Evolution of a Recording Curve

R. C. MOYER

A discussion of the reasons for the existence of "recording curves" and a presentation of the official specifications for the "New Orthophonic" curve currently used for RCA Vic- tor records and well on the way to universal adoption by all record manufacturers.

TPRIMARY FUNCTION of any home cian. While these ideas of the past have record is to provide en- Hints for playing been changed materially through the of RTMA, AES, and the record tertainment for the consumer. That RCA VICTOR records efforts this entertainment may be provided in companies themselves, there still exists its best possible form has been one of Use of the "New Orthophonic" in the mind of the public considerable the prime objectives of every record curve is recommended for all RCA confusion on the subject. manufacturer since the start of the busi- Victor records and records released ness at the turn of the century. The de- by RCA Victor since August, 1952. Definitions gree of success attained in this direction With a few exceptions in the early Much of the past confusion in the rec- is judged largely by the of the fin- 6000, 7000, and 9000 series, this ap- ord and phonograph industry has risen ished record as reproduced on a typical plies to all LM, WDM, and DM rec- from the lack of satisfactory and gen- or standard reproducer. The quality and ords or above 1701, and LCT erally accepted definitions of expressions balance of this sound is determined by and WCT above 1112. It also in- commonly used by the recording engi- the characteristics of the reproducer, the cludes all LHMV, WHMV, LBC, neer; and still complicating the problem recording system, and to a very large WBC, and Extended Play 45's. Rec- (both nationally and internationally) is extent, placement, orches- ords issued prior to that date should the difficulty in making absolute meas- tra seating, and studio . Thus be played with the same crossover urements. there are in effect three areas, any one and high- characteristic, but If a recording- reproducing system is or all of which may be made variable, without the roll -off at low frequen- divided into sections according to func- to change the sound heard by the lis- cies. A 4- to 5 -db increase in response tion, four general areas will result as tener. It is the first two of these three at 50 cps, usually obtainable with a shown in Fig. 1. Area 1 includes the areas, namely the over -all reproducing low -frequency tone control, is sug- studio, , orchestra seating, and recording characteristics, with gested for these records. mixers, variable equalizers, and ampli- which we are primarily interested. fiers feeding the recording bus and During the period from approximately monitor speaker. Area 2 contains tape 1900 to 1925 when acoustical recording intro- or disc recorders and their associated ords was also provided with the mag- was used, both of these areas were rela- duction of the electric phonograph with which produce certain tively fixed ; that is, balance, separation, tone and volume controls. netization vs. frequency characteristics etc., were being accomplished as well as improvements in gen- on tape or velocity vs. frequency char- Although the possible by placement of the artists in eral quality and frequency range ob- acteristics on disc with constant relation to the horn of the recorder. new equipment were applied to the recording bus. Area tained with the may be With the advent of electrical recording outstanding, this added flexibility led to 3, the manufacturing operation, in this discussion func- equipment, great flexibility which had a period of confusion for the disc manu- disregarded -its hitherto been impossible was provided manufacturer, tion, of course, is to provide finished facturer, the phonograph the quality and for the recording director in making and the consumer. The difficulty was records which duplicate records. Flexibility in reproducing rec- companies tried to make frequency range recorded on the origi- that the record 4 includes the discs sound right on what they consid- nal lacquer master. Area RCA Victor Division, Indianapolis, Ind. best of the day, finished record and the reproducer. ered the recording is often while the phonograph manufacturers The fidelity of a AREA of the 2 models which, in judged in terms of the naturalness were bringing out new the degree that their opinions, sounded best with all reproduced sound and CORDIRR Sis heard in the of records. Whether or not this it recreates the sound types or the hall. The objective was a healthy condition is questionable. studio - in modern phonograph recording is not AYRIFIERS However, eventually a certain degree of PRE -AMP MIXER R[- EMPRASS always in that direction, however. Spe- ETC standardization resulted largely because V EOWIOERS -{!I cial acoustic effects, changes in normal ETC all companies had a objective, customer the best balance among instruments and soloists, namely, to bring the in "pop" possible sound from the available repro- and in some cases-especially ducers.

