The Measurement of

An AES Paper Presented I March 31. 1951 Audio Volume H. A. CHINN"

A comprehensive discussion of the problems involved and the instruments employed to indicate program level and sine-wave tones in broadcast and recording circuits.

N THEORY combinations in order to provide for by simultaneous measurements at a num- there are three related values of a new points of origin of the programs, ber of points on particular peaks or im- I sine wave by which its magnitude for the addition of new broadcasting pulses of the program wave which is be- may be expressed. These yethe average stations, and for' the removal of others ing transmitted. They are also used for value, the r.m.s. (or effqctive) value, from the network. In whatever combi- sine-wave transmission measurements and the peak (or crest) value. Certain nation the parts of the system may be on audio systems and circuits. fundamental electrical measuring de- assembled, it is necessary that the am- In spite of its importance and its ex- vices provide means for determining plitude of the transmitted program waves tensive and universal use, the volume these values. Complex, non-sinusoidal -at all times and at all parts of the sys- indicator is probably the least under- periodic waves also have the same three tem-remain within the limits which the stood of all audio measuring instruments. read~lymeasured values. As a rule, the system can handle without impairment For this reason the standard volume in- problem under consideration determines from overloading or from noise. To ac- dicator, its reference level, its method of whether the average, the r.m.s., or the complish this, some convenient method calibration and the teminology used for peak value of the wave is of primary of measuring the of program volume measurements is covered in detail importance. waves is needed. in the 'following paragraphs. These and similar considerations led Concept of Audio Volume to the conception 'of a fourth electrical Peak vs. r.m.5. Volume Indicators In the field of communication engin- quantity, known as "v?lume", whereby In the study that led to a standard eering, waves which are both very com- the magnitude of waves encountered in volume indicator1 the decision had to be plex and. non-periodic are encountered. electrical communications, s u c h as made as to whether the standard volume When an attempt is made to measure speech or program waves, may be readily indicator should be of the r.m.s. or of such waves in terms of average, r.m.s., expressed. This quantity is a purely em- the peak-reading type. These two types or peak values, it is found that the results pirical value, created to meet a practical of instrument represent two schools of can no longer be expressed in simple need. It is not defimble by means of a thought. The peak-reading instrumept numerical terms since these guantities precise mathematicaL formula in terms is favored for general use by many are not constant but variable with time. of any of the familiar electrical anits of European engineers and is specified by Moreover, the values appear to be af- , voltages or current. Volume is the F.C.C. for use as modulation moni- fected by the characterictics of the simply the indication of an instrument tors in this country. The r.m.s. type has measuring instrument and the technique known as a volume indicator, which has however, been employed in this country of n~easurement. The communications specified dynamic and other character- on broadcast program networks and for engineer, however, is vitally concerned istics and which is calibrated and read general telephone use. In view of the im- with the magnitude of these non-sinuso- in a prescribed manner. Because of the portance of the decision and the dif- idal, non-periodic waves since he must rapidly changing character of the pro- ference of opinion that has existed, the design and operate systems in which gram wave, the dynamic characteristics basis on which the choice was made is they are amplified by vacuum tubes, of the instrument are fully as important discussed below in some detail. transmitted over wire circuits, modulated as the value of sine-wave power used for In accord with common practice, the on carriers, and otherwise handled as instrument calibration. The readings of terms "r.m.s." and "peak-reading" are required by the various communication volume have been customarily expressed used rather loosely herein. The essential services. He needs a practical method of in terms of with respect to some features of an r.m.s. instrument are a measuring and expressing these mag- volume level chosen as the "reference" rectifier or detector and a d.c. milliam- nitudes in simple numerical fashion. level. '3 meter. The movement of the latter is not This need pay be better appreciated especially fast, generally requiring tenths by considering a typical example-the Volume Indicator Applications of a second to reach substantially full- communication systems employed for Volume indicators are used exten- scale deflection. Obviously, if a wave of broadcasting. These are often very com- sively to indicate the correct transmis- sufficiently low frequency is applied, say plicated networks spread over large sion level for speech and program waves one whose frequency is one or two cps, geographical areas. A typical network in audio systems employing , the instrument can follow it and the true may include 20,000 miles of wire line program-actuated automatic devices, peaks of the wave will be indicated. But and hundreds of amplifiers situated both program wire circuits, modulators, sound when much higher frequency waves are along the line and in the 100 to 200 con- recorders and reproducers, or wherever applied, such as the complex speech or nected broadcasting stations. Every 15 the transmission of speech and program program waves, the instrument is too minutes (luring the day the component waves are involved. In this capacity slow to indicate the instantaneous peaks, parts of such a system may be shifted volume indicators serve as a guide to the rather, it averages or integrates whole and connected together in different avoidance of overloading. Equally im- syllables or words. As shown by tests and practical experience, it is of sec- * Columbia Broadcasting System, 485 portant, they serve as a means of indicat- Nadison Ave., New York 22, N. Y. ing approximately the comparative loud- ondary importance whether the detector ness with which various elements of a actually has an r.m.s. (or square law) complete program will be heard when characteristic, or has a linear or some finally converted to sound. intermediate characteristic. paper rests upon the author, and state- [Continued on Page 281 ments contained herein are not binding Volume indicators are also used for checking transmission gains and losses 1 Chinn, Gannett and Morris; Proc. in program networks and audio systems Z.R.E., Vol. 28, No. 1, p. 1, Jan. 1940.

