000 ohms. ics, Inc., 34 DeHart Place, Elizabeth 2, N.J. For descriptive literature, write Dept. A -3, Within the range of its power rating the Indiana Steel Products Company, 6 N. Mich- new Model 12A series equals the perform- igan, Chicago 2, M. ance of Brook 30 -watt models in every respect. EQUALIZER PRE -AMP A new Brook circuit development, known In response to the need for a flexible, as the "transient peak" circuit, permits the compact pre -amplifier of moderate cost, amplifier to handle power peaks consider- Brociner Electronics Laboratory, New York The tone -generator circuit is an R -C type, ably higher than its 10 -watt rating, at the City, announces the Model A65 Amplifier allowing selection of ten frequencies from same time holding distortion within the and companion Model P6 -300 D.C. Power 50 to 15,000 cycles per second. The output figures published by the manufacturer as Supply. Permitting compensation for the recording characteristics in is metered and calibrated in dbm. An out- applying to constant power output. Inter - widely varying \iodel put of either 150 ohms or 600 ohms may modulation and harmonic distortion are re- use in the past as well as today, the be selected by means of a switch mounted duced to negligibility and frequency re- on the front panel. The equipment is sup- sponse is virtually flat from 20 to 20,000 plied with a black leatherette carrying case. cycles. The over -all weight with batteries is 9 Distortioin analysis and complete techni- pounds, 4 ounces. cal specifications will be supplied by the manufacturer without charge or obligation. LOW -COST DYNAMIC NOISE NEW RECORDING HEAD SUPPRESSOR The Indiana Steel Products Company Low cost, remote control, and easy in- A65 Amplifier provides 18 combinations of stallation are features of the newly announced announces completion of the development bass and treble curves. Turnover frequen- model of the famous H. H. Scott Dynamic program for their new magnetic tape re- 500, and 800 cycles Noise Suppressor, Type 110 -A. This unit cording head, Model TD -704. Thoroughly cies of 300, are accom- modated, the control may be added to most existing a -c phono- tested and approved, this new head, used and high frequency graph or radio -phonograph systems. Designed ' for both recording and playback, features permits adjustment to any of, six response flat response to primarily to minimize the background noise high output, maximum frequency response, curves, ranging from down NAB slope -off. amp- on records that is so annoying to the dis- extremely low hum level, compact size, and slightly more than The criminating music lover, the unit can be used efficient performance with all available quar- lifier measures only 8% long x 2N high x 6 mounted a 3- for both AM and FM reception. ter -inch tapes. . inches over -all; can be on -704 is inch relay rack panel, and is small enough Further simplifying the dynamic band - This new Indiana Model TD used to fit into the phonogrph compartments of pass principle of suppression, the Type 110 -A in high impedance circuits; and gives op- covers a frequency range equal to or Using tape with a coercive force of 300 most cabinets.

PROFESSIONAL DIRECTORY

C. J. LEBEL AUDIO CONSULTANT Custom-Built Equipment Winston Wells 370 RIVERSIDE DRIVE 25, N. Y. NEW YORK U. S. Recording Co. Designer and Consultant Acoustical and Electronic Research 121 Vermont Ave., Washington 5, D. C. 307 East 44th St. MU 43487 PRODUCT DESIGN INSTRUMENTATION District 1640 PSYCHO ACOUSTICS SOUND RECORDING New York 17, N. Y. ELECTROMED:CA1 SUBMINIATURE AUDIO

32 AUDIO ENGINEERING NOVEMBER, 1948

www.americanradiohistory.comAmericanRadioHistory.Com FAIRCHILD FORMS RECORDING CORPORATION Formation of Fairchild Recording Equip- Sha1Icross ment Corporation to combine the manufacture and sale of a new professional studio quality magnetic tape recorder with the extensive line of professional disk recording and sound equipment of Fairchild Camera and Instrument Corporation, has been approved by the latter's board, it was announced by James S. Ogsbury, president. The new corporation, with offices at 30 Rockefeller Plaza, New York City 20, will serve the specialized requirements of the radio broadcast industry, as well as those of record' and trancription manufacture, and will also apply magnetic tape recording to industrial, motion picture and aviation uses. Furthermore, recent Fairchild developments have made it possible to meet all the exacting requirements of micro- groove recording for the production of the new long -playing records. Fairchild Recording brings together many well -known figures in the recording equipment field, with Sherman M. Fairchild in- ternationally known for his photographic and aviation developments, heading the croup as president. Wentworth Fling, who has been engineering head of Fairchild Cam- era's recording equipment division, is operating vice president. Jay H. Quinn has re- Maximum Minimum Resistance signed sales Type Sections Size Watts Resistance as manager of Gray Research per section per section and Development Corporation to become Ohms Ohms director of sales and advertising. C. V. 136 1 13/32" x 1/4" 0.25 150,000 1. Kettering, in charge of recording equipment 137 2 45/64" x 1/4" 0.25 150,000 1. sales at Fairchild for many years and an 133 3 1- 5/32" x 3/8" 0.25 550,000 1. authority on the use of this equipment in 134 education, heads a sales division specializing 4 1 -1/4" x 3/8" 0.25 375,00D 1. in educational and industrial applications; Theodore Lindenberg, the inventor of the Fairchild dynamic pickup, heads the instrument laboratory; and Gordon Mercer, for many years in charge of manufacturing for AKRA -OHM PRECISION RESISTORS Recordisc Corporation, then engineer of Musicraft Records, Inc., later with the Gray for "miniaturization" programs company, and recently with Fairchild, is in new charge of the electronic laboratory. John These Shallcross Akra -Ohm Wire - own tinned copper leads, or may be secured B. Wolf leaves the Wound Precision Resistors have been de- with mounting screw. camera company to be- signed to meet the needs of modern, mini- come secretary- Other Shallcross Akra -Ohm Precision treasurer. ature equipment. Standard tolerance is 1% Resistors include types, shapes, mounting Dr. D. G. C. Hare, recently president of and closer tolerances can be furnished on arrangements and ratings for every close - Deering -Milliken Research Trust and an special order. tolerance requirement and are designed to outstanding authority on magnetic recording, The units offer unusually high and ac- meet JAN specifications. Write for Bulle- curate resistance values has been retained by the new corporation in small space and tin RG, giving complete precision resistor are light enough to be suspended by their data in convenient chart form. as technical consultant and advisor. During World War II, Dr. Hare was director of the Airborne Instrument Laboratory at Mineola, N.Y., where magnetic recording techniques were applied to anti -submarine detection Complete Service and guided missile and proximity fuse counter measures.. measurement facilities Mr. Ogsbury said the steady growth of, IN A SINGLE INSTRUMENT the recording equipment division and the desire for further expansion, especially in The improved Shallcross 614 -A Service meter covers a wide range of measurements. These include d -c and a -c voltage, the magnetic tape field convinced the man- capacitance, and d -c resistance. Also it can be used for agement of Fairchild Camera that organiza- approximating an artificial load. Auxiliary scales provide an tion of inductance range of 1 to 100; 1,000, 10,000 henries, and an a separate company would make for a -c resistance range of 25 ohms to 3 megohms. Only two a more efficient operation and closer co- switches are used for 25 ranges. The instrument is self- contained, housed in a metal case with handle and weighs operation with users of this type of equip- only 121/2 lbs. Write for details. ment. Personnel of the new company has been carefully selected for knowledge and ability in recording and reproduction of sound on SHALLCROSS MANUFACTURING COMPANY both disk and tape, and the staff will devote Dept. A -118, Collingdale, Penna.