AREA 3 The recent increasing interest of the 35 YAROFACTMIRR of wide - RANT and owners 30 range phonographs in the subject of disc E recording and reproducing character- ¡. 25 20 11\ istics, and the many conflicting opinions 1 prevalent on the subject make it 15 I Ip desirable that the past and present prac- I0 PROMOCAAPR tices of one of the oldest record manu- 0 5 AREA facturers be presented. Traditionally, the IV 02 3 5 7 3 5 7 2 exact recording characteristic in use has 1000 10,000 20000 Fig. 1. The actual disc recording character- 200 been a closely guarded secret of each FREOUENCY IN CYCLES PER SECOND istic is defined as the characteristic of Area 2 ; in early days of alone. It includes the fixed high- and low -fre- company just as the box 2. of an early quency pre -emphasis and crossover curve. All disc recording a particular sound Fig. Relative acoustic phonograph based on an ideal variable components are grouped in Area 1 and was often the personal property and Victor with 500 cps crossover. are used to obtain desired musical effects. secret of success of a recording techni- frequency record

AUDIO ENGINEERING JULY, 1953 19

www.americanradiohistory.comAmericanRadioHistory.Com recordings -unusual electronic sound . Low -frequency pre- empha- effects are often used as devices to create sis in recording, also possible because a particular over -all effect desired by of actual attenuation of very low fre- the conductor, artist, or musical direc- quencies in , permits tor. In these cases, all of the variable the use of a corresponding low- frequency attenua- items included in Area 1 are used as tion in the reproducer, thereby reducing tools to obtain the desired result on the R -F POWER monitor speaker. The criterion, then, for GENERATORi hum and rumble. judging recording fidelity is the degree When these two or three curves are that the reproduced sound matches the added together the resulting curve gives sound heard in the monitor speaker at what was generally considered to be the the recording session rather than the recording characteristic. This was true sound that was heard in the studio itself. for all practical purposes when record- From the foregoing discussion it can ing on wax discs, but is not necessarily be seen that two definite advantages re- true at high frequencies for lacquer sult from a grouping of recording com- discs due to recording losses which will ponents as shown in Fig. 1. First, all Fig. 5. R.f. induction heating applied to sap- be discussed later. It is largely due to variable effects of studio, mixers, equal- phire recording . A small iron the existence of these recording losses izers, etc. may be evaluated at the moni- slipped over the sapphire supplies the to that the term "recording characteristic" the cutting tip by conduction. toring point and adjusted at will to ob- is now defined as the actual velocity vs. tain the desired sound. Secondly, the istic on the record. In other words, it frequency characteristic recorded on a disc recorder and reproducer, although was always part of the effective record- disc with constant voltage input applied physically separated, may be considered ing characteristic. to the recording bus. Specifically, it is as a unit whose sole function is to bring the over -all characteristic of Area 2 in the same sound heard in the monitor Recording Curve Fig. 1 which involves the response -fre- speaker into the home of the listener In discussing a recording curve it is quency characteristic of the recording through the medium of the record. important to keep in mind the elements after the bus, any fixed record- Unfortunately, the situation has not which are combined to form any con- ing equalizers, the recorder itself, and always been that simple and straightfor- ventional disc recording characteristic. the cutting properties of the stylus and ward. In many early recording installa- First, we have the electromagnetic cut- disc material. tions fixed and variable components ter response which ideally produces a It is important to realize that for any were, of necessity, often intermixed constant- amplitude cut at low frequen- given recording, the type of music, vari- throughout the system. An example of cies, changing gradually to a constant- ations in microphone placement, studio -this in the early Victor electrical re- velocity cut (decreasing amplitude) at acoustics, and recording equalizers will cording systems may serve to illustrate high frequencies. The primary reason affect the actual velocity- frequency char- the point. Condenser microphones used for maintaining constant-amplitude re- acteristic recorded on the final disc. time at that are known to have a sharply cording at low frequencies is to limit the However, if the consumer desires to re- . rising response characteristic at high amplitudes (lateral groove produce the sound as originally heard frequencies. The point has never been excursions) to some practical value from the monitor speaker, the recording completely resolved, but presumably due which can be successfully recorded and characteristic and the reproducing char- to speaker deficiencies, the high -fre- reproduced. The crossover (the transi- acteristic must remain fixed and com- quency balance was satisfactory with tion point between constant -amplitude plementary. these microphones. Liter when ribbon - and constant -velocity recording) is de- One might conclude then that, pro- type velocity microphones replaced the fined as the intersection of the asymp- vided they are matched, these character- condenser types, high -frequency pre -em- totes to the two straight -line portions istics in themselves are unimportant, phasis was added to in the of this curve. Secondly, we have the serving only as a means to an end. This, to preserve the former balance. Finally, high- frequency pre-emphasis and in in fact, would be true, were it not for the when wide -range monitor speakers were some cases a moderate low- frequency mechanical limitations of disc recording installed, the pre- emphasis was removed pre- emphasis which are both added elec- and reproducing and the question of from the and placed after trically in the recording amplifier. record and system noise. These then be- the recording bus. Thus, at different High -frequency pre- emphasis in record- come the real contributing factors in times we have had the saline fixed pre - ing is added in order to obtain a reduc- selecting a specific recording character- emphasis in three different parts of the tion of record noise by using a corre- istic. Gradually as techniques and equip- channel: in the microphone itself ; after sponding high- frequency attenuation in ment have improved, the range of re- the microphone, but ahead of the moni- the reproducer. This pre- emphasis in corded and reproduced frequencies has tor speaker; and finally after the moni- recording is possible because the high been increased, making certain changes tor speaker. In each case it contributed frequencies in actual music and speech in the basic recording characteristic the same effect to the over -all character- are attenuated with respect to the lower desirable for best over -all results.