AUDIO ENGINEERING SEPTEMBER, 1951 ment is far superior to the peak-reading type. - This is because phase and slight nonlinearity in the program circuits (the results of which are too small to be detectable by ear) change the wave shape of the program peaks suffici- ently to cause serious errors in the indi- Fig. 1. An installation cations of the peak-reading instrument of twelve standard .but have no noticeable effect on the r.m.s. volume indicators, on instruments. an equal number of The effect of a long program circuit outgoing program cir- on the indication of the peak instrument cuits in the CBS is partly due to the cumulative effects of shortwave master con- the slight nonlinearity in the many trol room. vacuum-tube amplifiers and loading coils in the circuit, and partly to phase changes which alter the wave front and amplitude of the peaks. It might be thought that phase changes which de- stroy some peakb would tend to create others. However, a Fourier analysis of a sharp peak will show that an exact phase relationship must exist between all of the frequency components. The A peak-reading instrument capable of law detector, or one of some intermediate probability that phase shift in a line will truly indicating the sharpest peaks which characteristic. The important difference chance to cause all of the many fre- occur in a high-quality program wave between the two types lies in the speed quency components of a complex wave would have to respond to impulses last- of response as measured by the length to align themselves in the relationship ing only a very small fraction of a milli- of impulses to which they will fully re- necessary to create a peak where none second. Cathode-ray or gas- spond, or what is the same thing, in the existed before, is very slight, indeed tube trigger circuits are capable of doing infinitesimal compared to the probability this and consequently, might be used as +of the- occukrence of a peak in the a peak-reading volume indicator. How- original wave. ever, the so-called peak-reading volume Data on peak checking showed such a indicators used in practice, designed to marked kdvantage for the r.m.s. type as give a visual indication on an instru- compared with the peak instrument, that ment, are far from having the above it was decided to employ the r.m.s. type speed although they are much faster of instrument, Other considerations in- than the r.m.s. instruments. They gen- cluded the possibility of employing erally respond to impulses whose dura- copper-oxide rectifiers and thereby tion is measurable in hundredths or eliminating vacuum tubes with their thousandths of a second. As a result they attendant need of power supply; an ad- truly indicate the peaks of waves whose vantage not shared by peak-reading in- frequencies do not exceed say, 50 to 100 struments. Thus, the r.m.s. instrument cps. They are similar to the r.m.s. in- has advantages of comparative low cost, struments in that they are not fast ruggedness, and freedom from the need enough to indiqte thk instantaneous Courtesy General Electric Co. of power supply, and can, moreover, be peaks of speech or program waves but Fig. 2. The "A type standard volume indicator readily made in portable forms when tend to average or integrate a number scale emphasizes the VU markings and has an desired. of peaks of the wave. inconspicuous voltage scale. This type of scale A feature of the usual peak reading is commonly used for transmission measuring Dynamic and Electrical Characteristics instrument which is superficially im- sets. It will be appreciated from the above pressive, but from which the analytical discussion that for a volume indicator to standpoint is of secondary importance, time over which the complex wave is integrated. be truly standard, both its dynamic and is that it is usually given a dynamic electrical characteristics must be con- characteristic of rapid response coupled Peak Checkirig s trolled and specified so that different in- with very slow decay. This is usually struments will indicate alike on the acomplished by a circuit wherein a ca- An important use of eolume indicators [Continued on Page 361 pacitor is charged through a full-wave is that of checking the transmission vacuum-tube rectifier, the rates of charge losses or gains along an audio system or and discharge being determined by re- a program network by measurements sistances. A d.