AUDIO ENGINEERING NOVEMBER, 1948 33

www.americanradiohistory.comAmericanRadioHistory.Com for both lateral and vertical its entire effort to provide the industry with RECO:ID:NG STANDARDS racteristics transcriptions as now used throughout the the finest and most up-to -date precision ® Reports on recording and reproducing industry was recommended to the committee. equipment, Mr. Fairchild said, adding that standards from four of nine project groups Slight changes in the working of this stand- the firm will soon announce a number of working on phases of the problem are being ard were proposed, which would more realis- new and advanced pieces of recently- devel- studied by the National Association of Broad- tically define the curves so widely accepted. oped items. casters' Recording and Reproducing Stand- The standard as to signal -to -noise ratio AES MOVES ards Committee. on disk recording was somewhat revised Audio Equipment Sales, a division of The executive group of the NAB commit- and adopted as a proposal. The "Glossary F. Sumner Hall, Inc., has announced the re- tee, meeting in New York, has also adopted of Terms and Definitions for Disk Record- moval of its executive offices from 923 audio frequency response limits for primary ing" was studied in connection with stand- Eighth Avenue, New York, to larger quarters and secondary magnetic tape recording standards on acoustical terminology recently pro- at 153 West 33rd Street, New York 1. ards, to be associated with tape speeds al- the American Standards Associa- Audio Equipment Sales manufactures teady adopted. posed by tion (ASA). Audio -Line Jack Strips, Patch Cords, Jacks, The tape speeds, approved at a previous The section of the ASA proposal concern- Plugs, and the Audio -Rule a new device meeting, were 15 inches and 7% inches per - ing record terminology was found to depart for quickly determining the exact values of second, the former for high fidelity work, respects from NAB proposals. It resistance for attenuators, without calcula- the latter for portable and mobile equipment. in some it was advisable to re -study tion. Corti,-rance of the NAB recording cha- was agreed that the NAB and ASA proposals with a view toward the early adoption of a coordinated terminology. The various groups were given specific suggestions to aid them in preparing final dey WARD LEONARD yúfed you proposals for the next meeting of the executive committee, scheduled to be held in New York, October 15. Proposals prepared at the meeting will be sent to the NAB Engineering Executive Vitrohm Committee for presentation to the Board of Directors at their November meeting. Preesnt at the meeting were Robert M. Morris, ABC, Chairman; W. S. Bachman, Columbia Records, Inc.; H. A. Chinn, CBS; Plaque C. J. LeBel, Audio Devices, Inc.; R. A. Lynn, NBC; G. M Nixon, NBC; W. E. Stewart, RCA- Victor; and Neal McNaugh- ten, NAB. Resistors TWO -WAY SPEAKERS page 30] For High [tram should be used in constructing the spools, and they should be mounted Frequency Circuits with brass screws through the center. One pound of wire will make both of the coils for any combination shown except the last one in the table, which will require almost 1% pounds. FOR RHOMBIC ANTENAS These non -inductive Plaque resistors are Paper capacitors must be used for For use as a terminating resistor, the especially suitable where a combination 125 wan size is available in 800 frequencies exist. these networks, and low -voltage types fór of power and high ohm, 1600 ohm. and 2400 ohm, - since there will individual use on low power rigs, Deep insulating barriers separate non are quite acceptable, and parallel or series- parallel net- inductive winding. Special Ward Leon- never be more than 25 volts across works on high power transmitters. ard vitreous enamel- tough, crazeless, acid and moisture resisting -is fused them, even at 30 -watt levels. Many of over the base, wire and terminals. Avail- the surplus houses list suitable values able from stock in 20, 40 and 125 watt at low prices. If it is not possible to sizes, in a wide range of resistance values. obtain a single capacitor of the desired AUTHORIZED DISTRIBUTORS EVERYWHERE value, the total may be built up by eon - neeting smaller values in parallel. WARD LEONARD ELECTRIC COMPANY Performance Tests & Electronic Distributor Division Radio In order to stake comparisons of the 53 -R West Jackson Blvd., Chicago 4, U. S. A. GANG MOUNTING performance of these systems, two types Two or more Vitrohm Plaque Re- were assembled. Figure 8 shows a single sistors may easily be ganged to obtain other desired wattages and re- baffle using the Racon unit with a 15- sistances. Illustration above shows while the dual Uni- method of mounting two units to- inch Jensen cone gether. versity tweeter and a 12 -inch cone were ewe shown in Fig. 4. For testing, the Ra- SEND FOR HELPFUL WARD LEONARD con-Jensen combination was mounted CATALOG on a 6 cu. ft. enclosure and the response Catalog D -130 gives com- RELAYS RESISTORS plete data and listings on measured using a warble -tone frequency stock units available in Re- sistors, Rheostats, and Radio record through a flat amplifier as the Amateur Relays. Send for your copy today! étt_5t3t .-' GINE5RED CONTROL C'tVICES source, the sound output measurement being made with an Electro -Voice

AUDIO ENGINEERING NOVEMBER, 1948 34

www.americanradiohistory.comAmericanRadioHistory.Com Model 630 dynamic microphone working directly into a high impedance a -f voltmeter. This microphone has an excellent response up to around 11,000 Potojeddeaaai cps and is quite adequate for measurements of fair accuracy. TAPE RECORDING aaÆFM QUALITY curd EASY PORTABILITY

Fig. 8. Baffle with two -cell Racon horn and I5" Jensen cone and dividing network for I,200-cps crossover frequency. The basic Magnecorder The smaller unit was measured in its cab- recorder mechanism (PT6 -A) inet and the Atlas unit was connected with the Jensen speaker, using the Atlas dividing network and with the Racon unit disconnect- The New 3 Element ed. After adjusting levels, the runs were made, and the results were consistently flat with all models. All three units gave appreciable output up to 13,000 cps, so the h -f ECORDER performance was quite acceptable through- MAGN out. Naturally, the small box was deficient in the lower register, due to its inadequate The Magnecorder meets the highest broadcast standards, size. The measuring set -up is certainly open it costs less. You buy and combine only the to question, since all measurements were and you made in a normal living room, but the prin- units you need: cipal reason for making them was to check Magnecorder (PT6 -A) - Basic recorder mechanism. performance through the cross -over region. Portable Mixer -Amplifier (PT6 -P) - Recording and repro- Further checks were made with an oscillator, used - but standing waves made these measurements ducing portable field amplifier. Can be as high less reliable. quality remote amplifier. Mixes three low -level micro- Subjectively, the two systems with the phones. lower cross -over had more presence on speech Rack Mount Amplifier (PT6 -R) Recording and reproduc- than the University model, but on music - there was little noticeable difference- except ing amplifier for studio rack mounting. With PT6 -A for the poorer bass from the small box, which makes complete studio recorder -reproducer. is only 14 "x18 "x7" inside. However, any of the three systems was noticeably more realistic than a good single speaker, and the Ved'o'ta&e! %d 71ex fie! time spent in assembling any of the types Weight PT6 -A, 23 pounds; PT6 -P, 29 pounds. will be well repaid in listening pleasure. - Wow and Flutter - .2% Frequency Response - 40 to 15,000 cycles; -{- or - 2 db. Tape Speed - 15 inches or 71/2 inches per second (Inter- changeable). RECORD REVUE Motor - Synchronous 1/50 HP. [from page 28] Single Control control on the amplifier, and /or 2) via a Rewind - 45 seconds. separate filter system with step positions. Reels - Standard 7 -inch 8MM film reels. There is little doubt that some form of Current failure never throws tape. Instantly interchange- control of this gate -open (ceiling) situation able rack mounting. is necessary in any system that will repro- from portable to duce over 6 or 7 kc. Just where it is to Write today foi detailed specifications appear. technically, would depend on the response curves attainable in the design. Given similar slopes, the fixed -position and continuously- variable controls are not different in any important way; but if the fixed -filter curves cut off more steeply, there can be a considerable difference in effect between the two. (Note that many ampli- INCORPORATED fiers include both types of action, the boost - attenuate knob for treble, and another for 360 N. Michigan Ave. Dept. A Chicago 1 bass, plus the fixed cut -off, usually in the semi -standard positions of 5, 8, 10 and 20 kc.) In my own amplifier I would definitely want