.20 15 I E .._._.-.. .O *5 .... ó 0 .._.-.- ue111p _ A 5 ..`rM 5 ; ---- 10 -Bi' A B 1928-1958 113 á -10 C ...Ì1 938-1917 n-ORIGINAL W.E. WAX RECORDER 13 E2]2.1/111, D 1947-1952 B -MODIFIED WE. WAX REC OROER -15 I= E NEW ORTNOPNONIC -20 ...C - 938 RCA MODIFICATION . ,._I TOLERANCE -20 _1 p -D t 3 5 7 3 5 7 I 3 5 7 1 2 1 3 6 7 3 8 7 1 3 7 20 100 1000 10,000 20000 5 2 20 100 1000 10.000 20.000 FREQUENCY IN CYCLES PER. SECOND FREQUENCY IN CYCLES PER SECOND

Fig. 3 (left). Relative frequency response of "Wax Recorder" in original form and as modified by Western Electric and later by RCA Victor. Fig. 4 (right). Recording characteristics used on Victor records from 1925 to present. Dashed part of Curves A and B represent cutter characteristic. Filters were generally used to remove bass when using these cutters

20 AUDIO ENGINEERING JULY, 1953

www.americanradiohistory.comAmericanRadioHistory.Com of these curves is subject to some ques- of Curve C, Fig. 4. With the introduc- tion, however, since it was common tion of improved reproducers and manu- practice to use a rather elaborate "bass facturing techniques after the war, the filter" to reduce the low- frequency re- 8,500-cps filter 'was removed from the sponse in order to obtain the best sound channel, resulting in Curve D. Finally, on average reproducers. after the installation of feedback re- The change from condenser micro- corders with further improved high - phones to ribbon -type velocity micro- frequency response, the "New Ortho- phones and new preamplifiers with high - phonic" characteristic has been adopted. frequency pre-emphasis built in was ac- Examination of the curves of Fig. 4 will IA) (B) complished during 1932. The resulting show that -except for the questionable over-all curve remained essentially un- low-frequency portion used in the early Fig. 6. The recording loss at 10,000 cps across changed. days of electrical recording- character- a 12 -in. LP record using a burnishing -type In 1938 the improved RCA version istic changes introduced throughout the stylus is illustrated in (A). Use of a heated of the wax recorder and completely new years has been essentially extensions of wax -type stylus eliminates this loss as shown and improved recording channels were range, the general curve throughout the by the light pattern in (B). The outer and middle range of frequencies being held inner bands are 1,000 cps tones recorded for placed in operation. At that time the reference purposes. adjustable bass filter was discarded, constant. Furthermore, it may be as- pre- emphasis was removed from the sumed that the early electrical record- preamplifier and added after the record- ings (Curves A and B) also had ap- Early Victor Characteristics ing bus, and an 8,500 -cps low -pass filter proximately a 500 -cps crossover point, was added primarily to reduce noise and since the change to the new equipment As previously stated, with early me- in 1938 was accomplished with no loss chanical recording techniques, playing of bass on the finished records. Recent the record on a phonograph offered the studio experience in rerecording many only satisfactory method of judging of these older records for the Collectors record quality, since monitoring at the WAX TYPE STYLUS and Treasury series has shown recording session was impossible. Con- / LI EQUIVALENT RANGE that a 500-cps crossover frequency rep- tinuous experimentation with sound FOR "4S "RECORDS resents about the best average charac- boxes, horns, recording and reproduc- teristic for satisfactory reproduction of ing styli, etc. resulted in gradual im- BURNISHING EDGE STYLUS these