~. and instrument made on the program material being indicate the charge on the capacitor. transmitted. The circuits which make up The advantage of making the discharge broadcasting networks, for instance, are rate of the capacitor very slow is that in continuous use for many hours each the indicating instrument itself need not day and during that period are switched then be particularly fast and, moreover, together in as many combinations as the ease of reading it is greatly in- called for by the operating schedules. creased. It is seldom possible to free a circuit for From the above analysis it is seen sine-wave transmission measurements. that the r.m.s. and the peak-reading in- Therefore to check the transmission con- struments are essentially similar and ditions during service hours, it is the differ principally in degree. Both indi- custom to take simultaneous readings at cate peaks whose durations exceed some two or more points in the program net- value peculiar to the instrument and both works on particular impulses of what- Courtesy General Electric Co. average or integrate over a number of ever program wave is being transmitted Fig. 3. The "B" type standard volume indicator peaks the shorter, more rapid peaks en- and to coordinate these readings by scale emphasizes the percentage scale. This countered in spgech or program waves. means of tqephone communication. On scale is used, extensively for program trans- Either may have an r.m.s. or square- such readings, the r.m.s. type of instru- mission applications.

AUDIO ENGINEERING SEPTEMBER, 1951 when the sine wave mentioned in the Both vu markingsZ and markings preceding paragraph is applied. proportional to voltage are incorporated AUDIO The question of whether the rectifier, in the new instrument scale. The need which is a part of the standard volume for the former is obvious, but the MEASUREMENT indicator, shduld be half-wave or full- philosophy which lead to the inclusion of [from page 281 wave needs little discussion. As is well the latter may require some explanation. known, many program waves (par- It is evident, assuming a linear sys- rapidly varying speech and program ticularly speech) show a marked lack of tem, that the voltage scale is directly waves. In deciding upon the dynamic symmetry. Obviously, if an instrument proportional to percentage modulation characteristi,cs, an important factor in- is to give the same indication, no matter of radio transmitter or recording system cluded in the consideration was the ease which way it is poled, a balanced full- upon which the program is finally im- 1 of reading the instrument and the lack of wave rectifier is required. pressed. If the system is adjusted for I eye strain in observing it for long Throughout this discussion, the term complete modulation for a deflection to periods. "r.m.s." has been used loosely to de- the 100-per cent mark, then subsequent For ease of reading and minimum of scribe the general type of instrument indications show the degree of modula- eye fatigue, the movement should not be under consideration. The equation *at tion under actual operating conditions. too fast. As a result of, observations relates the instrument coil current to the In the interests of best operation, it may under service conditions, and other tests, potential applied to the volume indicator be desirable, of course, to adjust the the requirement was adopted that the is : system for somewhat less than complete sudden application of a 1000-cps sine i=keP modulaton when the 100 per cent indica- wave of such amplitude as to glve a where i = instantaneous coil current. tion is reached. steady deflection at the scale point where e = instantaneous potential. In any event, the indications on the k voltage scale always show the percent- the instrument is to be read, shall cause = a constant. age utilization of the channel. This is a the pointer to read 99 per cent of the The exponent p in the above equation decided advantage because everyone final deflection in 0.3 second. is 1.2 for the standard volume indicator. concerned (both technical and non- It was also noted that on speech and Therefore its characteristics are inter- technical personnel) has a clear con- program waves, instruments which were mediate between a linear (p= 1) and a ception of a percentage indication. critically damped or slightly overdamped square-law or "root-mean-square" (p = Furthermore, since the scale does not had a more "jittery" action than instru- 2) characteristic. extend beyond the 100 per cent mark ments slightly underdamped. Conse- In many applications the rectifier law (except in the form of a red warning quently the strain of reading the former is just as important as the other electrical band) and since it is impossible to ob- type is greater than for the latter. A and- dynamic characteristics of the tain more than 100 per cent utilization theoretical study of the problem verified standard volume indicator. Unfortu- of the facilities, there is less incentive the validity of this subjective observa- nately, there is a tendency to overlook on the part of non-technical people con- tion. The requirement was therefore this fact in many instances and to simply nected with program origination, to re- adopted that the standard volume indi- specify that the indicating instrument of quest "an extra-loud effect" on