AUDIO ENGINEERING NOVEMBER, 1948 35

www.americanradiohistory.comAmericanRadioHistory.Com to have both types available, for use separately or in combination. Why a ceiling control? Let us examine the ceiling studiously. This is, again, the situation where we have full volume, gates wide open, no suppression. There are three factors in the input signal, important in reverse order: 1. Tonal range of the useful

signal 1 the music I. 2. Noise input. 3. Dis- tortion in the useful signal. 1. Tonal range is a fairly simple business. Since a large number of records have little on them in the higher tonal ranges -virtually all pre -war records-it may be desirable to cut down maximum gates -open (ceiling) tonal response and thus eliminate ßOJ.nE W.YE' ß(.M2MlC=R a lot of noise that comes through where there is virtually no useful sound. Here the ceiling control acts as an alternative automatic filter setting (the floor control being the other) for high-level conditions, when we may want the full range of available musical signal -but not more. This, again, is simply common fixed-filter usage, applied SPECIFICATIONS in this case to full -volume conditions only MODEL 71 2. FREQUENCY RANGE: 5 to 100,000 cycles. But suppose we have a really wide WAVE SHAPE: Rise time less than 0.2 microseconds range signal from a record- beautifully with negligible overshoot. encased in noise. Not so simple. Here it is alas, a matter of having your cake eat- OF OUTPUT VOLTAGE: Step attenuator giving 75, 50, 25, and MANUFACTURERS 15, 10, 5 peak volts fixed and 0 to 2.5 volts continuously ing it; but what we most need is flexibility, Generators Standard Signal variable. to deal with the cake as our taste demands. Generators Pulse SYNCHRONIZING OUTPUT: 25 volts peak. With ceiling unlimited, every full -volume FM Signal Generators R. F. MODULATOR: 5 volts maximum carrier input. peak opens up all the way and allows the Sauare Wave Generators Translation gain is approximately Output imped- full noise content to come through-in this Vacuum Tube Voltmeters ur1ity- Noise I Field ance is 600 ohms. case plenty of it. And here a vital psycholog- Meters UHF Strength POWER SUPPLY: 117 volts, 50 -60 cycles. ical principle operates: continuous, smooth tiparid "Iss DIMENSIONS: 7" high x 15" wide x 7'/2" deep, overall. noise is far less consciously noticeable than teegohm Meters noise that abruptly or rhythmically changes. Indkators Phase WPM, Thus to suppress some noise and let through Flit Tat Television the rest full- strength actually makes the noise more objectionable than with no suppressor action. This is a delicate situation, and one in which the suppressor is at its weakest. By taking advantage of smoothness in noise content we can usually overlook more of it (and get more highs to our ears) than by suppressing some, but not all. And the GRAY TRANSCRIPTION ARMS and EQUALIZERS farther we suppress some of it (the lower we make the floor) the more noticeable, by cantrast, is the unsuppressed noise that comes through at the ceiling. rn such embarassing cases perhaps it is best to cut out the suppressor, entirely and work on the fixed- fiilter principle, for smooth, steady noise. But, I find, it is often possible to gain an advantage over this-if you have adequate flexibility in both ceiling and floor, as well as gate sensitivity. You may decrease the gate sensitivity; that allows fewer peaks to get through. But, when the gates do open up, especially in sudden, percussive peaks, the noise is still suddenly for the Records too great. Far better, I find, to cut down IDEAL New LP the ceiling, and thus shave off any and all sudden peaks to a mean top level that is The GRAY TRANSCRIPTION ARM 103 -LP, with Selected GE Variable guaranteed (according to taste) not to throw sudden blasts and swishes of noise at you, Reductance Cartridge with I mil Diamond Stylus, has been especially designed and yet to give an optimum quantity of highs for use with the new LP Micro -Groove Records. Due to such features as under the circumstances. The over -all technique here is to reduce the stretch between adjustable stylus pressure, frictionless motion, self -leveling base and the floor and ceiling until the quick changes in accommodation of any standard cartridge, arm obsolescence is precluded. noise level are not too objectionable. But with no ceiling limit this is obviously Arm, with I mil Diamond Stylus Cartridge, $77.95. impossible. , The GRAY #601 4- position EQUALIZER for GE Cartridge, finest performance 3. A similar effect that is considerably more annoying is the distorted signal so and workmanship, ideal response curves. Adopted by radio networks. Matches often combined with loud noise as above. pickup to microphone channel. Complete, $49.50. Without distortion we might tolerate a lot of smooth hiss and even quite a bit of swish Inquiries invited for development and manufacturing. (changeable hiss) in order to get through the clean and pleasant highs of a good recording. But there are still a fabulous new, that are GRAY RESEARCH & DEVELOPMENT COMPANY, Inc. number of recordings, old and distorted, often rather badly -and this dis- 16 ARBOR STREET HARTFORD 1, CONN. tortion is frequently at its worst in the [Continued on page 38]

36 AUDIO ENGINEERING NOVEMBER, 1948

www.americanradiohistory.comAmericanRadioHistory.Com RECORD LIBRARY In this spot a continuing list of records of interest will be presented. This list specifically does not suggest "the" best recordings or versions. It will draw predominantly but not entirely from postwar releases. All records are theoretically available, directly or on order: if trouble is experienced in finding them AUDIO ENGINEERING will be glad to co- operate. Records are recommended on a composite of musical values, performance, engineering; sometimes one, sometimes another predominates but records unusually lacking in any of the three will not be considered. Number of records in album is in parenthesis.

Pre -War Classics - exceptional recording (restricted range); highest musical quality. Bach, Organ Works, vol. I. Albert Schweitzer, organ of All Hallows, Barking. Columbia M 270 (7) Bach, Cantata No. 4, "Christ lag in Todes- banden". Bach, Cantata No. 140, "Wachet auf ". Orfeo Catala; Barcelona RCA Victor M 120 (5)

Beethoven, Symphony No. 3 ( "Eroica "). Columbia M 285 (6) Beethoven, Symphony No. 7. Columbia M 269 (5) WHEN you invest in a Racon Horn or Beethoven, Symphony No 9 ( "Choral ") Vienna Philharmonic Orch. Weingartner. Speaker you get all- around super- Columbia M 227 (8) efficiency- qualities that can not be duplicated by units of conventional type. Such Beethoven, Symphony No. 2. units may resemble flacon units externally Boston Symphony, Koussevitsky. but only a Racon has the internal construc- RCA Victor M 625 (4) tion features that bring you top per- Beethoven, Septet, opus 20. formance. (solo players) Racon superiority is the result of RCA Victor M 571 (5) Advanced Engineering -improvements developed in the Racon Laboratories by Handel, Water Music Suite (arr. Harty). Racon Engineers through tireless research PERMANENT MAGNET London Philharmonic, Harty. and laboratory testing. HORN UNITS Columbia X 13 (2) RACON DOUBLE RE- ENTRANT These units, highly popular in all Haydn, Symphony No. 45 ( "Farewell"). T ".rUMPET types of service, embody many im- London Symphony, Sir Henry Wood. (Illustrated above) provements. Two groups, one with Columbia M 205 (3) An excellent example of Racon scientific Alnico V Magnets, and one with Haydn, Symphony No. 92 ( "Oxford "). engineering. Designed to deliver highly Alnico Blue Dot Magnets. All steel Paris Conservatory Orch. Bruno Walter. concentrated sound over long ranges with parts plated to prevent corrosion. RCA Victor M 682 (3) maximum efficiency. Seven models, ranging Also fitted with corrosion proof metal or plastic diaphragms. Voice coil Mozart, Piano Concerto in A, K. 488. in length from 6 -5/8" to 28 ". All with impedance on all units: 15 ohms. Marguerite Long, orch. cond. Gaubert. aluminum casting inside tone arm and bell of gauge spinning. Special ohmages on request. Columbia M 261 (3) heavy aluminum Large sizes built with center reflecting section of FURNISHED WITH Mozart, Sinfonia Concertante in Eb, K. 364. Racon Acoustic material for preventing NOW WATERPROOF CASING Albert Sammons, violin, Lioalele Tenis, resonant effects. Smaller sizes have heavy viola, London Phil. Harty. gauge aluminum spinning for center section. All units, from junior to giant Columbia M 188 (4) Strong construction, practically abuse size, may now be had with heavy close Mozart, Symphony No. 29, No. 34. proof. Fitted with swivel mounting ratchet spun aluminum case, forming a Boston Symphony Orch. Koussevitsky. wall bracket. For larger sizes U- bracket fitting hermetically sealed watertight RCA Victor M 795 (5) mounting will be supplied on request, at no housing for outdoor use. Waterproof extra cost. type at slight extra cost. Mozart, Symphony No. 35 ( "Haffner "). London Philharmonic Orch. Beecham. Write for Catalog of complete Racon line. Columbia M 399 (3) Mozart, Quartet No. 17 in Bb, ( "Hunt "), RACON ELECTRIC CO., INC. K. 458. 52 E. 19th Street, New York 3, N. Y. Budapest Quartet. RCA Victor M 763 (3) Mozart, Serenade for Thirteen Wind Instru- Speakers ments, K. 361. Harn ¿/M#$ Chamb. Orch., Edwin Fischer. RCA Victor M 743 (3) Horns