records. provements in clarity of tone and in- á W PLAYBACK LOSSES A: creased volume. In each case, however, 10.000 CYCLE TONE SS /SRPMI Hot-Stylus Recording the result of a change in equipment or METAL MOLO / - VINYL PRESSING in studio setup was evaluated in terms With the introduction of lacquer re- I I I I I of playback results; the objective, of B e 7 S 9 IO II cordings for instantaneous playbacks course, being to provide the consumer DIAMETER - INCHES and rerecording purposes, it became ap- with the best possible sound. parent that high -frequency recording Specific information concerning the Fig. 7. At the lower recorded velocities, play- losses existed which were not present of the various acoustical back from the metal mold and light pattern on wax. These losses characteristics measurements are in good agreement. At 5.5- when recording recorders used for the early Victor re- cm /sec. level obtained with the heated stylus, are due largely to the elastic properties cordings is limited. Recent measure- playback output falls slightly below the indi- of lacquer recording materials and to ments of the frequency response of an cated velocity due to curvature limitations to- the burnishing edges of lacquer record- early Victor acoustic phonograph shown ward the inside of the disc. Playback losses ing styli which are required to obtain in Fig. 2 give some idea of the over -all due to deformation of the record material exist quiet cuts. Once again modifications were results obtained in those days. Actually in either case and are overcome by diameter made to the wax recorder which largely the recorded range was somewhat when recording music or speech. overcame these recording losses on greater than indicated by these repro- lacquer at 78 r.p.m. However, at the ducer curves. On December 29, 1924 the distortion effects resulting from play- lower groove velocities encountered in last Victor acoustic recordings were back turntable flutter, pickup tracking, 33 -1/3 r.p.m. recordings it was found made, and on May 5, 1925 the first re- and manufacturing methods. The re- that the desired high -frequnecy equali- cording session with electrical record- cording characteristic then became that zation could be maintained on the disc ing equipment was held. The electromagnetic recorder used was developed by Bell Lab- NETWORK ADMITTANCE IMPEDANCE oratories. It is usually referred to as the "wax recorder" or "rubber -line recorder." It had a cross -over frequency of about 200 cps and a high- frequency (A) cut -off of about 4,500 cps, as shown in Fig. 8. Impedance or / }0l Fig. 3, Curve A. Subsequent modifica- admittance e i t h e r tions by Bell Telephone Laboratories rise or fall at the rate of 6 db per oc- resulted in an extension of the high - frequency range to about 5,500 cps as tave. The asymptotes to the two straight - shown in Curve B. During the early line portions of these and middle '30's further development by curves intersect at a RCA Victor engineers resulted in an frequency f1tI, often extension of the high -frequency range referred to as the to 10,000 cps or better and a smoothing turnover or crossover out of the low -frequency range as shown frequency. The exact in Curve C. The fact that intermodula- frequency is deter- tion distortion at full 78- r.p.m. level is mined by the time constant of the net- of 2 3 in the order to per cent gives work. ample proof of the excellent design and performance of these recorders. The over -all recording characteristic (0) -fi "...,.._ using the wax recorder and condenser microphone are shown in Fig. 4, Curves A and B. The actual effective low end