special cator movegent shall be slightly less some particular piece 03 measuring occasions. than critically damped, so that the equipment "shall have dynamic charac- Actually, two scales, each containing pointer will overswing not less than 1 teristics identical to that of the standard both vu and voltage markings, have been per cent nor more than 1.5 per cent volume indicator." standardized. One of these known as the type A scale, Fig. 2, emphasizes the vu Instrument Scale markings and has an inconspicuous volt- Among the more important features age scale. The second, known as the to be considered in the development of a type B, Fig. 3, reverses the emphasis volume indicator is the design of its on the two scales. This arrangement scale. In broadcasting studios, volume permits the installation of the instru- indicators are under observation almost ment which emphasizes the scale that is continuously by the control operators. most important to the user, while retain- Consequently, the ease and accuracy of ing the alternate scale for correlation reading, and the degree of eye strain are purpses. of major importance. Ever mindful of the possibility of eye It is evident that the instrument scale fatigue even the color of the scale card should be easy to read in order that the has been standardized. It is a light peak reached by the needle under the oranz-e-vellow. which seems to be a impetus of a given impulse may be ac- satis~ac~orycompromise between high curately determined. The instrument contrast and reduced eye-strain. This scale, therefore, should be as large as choice was based upon the preference practical since, in the case of the broad- of a large group of skiIled observers and cast and recording applications, atten- upon the reports of certain societies for tion is often divided between the action the improvement of vision. The use sf in the studio and the volume indicator. matte-finished instrument cases having Volume level indicators are used (a) fairly high reflection coefficients, such as as an aid to tailoring the wide dynamic light gray, is also desirable for ease of range of an original performance to vision. that of the associated transmission Finally in studio applications the scale medium and (b) for locating the upper must be properly illuminated so that the part of the dynamic range just within relative light intensity on the face of the the overload point of an equipment instrument is comparable to that on the during its normal operation. For the first sound stage. Unless this condition pre- of these uses, a scale having a wide vails, the eye may have difficulty in ac- range is preferable. For the commodating itself with sufficient ra- latter purpose, a scale length of 10 db is pidity to the changes in illumination as usually adequate. Since a given instru- the operator glances back and forth from ment may be used for both applications the studio to the volume-indicator in- neither too large nor too small a range strument. is desirable in volume level indicators (To be conclzcded) for the above purposes. A usable scale 2 Terminology is explained in a folIowing length covering 20 db appears to be a section. satisfactory compromise. AUDIO EN GlNEERlNG SEPTEMBER, 1951 / AUDIO engineering society , Containing the Activities and Papdn of tho Society, and pub- lished monthly as a part of AUDIO ENGINEERING Magazine OFFICERS w John D. Colvin ...... President Bob Hugh Smith Western Vice.-Pres. Audio Enginbring Society, C. C. McProud Exekutive Vice-Pres. Lawrence Shipley Central Vice.-Pres. Box F, Osern~,N. Y. Norman C. Pickering . . . . Secretary Ralph A. Schlegel ...... Treasurer The Measurement of Audio Volume lj. A. CHINN" Part II-A comprehensive discussion of the problems involved and the instruments em- ployed to indicate- program level and sine-wave tones in broadcast and recording circuits. THOROUGH COMPREHENSION of the is calibrated so as to read zero vu on a brating voltage, inasquch as a volume connotations of the term "reference sine wave power of, say, one milliwatt in indicator is generally a high-impedance, A volume" is fundamental to any a stated impedance, a speech or program voltage-responsive device. A reference studio engineering endeavor. , Unfortu- wave in the same impedance whose in- Eve1 could conceivably be established nately, experience has shown that this tensity is such as to give also a reading based on voltage and the unit of subject is often completely misunder- of zero vu will have instantaneous peaks measurement might be termed "volume- stood. It is hoped that the following of power which are several times one volts." However, volume 'measurements will dispel the vague understanding that milliwatt and an average power which are a part of the general field of trans- sometimes surrounds this simple subject. is only a small fraction of a milliwatt. mission measurements, and