AUDIO ENGINEERING NOVEMBER, 1948 37

www.americanradiohistory.comAmericanRadioHistory.Com peak volume passages, the very ones that are passed through untouched by the suppressor with unlimited ceiling. Moreover, distortion AUDIO to being what it is, even those records that have From a highly restricted legitimate tonal range still send distortion through at the higher TELEVISION levels, well above the nominal undistorted range of the music! The trouble that distortion in the signal Molded -iron cores and core assemblies makes in any suppressor without ceiling control is 'only too obvious when you con- in a wide range of sizes and styles for sider that since most distortion is worst in 100 200 mc. the peak passages. the suppressor actually frequencies from cps to works in reverse -it suppresses the less mo- Lenkurt Trancors excel because they are ticeably distorted highs at low -level passages, lets through to a maximum of 10 kc made to the exacting requirements of or so every good burst of loud distortion that comes along. Is it any wonder that Lenkurt carrier telephone gear. Lenkurt many demonstrations of the dynamic suppressor have gone sour? When high dis- knows how to reduce losses, minimize tortion is combined with high noise level, the effect of the unlimited- ceiling action is distortion, tailor permeability and Q to nothing less than dreadful. The number of needs. for catalog. musically good records that boast this un- your Write lucky combination of noise and distortion at peaks is appalling. The most vital function of the ceiling control, then, is to limit distortion. Without any question, the most important single difficulty I've run into is the highly unpleasant effect of those unchecked bursts. let LENKURT suddenly through the gates when there is no ceiling to control them-and sounding ELECTRIC CO. the worse in contrast with the low -level SAN CARLOS, CALIFORNIA passages where distortion is sharply controlled by the closing of the gates. For most purposes a single ceiling filter position at 8000 kc cut-off would take care of a very large part of the distortion problem and a good deal of the sudden swish from KNOWS 110W too sudden letting through of high- frequency LENKURT noise. But for records with limited tonal range one is likely to want a lower cut -off, probably at the usual 5000 position; and so a ceiling control on my theoretical suppressor- amplifier would probably have graduated steps at the standard positions of 20 (unlimited), 10, 8, 5 kc, in addition to the attenuator control. Announcing * r e Needless to say, this discussion of the three basic outward controls desirable on a really flexible suppressor -amplifier ignores TELEVISION INTERFERENCE many internal problems of design which, though they obviously affect performance of any of these functions, are beyond the scope Its Causes and Cures of any but the laboratory designer himself. I mention briefly two, for reference, on which there is significant disagreement. First, the A new Handbook by Radio Magazines, Inc., covering in detail the delay in release time in the gate action. important facts of TVI. The TVI Handbook is edited to fill the pressing (Attack time isn't particularly controversial; of amateurs and other technicians confronted with the fast enough to open for all music, but slow requirements and pops). Re- television enough to skip over ticks problems of TV interference, or otherwise unsatisfactory lease time, which is easily adjusted via con- reception. Included in its thorough treatment of causes and cures are denser values, is bound to be a compromise a comprehensive set of TV screen photos depicting all types of to suit many different requirements. The suppressor (August reception, many case histories, preventative design data and other McProud simplified issue), for instance, has to my ear an un- equally pertinent facts. It is a vital publication for radiomen wherever usually long delay- excellent for percussive TV is on, or about to go on the air. music (piano, pizzicato strings, etc.) but possibly not as well suited to other musical Price 50c postpaid, or order from your local dealer. situations. Delay time clearly has much to do with the quality of "swish" achieved when noise bursts through. Were it possible, CQ -Radio Magazines, Inc. I would like to see on my super -duper com- am adjustable de- 342 MADISON AVE., New York I7, N. Y. plete amplifier-suppressor lay control, probably with, say, three settings. A simple and inexpensive addition delight in extra controls. Enclosed find $ for copies TVI Handbook for those who (Now it's four!) Name A final area of doubt and disagreement is between those who count on as slow, wide Address spread to the gate opening action (so that most of the time the gates are working City Zone State within the partly -opened area) and other designers who deliberately arrange for a

1948 38 AUDIO ENGINEERING NOVEMBER,

www.americanradiohistory.comAmericanRadioHistory.Com very rapid opening action. Either is quite simple to design - - -the difference is one -hundred per cent a matter of musical effect, in the actual operation of the spppressor. the House on Cortlandt Street McProud, it will be noted, goes so far as to take his control signal from the output, thereby making it "boost itself" so that the gates open very quickly once action is ini- tiated. With the material of this article in mind it can be seen that this involves some pretty fundamental questions of operational theory. I would like to hear Mr. Goodell and Mr. McProud have this one out, for the benefit of us all! I will venture no opinion at this point, being occasionally a cautious soul.

Recent Recordings

Albeniz, Iberia, Books I and II. Claudio Arrau, pianist. Columbia 757 (5) Two of the four "books" of this series of brilliant Spanish piano works, the top output of their sometimes -trite composer and piano music that rates high in content and energy, as well as mood and entertainment value. This album contains some of the less familiar of the set -El Puerto, Rondèna, Almeria, as well as the often heard Fête -Dieu New York's a Seville Evocation, and Triana, the best known of all. If you like the Spanish idiom -at least the Spanish as we outsiders know largest distributor it,-this music will appeal enormously. The playing by Arrau is just a bit chilly. of high fidelity De Falla, Seven Popular Songs Carmen Torres, soprano. John Newmark, sound equipment piano. RCA Victor DM 1223 (2) and its Here is a wilder, more genuine Spanish Terminal Radio Corporation CORTLANDT ST. style, full of the strong oriental feeling that staff of sound experts are sincerely is so remarkably potent in Spanish folk song and dance. These are actual folk songs, interested ill helping you make the proper set with elaborate accompaniment for piano, beautiful works in themselves, by Manuel choice of coordinated equipment to insure De Falla. The strange melodies here will satisfaction. remind one of other music of De Falla, your complete notably the Amor Brujo music, of which two Our Sound Studio is especially equipped recent recordings have been released. Car- men Torres is Spanish, has a "white" kind to enable you to conveniently see, of voice without as much exotic color as we somehow expect from that country; but listen to, and compare equipment she knows the fancy and difficult ornamen- before purchase. tation of these songs obviously well. Her mike technique is a bit bumptious -extreme low levels and very high ones -but nothing to spoil the music. Very fine piano record- Catalog. the accompaniment. our Sound ing in Write for Sound "For True Schumann, Liederkreis, opus 39. Our brochure equipment Helen Traubel; Coenraad Bos, piano. contains Columbia MM 752 (5 I0 ") Reproduction" recom- selected and One of Helen Traubel's earliest recorded carefully audio re- albums was of Schumann songs; here she is fidelity for high and back at the same stand, after countless re- mended quality from Isolde's Love Heavenly cordings of everything quirements. on amplifiers, Home, Sweet Home. A most com- prices Death to ground -level micro- plete contrast to the Spanish music above; speakers, this is fresh German Romantic, youthful, a tuners, pickups, etc. European. Traubel does cabinets, bit naive, highly phones, records, a better job here (with her huge voice) in accuracy and control and economical ex- pression than she did in the earlier work, but it is still a bit monolithic, without the wonderful bounce and flexibility of meaning that Lotte Lehmann or Elizabeth Schumann can put into this sort of music. Not bad at all, though. The recording is vocally beautiful; but -power of a name-the piano is almost entirely inaudible, a faint, tinny clanging somewhere off in the far back- RADIO CORPORATION ground. How long will it be before recording engineers learn to keep a stiff upper lip 85 CORTLANDT STREET when in the Presense of a Name! (This album recalls the famous Flagstad- Melchior NEW YORK 7, N. Y. - Phone: WOrth 4 -3311 Wagner duet album, with a faint and tinny Symphony Orchestra somewhere far, far