AUDIO ENGINEERING JULY, 1953 21

www.americanradiohistory.comAmericanRadioHistory.Com only with additional electrical high - octave slope at one end and a limiting frequency pre -emphasis in the record- TABLE I of fixed value at the other end, as shown ing channel. The amount of additional Relative Velocity vs. Frequency in Fig. 8. The transition frequency or pre- emphasis varied with styli and, of "New Orthophonic" Curve point of intersection of the two asymp- course, increased considerably with de- feps Vdb feos Vd1, totes to the curve is determined by the creasing groove velocity. Since the re- time constant of the circuit as follows: cording amplifiers 5000 + 17.2 3000 + 4.8 were capable of more 4000 + 16.6 2000 + 2.6 than adequate power, no harmful dis- 3000 + 16.0 f(r) 1 T = RC or LR 1000 + 0.0 - 2nT' tortion effects could be detected from 2000 + 15.3 the use of what appeared to be more 1000 + 14.5 700 - 1.2 Where fr = Transition frequency in cps than normal pre -emphasis at high fre- 0000 + 13.7 400 - 3.8 T = Time constant in micro- quencies. Due to the recording losses, 9000 + 12.9 300 - 5.5 seconds this pre -emphasis did not appear on the 8000 + 11.9 200 - 8.2 R = Resistance in disc. 7000 + 10.8 100 - 13.1 C =Capacitance in microfarads 6000 + 9.6 70 - 15.3 L = Inductance in microhenries During '1950 recording on lacquer 5000 + 8.2 50 - 17.0 with an electronically heated wax -type 4000 + 6.6 30 - 18.6 At the frequency stylus was introduced. fr the magnitude of R.f. induction the reactance of the or in- heating is used as shown in Fig. 5 where ductor is equal to the magnitude of the the work coil, sapphire stylus, and thin New Orthophonic Characteristic resistor. Also at this frequency surrounding band the ab- are illustrated. One solute value of the impedance or admit- of the outstanding advantages of the It has been customary to refer to re- cording curves in terms of crossover tance is either 0.707 or 1.414 times its heated wax -type stylus over the burn- constant frequency and amount of pre -emphasis -impedance value, i.e. 3 db above ishing -type stylus is that high -frequency or below the "flat" recording at 10,000 cps relative to 1,000 portion of the curve. losses when cutting lacquer cps. Un- These curves have at low groove velocities fortunately, these two factors a general shape are completely alone do that fits the requirements of disc record- eliminated as can be seen from the light not adequately define a recording char- ing pattern photographs and reproducing characteristics. in Fig. 6. The up- acteristic. When more information about They are also per part of curves that are easily this figure shows a heated a curve is required, a graph showing obtained in amplifier designs either stylus recording of a 10,000 -cps tone relative velocity vs. frequency is usually recorded separately or in combination. It follows in bands across the entire play- supplied. The obvious difficulty with a then that impedance ing surface of or admittance a typical twelve inch curve alone is that the true crossover curves provide an ideal 33 -1/3 r.p.m. LP record. The frequency method of ex- photo- and pre- emphasis are usually pressing or defining a recording or re- graph in the lower part shows a similar obscured, making the design of suitable recording producing characteristic. made with a cold burnishing - equalizers possible only by the cut and To illustrate type stylus. It can readily their use, three specific be seen that try method. To overcome these difficul- examples are cited here. These examples the width of the reflected light bands ties, recording curves now are often remains constant when combined form the "New Ortho- in the first case, while defined as conforming to the impedance phonic" recording characteristic. in the second photograph the width or admittance of one or more electrical The - decreases