the same It is important to appreciate that It is therefore erroneous to say that reasons apply here for basing them on reference volume is a practical and use- reference volume is one milliwatt. power considerations as in the case of ful concept, but one which is quite arbi- Moreover, it should be emphasized ordinary transmission measurements us- trary and not definable in fundamental that although it is convenient to measure ing sine waves. If the fundamental con- terms. As already mentioned, it cannot the performance of amplifiers and sys- cept were voltage, apparent gains or be expressed in any single way in terms tems by means of single frequencies losses would appear wherever impedance of the ordinary electrical units of power, there is no ex&f universaP relatiofiship potential, or current. Reference volume between the single-frequency load-carry- is describable only in terms of the elec- ing capacity indicated by such measure- trical and dynamic characteristics of an ments, and the load-carrying capacity instrument, its sensitivity as measured by for speech and program waves expressed its single-frequency calibration, and the in terms of volume level. This relation- technique of reading it. In other words, ship depends upon a number of factors reference volume may be defined as that such as the rapidity of cutoff at the over- level of program which causes a standard load point, the frequency bandwidth be- volume indicator, when calibrated and ing transmitted, the quality of service used in the accepted way,, to read zero to be rendered, and similar factors. vu. The question may well be raised why The sensitivity of the standard volume reference volume has been related to a indicator is such that reference volume calibrating power rather than to a cali- corresponds to the indication of the in- strument when it is bridged across a I 600- resistor3 in which is flowing Courtesy General Electric Co. one milliwatt of sine-wave power. Fig. 3. The "B" type standard yolume indicator It is especially cautioned that refer- scale emphasizes the percentage scale. This ence volume should not be confused with scale is used extensively for program trans- the single-frequency power used to cali- mission applications. / brate the zero volume setting of the volume indicator. If a volume indicator transforming devices (such as trans- formers) occur in a circuit. This diffi- * Columbia Broadcasting System, 485 , culty is avoided by adopting the power Madison Ave., New York 22, N. Y. concept, making suitable corrections in a A standard impedance of 600 was the readings when the impedance is chosen originally since, keeping in mind other than 600 ohms. the telephone plant, there was more audio equipment designed to this impedance than Volume Measurement Terminology to any other. Courtesy General Electric Co. (a) VU. The terminology that is used to express volume measurements was Fig. 2. The "A" type standard volume indicator created to avoid confusion as to the type scale. emphasizes the VU markings and has an inconspicu~~isvoltage scale. This type of scale of volume indicator used and 'the refer- is commonly used for transmission measuring ence level. The term "vu" (pronounced sets. "vee-you") is used; the number of vu , '24 AUDIO ENGINEERING OCTOBER, 1951 time, the length of time depending upon the program material. For speech a 5 to 10 second period of observation may be sufficient whereas for symphonic music 1 to 2 minutes may be necessary. During this time the adjustable attenuator, which is a part of the volume indicator, is adjusted so that the extreme deflec- tons of the instrument needle just reaches Fig. 4. The basic circuit for the standard volume indicator has an input impedance of 7500 ohms. the reference point; i.e., a scale reading For maximum sensctivity the loss in the adjustable attenuator is reduced to zero, but in order to of zero on the vu scale or 100 on the maintain proper dynamic characteristics the indicating instrument itself must always fqce approxi- percent voltage scale (see Figs. 2 and mately 3900 ohms. 3). The volume level is then given by being numerically the same as the num- the "vu" level used for program trans- the designations @umberedon the attenu- ber of db above or below the reference misson peaks and the "dbm" level used ator. If, because of the coarseness of the volume level. The use of this term is for system measurements. In practice it adjustments ,provided or for other restricted to the ASA standard volume has been found that with typical pro- reasons the deflections cannot be brought indicator described herein. A volume gram material of a given crest amplitude, exactly to the 0 vu or 100 per cent mark, level reading can be correctly expressed the standard volume indicator reaches the reading obtained from the setting of in terms of vu only when it has been an indication 8 to 