AUDIO ENGINEERING NOVEMBER, 1948 39

(if racks are employed), and terminal Machines; Auricon SS camera & recording equip- block data are all that is needed to un- ment; Bardwell & McAlister lighting equipment; dertake the assembly and wiring of the Moviola and Micro Engineering editing equipment. audio facilities involved. Finally, practical design considera- FRANK C. ZUCKER Complete staff of trained technicians tion dictates the avoidance of appara- CAMERA EQUIPmEI1TC. available for consultation. are to tus components that operated ó00 BgCQCWFy rIEW tots CITY

AUDIO ENGINEERING NOVEMBER, 1948 41

www.americanradiohistory.comAmericanRadioHistory.Com away. Same story and there are plenty of others, notably among recordings by famous violinists.) Vaughan Williams, Concerto for Oboe and Strings Mitchell Miller, Seidenberg Little Symphony. Studio Microphones Mercury DM 7 (3) Vaughan Williams, Mass in G Minor 0923). at P.A. Prices Fleet Street Choir London I. T. B. Lawrence IlarrA I nneinn FrIA F7 I l l

ation at 15000 cycles. An image .0004 inch wide gives similar results in 1.6- Introdttcin? mm reproduction. At one -half the frequency of infinite attenuation the attenuation due to scanning beam width hv 11 J.a('(1 ` is 3.9 db. The scanning light beam in a film reproducer must be quite accurately adjusted if good results are to be obtained. The important adjustments are for focus, lateral position, and "azimuth ". The last term refers to the adjustment by which the line image is placed accurately at right angles to the direction of motion of the film. Departure from the right angle by more than about five minutes of arc introduces noticeable wave form distortion in the reproduction of variable area tracks. In the reproduction of variable density tracks no wave form distortion is introduced, but high frequency attenuation is increased. 1HIGH FIDELITY reprod.reer that sets In reproducing variable area records AA a new standard of cfficiency in electro- is have illumi- acoustical conversion. Small enough to it necessary to constant fit into the home console. Sound output ample nation over the len,th of the scanning for studio and industrial requirements. Has 1200 horizontal angular dispersion. Special light beans. This requirement is not type cone speaker for low reproduction, and easy to meet in practice. Fortunately cellular s eaker for wide angle coverage. High frequency response, and many other a falling off of about75 ver cent at the exclusive features. Frequency range 50 to ends of the beam does not produce ob- 12.000 cps. Size 11" x 15" x 11" deep for 6 watt operations, size 15" x 21" x 12" deep jectionable distortion, and most sys- for 20 watt operations. tems perform within this limit when Write for free catalog the light source is properly centered. RACON ELECTRIC CO., INC. The photo cell and the circuit by 62 E. 19th St. N. Y. 3, N. Y. which it is coupled to the amplifier deserve more attention than is sometimes given. If the modulated light PRECISE AUDIO beam is brought to the cathode surface of the cell in such manner that the MEASUREMENTS illuminated area on the cathode any un- Pickering and Company, Inc., leaders in changes with the modulation, the audio field, make available to the evenness of cathode sensitivity will in- industry their specialized facilities for troduce wave form distortion. If the audio measurements. land or cell is coupled by a resistance of sev- We are prepared to make guaranteed sea .. measurements on any type of audio Sound wherever eral megohms, as has been done in is used, equipment for manufacturers, dealers, or can count many cases, there is considerable at- consumers. on unblemished frequencies as low as 5000 faithful performance. tenuation of cycles, and harmonic distortion up to PICKERING & CO. Inc. a decade For Oceanside, N. Y. Atlas Sou several per cent may be introduced at been noted forf nd has this point. If the operating bias on Equipment Sound . the cell even slightly exceeds the rated tratea Alnico the ex- maximum (usually ..90 volts) the cell 13 V Plus extra feature hiss will be high, and the cell may clarity, "flashover" due to gas ionization when Christmas power, extra long-life. no film is in the light beam. Under Seals this condition cells deteriorate rapidly. All these defects can be avoided in a properly designed system. Performance / Photographic recording proceE.ses are s ATLAS capable of extremely high fidelity. For example, as reported by Hilliard6, ND cORp, push -pull 35 -mm sound tracks totaling .200 inch in width, which are gmployed for original motion picture studio recording, are capable of a signal to noise STREET 1445 39th ratio of 74 db. Single 35 -mm sound BROOKLYN 18, N. Y. tracks recorded by similar methods

44 AUDIO ENGINEERING NOVEMBER, 1948

www.americanradiohistory.comAmericanRadioHistory.Com give signal -to -noise ratios of 60 to 6:i db. Intermodulation values as low MODEL 4031 -1B PROGRAM EQUALIZER as 4 per cent are not unusual. The recorded frequency range in 35 -mm practice is usually 8000 cycles, although on several occasions records which included the range to 15000 cycles have been demonstrated. Ex- tension of the frequency range is a commercial rather than a technical problem, since losses in the 35 -mm film amount to no more than 6 db at 10,000 cycles or 11 db at 15,000 cycles when the usual methods of printing are INSERTION LOSS ONLY 14 1113.! used. Capable of providing vari-bie regulation over a range of 16 DB. attenu- action and 12 DB. equalization in calibrated and detented 2 DB. steps at In theatrical motion picture practice both ends of the audio spectrum, the 4931 -B Program equalizer is one of the quality standards are considerably the most advanced units on the market today. lower than those indicated above. The Designed to fit the needs of motion picture, recording and broadcast industries, this distortion -free equalizer can be cut in or out without standard reproducing frequency char- changing the signal level at the center of the range. Variable high -fre- acteristic is shown in Fig. 8. The vol- quency positions are selected at 3, 5 and 10 KC. by a panel key. A separate ume range of sound in release prints control permits independent equalization and attenuation over the low - is usually limited to about 40 db, since frequency spectrum. Designed with a constant "K" circuit, the impedance remains constant over the entire range of control. experience has shown that " low level are hum pickup. Controls to be Toroidally wound coils used to eliminate passages in recording are likely may be operated while the program is in progress without causing clicks masked by audience noise. The sound or apparent change of average level. negatives for release prints are pro- Send for literature and complete set of curves. duced by re- recording from the push - pull originals to single tracks, with, naturally, some increase in distortion. Often as many as twenty original CINEMA tracks are combined into one in this ENGINEERING COMPANY re- recording process. 1510 W. Verdugo Ave. Burbank, Calif. Original single track records on 16- Cable Address: CINENG BURBANK mm film, as the writer has demon - strated,7 can carry a frequency range THE H. H. SCOTT 210 -A AMPLIFIER IS THE LOGICAL CHOICE IN of 10,000 cycles provided the print is made by the optical method. At 10,000 cycles the film losses amount to CUSTOM 15.5 db. As is well known, it is practical to use this amount of "tip up" INSTALLATIONS of the high frequencies. The characteristic required is not much dif- /04. izealiazie - ferent from the NAB characteristic for disc recording. The signal -to -noise Broadcast and Recorded Music ratio with this frequency range is mechanical, and musical excel- 45 to 50 db, and harmonic distortion Built to laboratory standards of electrical, lence, this 20 -watt amplifier includes these features . proved essential does not exceed 2 per cent with prac- to satisfactory custom performance in both FM and AM radio reproduction tical tolerances in film processing. Most as well as in record playing. 16 -mm reproducing systems, however, Extended listening range ... 30 to 20,000 cycles, with negligible inter - are equipped with inexpensive loud modulation or transient distortion, speakers and for this and other pri- *Dynamic noise suppressor ... designed particularly for this unit ... marily economic reasons have been allows widest use of high- and low- frequency gain, designed to respond well only up to Twenty -watt output ... with less than 2% harmonic distortion; range and power -handling capability exceed RMA broadcast station about 5500 cycles. Because of this requirements, limitation in reproduction it has been Minimum controls for maximum flexibility . . . easy selection of general practice to limit the frequency exact tonal balance to please every ear. range in recording to approximately GUARANTEED FOR A FULL YEAR 6000 cycles. Since the 16 -mm projector FOR RADIO RECEPTION IN RECORD REPRODUCTION is in the same room usually operated . the H. H. Scott 210 -A enhances . . . the H. H. Scott 210 -A lab- with the audience, it is usual to limit enjoyment of both FM and AM oratory amplifier provides unsur- the volume range in 16 -mm recording broadcasting, minimizing clicks, passed realism, with freedom from to not more than 25 db (in some cases whistles, and noise. scratch, hiss and rumble. to no more than 15 db !) in order that For full details, request Bulletin 4811- AE *Licensed under U.S. and foreign patents pending and issued. the sound level will always override the machine noise. It is to be expected that the now rapidly increasing use of 16 -mm sound film in television broadcasting where these reproducer limita-