toward the networks. expression "75- microsecond pre center of the emphasis" indicates that high frequen- record, indicating recording losses at The impedance or admittance curves the lower groove cies are pre -emphasized according to a velocities. of simple two -element networks con- curve which Playback losses, of conforms to the admittance course, still exist sisting of a resistor and capacitor or a of a parallel resistor and capacitor and are compensated for by the resistor net- addition and inductor are all similar in work, (B) of Fig. 8, with a time of sufficient diameter equalization in shape con- re- when plotted with frequency on stant of 75 microseconds. The curve is cording to maintain constant high -fre- a logarithmic scale and impedance or quency response across +3 db at 2,120 cps and + 13.7 db at 10,- the playing sur- admittance in on a linear scale. 000 cps relative to low frequencies. face of the record. These playback losses These curves approach a 6- db -per- are a function of pickup construction, An ideal cutter characteristic as de- stylus size, and record material. The fined earlier may be represented by a output obtained from TRIODE EQUALIZER: curve which conforms to the admittance a modern of high -quality pickup when playing the a series RC network, (A) of Fig. 8, RL where the time constant defines the cross- metal mold (sometimes called "mother R ") over frequency. a and vinyl pressing are shown in Fig. 7. For 500 -cps crossover The equivalent groove velocities and point T = 318 microseconds. In a simi- C2 lar manner, losses for 45 r.p.m. records are also low- frequency pre -emphasis indicated in this figure. It can readily may be expressed as a curve conform- be seen that the playback loss ing to the admittance of a parallel re- obtained inductor from the rigid metal record is negli- istor- combination, as in (D) R, 0.17 MEG OI O.I,1 of Fig. 8. For 3 -db rise at 50 cps, for gible, whereas the loss due to compound Rr 28,000 OHMS Cr 0.0015rf RB example, the time constant of the net - deformation amounts to some 5 or 6 db 0 68 MEG CS 001,1 work is 3,180 microseconds. from the outside to the inside of the PENTODE EQUALIZER: record. B Any of the above curves may just as L well be expressed as conforming to the Among the other advantages of heated C2 stylus recording are: impedance of suitable two -element net- R2 works, although use of the admittance 1. Elimination of horns at the top edges C, curves is more generally accepted. of the grooves which are character- By algebraically adding the ordinates istic of recordings made with burn- of these three curves, an over -all curve ishing -edge styli. This improvement CI will be obtained which accurately de- makes possible the use of slightly R, I5.000 OHMS CI 0.02,I RZ I fines higher recording levels without dam- 0 MEG C2 0.1,1 the "New Orthophonic" recording age to adjacent grooves and also re- R, 0 68 MEG CS 500r,1. characteristic. The relative velocity val- sults in finished pressings somewhat ues for the over -all curve arbitrarily less susceptible to scuffing. Fig. 9. When using magnetic pickups, simple referenced to "0" db at 1,000 cycles are 2. Reduced cutting noise, especially at RC equalizer circuits may be incorporated in shown in Table I. the low groove velocities encountered a voltage amplifier stage to obtain the "New In making comparisons between toward the inside of fine the -groove re- Orthophonic" characteristic. Additional ad- former RCA Victor curve and the cordings. This reduction in cutting justment of high- and low -frequency tone con- noise, of course, results in reduced trols should be made to correct for pickup and "New Orthophonic" characteristic, sev- surface noise on the final product. tone arm characteristics. (Continued on page 53)

22 AUDIO ENGINEERING JULY, 1953

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