14 db below that the attenuator may, if desired, be cor- made with an instrument having the reached with a steady tone of the same rected by adding the departure from 0 electrical and dynamic characteristics crest amplitude. To nominally take into shown on the vu scale of the instrument. described. account this 8 to 14 db difference in In the interests of accuracy the steps on (b) DBM. For steady-state measure- response, the established practice is that the adjustable attenuator should not ex- ments a reading in "vu" would denote a specific single-frequency , * 390Qn Adjusjobb for dynamic program indications "vu" denotes only a volume level. This dual meaning of "vu" is avoided by the use Copper of the term "dbm" for all steady-state measurements. As defined, a reading ex- Instrument pressed in "dbm" at once indicates the power level of a steady, single-frequency 1- signal where the number of "dlirn" is equal to the number of decibels above or Fig. 6. Volume indicator circuit having low input impedance which terminates source and also below a reference Dower of 1 milliwatt. provides increased volume indicator sensitivity. (c) DBM vs. VU. It is to be noted performance requirements must be met ceed 2 db. so that the departure from the that a "vu" reading can be made only at a single-frequency test-tone level that reference point never need exceed this on a standard volume indicator whereas is at least 10 db higher than the normal amount. Particular attention is called sine-wave power measured with the program peaking level (for example, in to the fact that, unlike almost any other standard volume indicator or with any a system that is to transmit program electrical indicating instrument, the voE other suitable a.c. instrument can be material at +8 vu, all single-frequency ume indicator reading is determined pri- expressed in "dbm"." measurements would be made at +I8 dbm marily from the setting of the associ- DBM is a unit of finite power whereas test-tone level). This procedure reason- ated range switch and, in effect, only a "vu~'is a measure of volume level and, ably insures that system performance is secondary correction is obtained by as already discussed, has no connotation within standards under normal operating observing the deviation of the needle of filcite power level. Thus no direct re- conditions. from exact coincidence with the refer- lationship between "dbm" and "vu" can ence point on the scale. t be established. Reading the Volume Indicator From a practical standpoint, however, Since program material is of a rapidly Features of the Standard Volume Indicator some relationship is desirable between varying nature, a reading of a volume The volume indicator that has been indicator cannot be obtained instantly. described has the statute of an American H. A. Chinn, AUDIOENGINEEBINC, Vol. Rather, the gyrations of the needle must Standard.= In the many years since it 32, No. 3, p. 28, March 1948 be watched for an appreciable period of was first placed into service there have been no ihanges, whatsoever, in either the fundamental requirements or the spe- cific features of the instrument that was developed to meet the basic needs. Be- cause of the importance and the wide- spread use of the instrument some of the detailed characteristics that are of con- cern to the design engineer are pre- sented below. (a) Response vs. Frequency Character- istic. The sensitivity of the volume indi- cator instrument shall not depart from that at 1000 cps by more than 0.2 db between 35 and 10,000 cps nor more than 0.5 db between 25 and 16,000 cps. (b) Input Impedatzce. The impedance of the volume indicator arranged for bridging across a line is about 7500 10 100 I000 [Continued ON $a&e 481 I CIRCUIT ITERATIVE IMPEDANCE - OHMS 5 Volume measurements of electrical Fig. 5. Volume indicator correction factors (to be added to volume indicator reading) for use speech and program waves, American when instrument is bridged across circuits having iterative impedances other than 600 ohms. Standards Association C16.5-1942 AUDIO ENGINEERING OCTOBER, 1951 stitute a complete volume indicator but AUDIO VOLUME must have certain simple circuits asso- ciated with it. The basic form which this MEASUREMENT circuit takes is illustrated in Fig. 4. This is a high-impedance (7500-ohm) ar- [from page 261 rangement intended for bridging across low impedance lines. As noted above, ohms when measured with a sinusoidal about 3600 ohms of series resistance has voltage sufficient to deflect the pointer been removed from the instrument and to the 0 vu or the 100 per cent scale must be supplied externally in order to marking. Of this impedance, 3900 ohms provide a point where the impedance is :s in the meter and about 3600 ohms the same in both direction, for the in- sertion of an adjustable attenuator. A must be supplied externally to the meter, portion of