AUDIO ENGINEERING NOVEMBER, 1948 45

www.americanradiohistory.comAmericanRadioHistory.Com tions reed not apply will lead to a grater utilization of the quality pos- sibilities of this medium. You Get All These Features [The second article in this series toil! NE,nionly discuss in detail the design of light modulat- ... in a REAL ing devices and optical systems for both variable area and variable density sound - on -flan. recording. Ed.] cost PRECISION that References an HALF l literature , technical 1. "Physik und Technik des Tonfilms" H. for rite today Lichte and A. Narath. Verlag vom S. INSTRitiIENT Hirzel in Leipzig, 1943. Reprinted by Edwards Brothers, Ann AMPLIFIER CORP. OF AMERICA Arbor, Mich., 1945. 398 Broadway New York 13, N. Y. 2. "Motion Picture Sound Engineering ". Academy of Motions Picture Arts and Sciences. D. Van Nostrand & Co., New 'NEW PRINTED LEARN York, 1938. 3. Chapter XXII- "Photographic Aspects FUN aRCU11S PROFIT of Sound Recording" in "The Theory of the Photographic Process" C. E. RESEARCH- DESIGN -REPAIR -MFG. - Kenneth Mees. The Macmillan Co.. MADE FASTER AND EASIER. Be first to New York, 1944. learn the new printed -circuit methods. Re- Most new radios have partly printed circuits. 4. "The ABC of Photographic Sound Paint working circuits over Sour rough cording" E. W. Kellogg, Jour. Soc. Mot. sketches. Kits of conductor and resistor Pict. Eng., Vol. 44, No. 3 (March, Paints : Standard $3, Super $5 (bas silver 1945) page 151. Also available as a paint). Complete repair and design- instruc- Model reprint from the Society of Motion Pic- tions. Quantity discounts. High instruction- al value for colleges. Free price lists and 748 ture Engineers. This paper contains an literature. Just enclose cash, M.O. or COD extensive bibliography. plus 25% with name and address, and this 5. "Optical Sound Track Printing" J. A. ad with desired kit underlined. Maurer. Jour. Soc. Mot. Pict. Eng., n Standard Super Literature Vol. 50, No. 5 (May, 1948) page 458. MICROCIRCUITS CO. PORTAL'_E HIGH FIDELITY 4-1N-1 6. "The Variable Density Film Recording Dept. 2C, NEW BUFFALO, MICHIGAN M. G. M. Studios" System Used at Name_ _ RADIO - PHONOGRAPH Soc. Mot. Pict. John K. Hilliard, Jour. Address 40, No. 3 (March, 1943) . Eng., Vol. City State "16 -mm Film as a Medium for Televi- - -- sion Broadcast Program Material" J. A. VERTICAL LATERAL 1948 Maurer. Paper presented at the REPRODUCERS RECONDITIONED of the Society of PUBLIC ADDRESS SYSTEM spring convention Types 9 -A, 9 -B, D- 93306, MI -4856, MI. 4875-G Motion Picture Engineers at Santa Complete Stock of New Replacement Parts Exclusive synchronized timing Monica, Calif. To be published in the for All Types the Society of Motion Pic- This service is being used by leading radio meter that indexes all recordings Journal of ture Engineers. stations and wired music companies from ...never before found in any coast to coast. Factory prices prevail. moderately priced wire recorder... VIBRATION SYSTEMS, INC. 1040 W. Fort Detroit 26, Michigan just one of the features you get in St. Pentron, new 4 -in -1 combination of wire recorder, radio, phono- RADIO SYSTEMS WANTED address graph and public system. [from page 23] G. I. Flyer motor model 818 In addition, in actual daily testing, Pentron instruments constantly the necessity for tone controls, and the double field two speed; also Blue model E. Contact measure up to a far higher standard writer is convinced that this circuit is Flyer of precision than any other make! one of the outstanding contributions W. Dacosta, Sound Dept., WOR Precision-built, 7 tube, high fidelity wire to the audio art ih some time. The recorder; records and plays back its own R35, R36, 022 serve to radio, phonograph or microphone. combination and W A N T E D Absolute one-time erasure adjust the total gain to a suitable Western Electric Vacuum Tubes, Types 101F, High speed, 6 to 1, rewind amount to take advantage of this type 102F, 272A or B, 310A or B, 311A, 313C, Neon recording level light of control, and the response curves of 323A, 328A, 329A, 348A, 349A, 352A, Two -way automatic shutoff Fig. 10 represent th13 output at various 373A, 374A, 393A, 394A, 121A Ballast Lamps. Wide, low loss recordings ... 70 to 7,500 cps. settings. C22 again adjusts frequency Box 100, Audio Engineering. Amplifier has 6 watts output response occasioned by the residual ca- Highly selective superheterodyne radio pacitance of the volume control struc- WANTED speaker Built -in 54" ture. W. E. Carrier Telephone and Carrier Tele- Tm- tracking phono arm, plays 10' or graph Equipment and components. Filters. 12" records, Astatic L70 cartridge di- The two level- adjusting voltage repeating coils, transformers, equalizers. Medium gain public address system viders used in this amplifier could have Type CFI., CF2, H, C, and other carrier Jacks for: External amplifier, Extension teen and the levels adjusted equipment, telephone and telegraph repeat- Speaker, F. M. Tuner 1 eliminated Approved by Underwriters' Lab. by selection of plate -load resistors, or ers. Box 101, Audio Engineering. 105 to 120 Volts 50 or 60 cycles AC I) \- various other means. However, for WANTED L(c experimenter it is thought better 12 sets of ARCS equipment as follows: Write for complete details R25 Receiver (1.5 - 3 mc.) MT7A Receiver to have some variable elements in the Rack MT5 Receiver Base T18 Transmitter DEALER INQUIRIES INVITED circuit. Once adjusted, these networks (2.1 - 3 me.) MT69 Transmitter Rack MT68 Transmitter Base MD7 Modulator DY8 cause sto trouble, yet they provide a Dynamotor MT76 Modulator Base MT77 THE PENTRON CORPORATION allow for fu- Relay Base T17 Microphone HS33 Headset measure of flexibility to E39 Receiver local tuning knob PL152 Plug 611 W. DIVISION ST. CHICAGO 10, ILL ture modifications. PL147 Plug PL151 Plug PL14S Plug 9377 In this amplifier, 6J7's were used Plug. Box 90, Audio Engineering. 4 46 AUDIO ENGINEERING NOVEMBER, 1948

www.americanradiohistory.comAmericanRadioHistory.Com because of the grid connection separation. The only tube with a -c filament supply is V7, which is not actually in coRNSTBN the signal circuit. The other heaters PI C K- U P LEADS are wired in a series -parallel arrangement, with the center tap brought out WITH REMOVABLE STYLUS to a terminal. Normally operating from a 12 6 -volt d -c supply, terminals A QUICKLY nEMOVABLE fTY " -äS SIZE O "r RECORDED GROOVE 2, 3, and 7 of the terminal strip may TO MATCH EVERY CLARKSTAN HIGH FIDELITY WILE RANGE PICKUP in NAB circuit plays new be connected together, and 6 and 8 fine groove long playing records : simply change stylus and adjust weight. The same strapped together, and all tubes fed pickup and turntable setap can be ,.scd for all types of records and transcriptions. from a 6.3 -volt a -c or d -c supply. The (' LARKSTAN PICKUP AND TRANSCRIPTION regulated 225 -volt plate supply for this ARM are ideally balal.ced f r reprodu t on of loop slaying micro groove records. Needle force of unit comes from the main amplifier - 5 -7 gm. gives optimum tracking. Use counter - needle force power Since 6SG7's have a semi - v.eicrht jí216.1 for qu'ck change of unit. en different typo records. remote cutoff, they are more desirable for the reactance tubes than either CLARKSTAN RV PICKUP covers all ranges included in highest types of recording -1 teral the 6SJ7 or 6SIi7 types. compliance and low needle force mean long record lite . . Stylii available w th .0J10 ", .0015", 0J22 ", No difficulties should be encountered .0025 ", .0030" ball point radius made in in adjusting this unit, provided the points of diamond, sapphire, ,. ^hy or precious metal. S -nd for stylus in'ormation. Circular No. 167. constructor has available an audio os- CLARKSTAN ESCI.USIVE DEVELOPMENTS: cillator and an output meter, prefer- Audio Sweep Frequency Test Records, Audio Sweep a a Frequency Generator and Clarkstan Gauge for Phone ably sensitive -f voltmeter. All Needle Force. tuning circuits in the suppressor are Net Price No. 201 Clarkstan Cartridge w /stylus $15.00 made adjustable, and the resistor values Sapphire Stylus for No. 201 2.40 for SW2 can be determined easily. No. 221 Equalizer 15.00 Diamond Stylus for No. 201 27.00 Since it is not expected that experi- No. 211 Clarkstan Pickup with Transcription menters will follow the circuit exactly, Arm 34.50 216.1 Counterweight for No. 211 1.20 this description has attempted to trace No. See Your Write For the steps taken in the design and ad- Favorite Catalog justment of the noise suppressor cir- Radio Jobber CORPORATION No. 41 Today! cuits and of the phonograph equalizer. 111927 W. Pico Blvd. Los Angeles 34, Cal. However, it is believed that if this en- CLA RKSTRN tire circuit is duplicated, equivalent results should be obtained. Components Resistor and capacitor value are 411 shown on the schematic, Fig. 3, but the characteristics of the transformers and inductances used are not so obvious. RCA In the unit shown Li is 2.9 II; L2 is 1.0 H; L3 and L4 are 0.45 H. For the these coils, UTC VIC adjustable in- presents ductances are suitable and readily obtainable. The types chosen should be used as near the minimum inductance LC -4B setting as possible, to insure the high- the est Q of the coils, which will then be Acoustically engineered to give finest in listening pleasure at the around 15. Therefore, Li should be a lowest cost. VI -C15; L2 should be a VI -C13; and L3 and L4 should be VI -C11 units. The input transformer used is a Langevin 401B, but suitable substitu- tions would he UTC A10, ADC 215A, From the laboratories and factory of RCA which have produced the sneaker equipment now used SPECIFICATIONS LC -4B Thordarson 20A05, Chicago BI -1, or Frequency range 60 to 13,000 cycles in some of the world's largest and finest theatres, Sensitivity 94 db (Measured with IMV Stancor A -4351. The output trans- auditoriums, and studios comes a NEW Console signal at 4 ft.) Impedance 7.5 ohms former used is a Western Electric LC -4B. This unit combines the Power Handling Capacity 10 watts. Reproducer -the Weight 44 lbs. 132C, but UTC LS -27 or HA -113, ADC exceptional frequency characteristics of a unique, Suggested list LC-4B, Complete $163.50 low distortion, low frequency speaker and a bril- 315A, Thordarson T- 22592, Chicago SPECIFICATIONS LC -4A liant, high efficiency, horn type tweeter in a func- BO -1, -or Stancor A -3315 should give Frequency range 75 to 13,000 cycles tionally designed cabinet with natural or walnut Sensitivity 97 db (Measured with IMV equally satisfactory results. finish. You can now realize the full tonal range, signal at 4 ft.) Impedance 7.5 ohms Power Handling Capacity 20 watts. The power amplifier an power sup- richness, and color of FM Radio and wide range Weight 44 lbs. ply units for this peular system recordings at a new low price. Suggested list LC-4A, Complete 5161.00 will be described in.(e Ttext article in Make the RCA Sound Products Distributors in your territo y YOUR SOUND HEADQUARTERS. this series, which will conclude with a SOUND PRODUCTS final article on a two -way corner RADIO CORPORAT /ON of AMERICA speaker. ENGINEERING PRODUCTS DEPARTMENT, CAMDEN, N.J.

In Canada. RCA VICTOR Company lim'i,ed, Manl,eol AUDIO ENGINEERING NOVEMBER, 1943 47

www.americanradiohistory.comAmericanRadioHistory.Com SPECIAL PROBLEMS %\1.O BETTER ELECTRONIC EQUIPMENT demand ADVERTISING ead T1 DARD ZED INDEX SPECIAL TREATMENT READY - TO -USE Engineering judgment is often the most important ingredient CABINETS in the design and construction Allied Radio Corp. 4.8 Altec Lansing Corp. 4.3 of apparatus for your specific Amperite Co., Inc. 40 CHASSIS needs. We specialize in - Amplifier Corp. of America 46 d4z ALL ip one -of -a -kind instru- Arnold Engineering Co. - Atlas Sound Corp. 44 PANELS ments P. A. for electronic or Audio Devices, Inc. Cover 2 electro- acoustic measure- NEEDS Audio Instrument .Co. 48 RACKS ments Par -Metal Camera Equipment Co. 41 Equipment Cinema Engineering 45 -recording or reproduc- Co. is preferred by Clarkstan Corp. 47 Service Men, ing systems with special Electro Motive Mfg. Co., Inc. 2 Amateurs, and characteristics Electro- Voice, Inc. 31 Manufacturers because they're subminiature audio Gray & - Research adaptable, easy - for Development Co. 36 to- assemble, eco- Hewlett- Packard Co. 10 nomical. Beautifully Laboratory, Production Testing, Hollywood Sound Institute 48 designed, ruggedly Broadcast Station, Recording constructed by spe- Jensen Manufacturing Co. 9 cialists. Famous for Studio, or Discriminating Home Langevin quality and economy. Manufacturing Corp. 40 Write for Catalog. LeBel, C. J 32 Audio Instrument Company Lenkurt Electric Co. 38 PAR -METAL Magnecord, Inc. 35 PRODUCTS 1947 Broadway, New York 23, N. Y. CORPORATION Measurements Corp. 36 32.62 -49th ST.,LONG ISLAND CITY 3, N.Y. Microcircuits Co. 46 Export Dept.: Rocke International Corp. Film Par -Metal Products Corp. 48 13 East 40 Street, New York 16 Cleaning Machines Pentron Corp., The 46 Pickering & Co., Inc. 44 Professional Directory 32 Sold out Racon Electric Co., Inc. 37, 44 before Radio Corp. of America (Broadcast Equip. Div.) ALLIED'S but we Cover 4 NEW 1949 just got Radio Corp. of America (Sound Equip. Div.) 47 RADIO more! Scott, H. H. Inc. 45 ..,,... Shallcross Manufacturing Co. 33 Shure Brothers, Inc. 8 CATALOG «~ Neumade Automatic Film Cleaning S. O. S. Cinema Supply Corp. 48 Machine -35mm model -Does full 1000' reel in in Radio B minutes, rewound perfectly dry, cleaned and Sylvania Electric Products Inc. 1 EVerything polished. Totally enclosed steel housing, sealed Supplies motor. Quantity limited. Tech Laboratories 43 Electronic Worth $330. Brand new $159.50J.7 J Terminal Radio Corp. 39 and Combination Model takes 800' 16mm reel Service or 1000" 35mm reel, otherwise United Transformer Corp. Expert same @ Completer specifications. Brand new $ 194.50 Cover 3 Engineers 2000 Watt Studio Spots large fresnel U.S. Recording Co. 32 for Radio lens, U type yoke mounts to any pedestal with Vr dia. stem. Laco Mogul screw or Creco Vibration Systems, Inc. 46 Mogul bi -post socket. Value $160. Ward Leonard Electric Co 34 Excellent $57.50a :1 You'll find everything you need in radio and S.O.S. EXTRA SPECIALS Wells, Winston 32 electronic equipment for laboratory and of the Month development work, in the new 180-page Synchronous Motors 1 /12HP 110V 60 cy., Western Electric Co. 4, 5 for cameras, recorders, film phonographs, ALLIED Catalog. Rely on one dependable projectors, etc. Worth $90. New $57.50 W 0 R 46 source for the world's largest stocks -thou- 35mm Sound -Film Recorders with sync sands of parts, tubes, tools, books, test in- motor 1546 struments, sound apparatus -ready for in- Houston 16mm MA film processing stant expert shipment at lowest market prices. machines (similar to present Model No Write today for your FREE copy of ALLIED's 11) rebuilt 3496.00 ENGINEERING SCHOOL newest Buying Guide. Berndt- Maurer AUDIO Model B 16mm Sound Re- A practical 9 months' course in Sound Funda- cording Outfit, with noise redaction amps mentals, Recording. and Sound Transmission Worth $3500 2275 measurements ; in a laboratory containing Cine Special Blimps, complete, New transmission sets, oscillators, square wave ALLIED RADIO (plus tax) 296 generator and intermodulation analyzer, and EVERYTHING FOR STUDIO. I CUTTING ROOM other equipment. 1 ALLIED R CORP., Dept. 174,-8 & LABORATORY- Lights, Mikes, Booms, Dol- Complete recording studios assimilating ,11lvd., .1 833 W. Jac -.). Chicago 7, III. lies, Lenses, Television Transcription, Cameras. broadcast, motion picture and commercial D Send FREIe -ALLIED Catalog. Moviolas, etc. Send for Catalog Sturelab -new sound recording. For information write : New edition now under way. H. M. Tremaine, Director Name HOLLYWOOD SOUND INSTITUTE, Inc. S. O. S. CINEMA SUPPLY CORP. 1040 -E No. Kenmore, Hollywood 27, Calif. Address Dept. AE 602 West 52nd St., New York IS

48 AUDIO ENGINEERING NOVEMBER, 1948

www.americanradiohistory.comAmericanRadioHistory.Com OR COMPACT HIGH Ultra for remote lightweight, I fn`U control IPM 20,000 design these EN Unit U' T cdc/es. s, the met C audio There fn hods and units is po vid anr are he high sa// employ e d to fidelity hi of another á i 9h p true :':0;j: n all open hen Unit diddal y, balancing All units,°ne Unit i Duren e ana mplif¡e dee -t- 7 p case, effects coil structurexcept those fo px :;;'" high. Weight, 51 /2 ind e s ¡ch, combined DC ,h oes. p¡ with stns ¡uns, g Maximuma wide

Unit shorn is FOR IMMEDIATE DELIVERY actual se.DV6 tube shown for comparison From Your Distributor only.

F1usl ITY AMMO usi HHIGH ± 2NDB fppCT rom Price Secondo COMPACT e primary Impedancc $15.00 ULTRA 30- 20,000 Impedance 50,000 ohms APPlication 333, 6.00 200. 250, 50- 10,000 Pipe 50, 125, multiple alloy mike, Pickup. 500 ohms 50,000 ohms imPedonce for tow_ to grid 500 ohms shield ultple., line 50, 200, low ickup extremely mike, hum pickup impedance 5.00 Low or 2 grids or line to 1 30- 20,000 overall 80,000 ohms 333, 200, 250, in Iwo sections 50, 125, 14.00 mike, pickup, 500 ohms 30- 20.000 w punt ohms overall, A -12 line to push 80,000 -15.00 oowmultple ohms rotin overall to 15,000 2.3:1 turn 30-20,000 grids 8,000 rids 200, 250, to two 9 50, 125, plate ohms 14.00 A18 Single to 15,000 500 ohms 50- 12,000 8,000 250, 333, multiple line 125, 200, plate to 50, 15.00 Single 15,000 ohms ohms ' A-24 8,000 to 500 30-20,000 ne 250, 333, multiple I 125, 200. to 50, 10.00 Single plate D.C. 15,000 ohms ohms A-25 8,000 to 500 D.C.. inductance 8 MA unbalanced to ohms plates each side 4 MA 1500 low level henrys @ push pull D.C 75 A-26 M A ohms multiple line 2 6000 300 henrys @ Audio choke, henrys many Ultra A -30 450 of the no D.C. only a few with includes for more details. listing . . write The above available . Audio Units -4 Compact 13, N. Y, NEW YORK CABLES: pPl11B STREET 16, N, Y., VtRICK NEW YORK 150 4055 STREET, 13 EAST DIVISION: EXPORT

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RCA Velocity RCA Polydirectional Microphay e Microphone Type 44 -BX Type 77 -D Effective Output Level, -55 dbm Effective Output Level, -57 db -n Hum Pick -up Level, -112 dbm Horn Pick -up Level, -126 dim.

-now available from stock

These are the network favorites. The 77 -D is the polydirectional type ... quickly ad- you want. A Year after year they serve more broadcast and tele- justable to any pick -up pattern 3- position enables you select the best oper- vision audiences than any other microphone'. Yet, voice -music switch to for voice and music. Hum pick -up despite their overwhelming popularity, RCA's engi- ating characteristic level, -126 dbm! neering continues to make both even better than before. f The 44 -BX is the bi- directional type- designed for AM, FM, and TV studios where highest quality repro- RCA 44 -BX and 77 -D microphones are yours for duction is desired. It provides high -fidelity output over immediate delivery. Simply call your RCA Broadcast the entire audio range -and is free from cavity or Sales Engineer. Or write Dept. 7 K, RCA Engineer- diaphragm resonance and pressure doubling. ing Products, Camden, New Jersey.

BROADCAST ¡NC to 91n Every lh F s The One Equipment Source for RCA BROADCAST EQUIPMENT RADIO CORPORATION of AMERICA ENG /NEER /NG PRODUCTS DEPARTMENT CAMDEN, N.J.

In Canada : R C A VICTOR Company Limited, Montreal