the series resistance is made this value of series resistance being re- adjustable as shown by the slide wire quired in order to meet the above d~- in the diagram. This is for the purpose of namic characteristics. facilitating accurate adjustment of the (c) Sensitivity. The application of a sensitivity to compensate for small dif- 1000-cps potential of 1.228 volts r.m.s. ferences between instruments and any (4 db above 1 milliwatt in 600 ohms) to slight changes which may occur with the instrument in series with the proper time. external resistance (3600 ohms) causes The maximum sensitivity possible a deflection to the 0 vu or 100 point on with this, the simplest circuit, is + 4 vu the scale. The instrument, therefore, has for indications at the 0 vu or 100 per only sufficient sensitivity at its normal cent mark when placed across 600-ohm reference point (0 vu or 100) to indi- line. The maximum sensitivity occurs, of cate a volume level of + 4 VU. It has not been found possible to design. more course, when the loss in the adjustable sensitive instruments while meeting attenuator is zero. The upper limit to the other requirements. range of measurement is limited only There should be no confusion because by the amount of loss introduced by the the instrument deflects to a scale mark- adjustable attenuator, its power handling ing of 0 vu when a level of +4 VU is capacity and that of the two series re- applied to it., The 0 point on the vu sistors. scale is merely an arbitrary point at The volume indicator of Fig. 4 can which it is intended nominally to read be bridged across circuits of other than .the instrument, and the rest of the vu 600 ohms, of course, but when this is scale represents deviations from the 0 done a correction factor must be applied point. The volume level is read, not from in order to determine the true level. the scale, but from the indications on the Figure 5 shows the magnitude of the associated sensitivity control when the latter is set so as to give a scale de- correction factor. It is to be noted that flection of zero (as detailed in a fore- the basic volume indicator circuit (Fig. going section). 4) has an input impedance of 7500 ohms (d) Overload Cajmcity: The instrument and should not be bridged across circuits is capable of withstanding, without in- of appreciably higher than about one- jury or effect on calibration, peaks of tenth of this impedance if undue loading 10 times the voltage equivalent to a de- of the circuit is to be avoided. On the flection to the 0 vu or 100 scale point other hand, when used across circuits of for 0.5 second and a continuous overload less than the normal 600 ohm value, the of 5 times the same voltage. sensitivity of the instrument is reduced, (e) Presence of Magnetic Material. It as indicated by Fig. 5. should be cautioned that the presence of Figure 6 shows an arrangement in magnetic material near the movement which, by adding a transformer, the of the instrument may affect its calibra- tion and dynamic characteristics. This sensitivity has been increased at the is because, to obtain the desired sensi- expense of decreasing the input imped- tivity and dynamic characteristics, it ance to a low value. The circuit is de- has been necessary to employ more pow- signed so that the impedance facing the erful magnets than usually required for instrument itself is the same as in the such instruments, and any diversion of basic circuit (Fig. 4). Thus the correct flux to nearby magnetic objects effec- dynamic characteristics are obtained. tively weakens the useful magnetic field The input impedance, on the other hand, beyond the point where these character- is low, hence the device cannot be istics can be met. The instruments bridged across a through line but must should not, therefore, be mounted on be used to terminate the circuit. In prac- steel panels. (The effect is only slight, tice, approximately a 10 db increase in however, if they are mounted on 1/16-in. sensitivity may be obtained by this ar- steel panels with the mounting hole cut rangement. away as far as possible without extend- In high-fidelity audio systems the use ing beyond the face of the meter case.) of a 150-ohm circuit impedance is be- In the instruments as now available, coming common practi~e.~However, the deviation of the sensitivity with tem- when the basic volume indicator circuit perature is less than 0.1 db for tempera- is used with 150-ohm circuits. a loss in tures between 50' F. and 120' F., and sensitivity of 6 db results. Therefore, on is less than 0.5 db for temperatures as a 150-ohm circuit the instrument is ca- low as 32O F. The instrument by itself does not con- Monroe and Palmquist, Proc. I.R.E., Vol. 36, No. 6, p. 786 (June 1948). AUDIO ENGINEERING OCTOBER, 1951 pable of reading levels only down to veloped that on the average, listeners c 10 vu. The loss in sensitivitv in situa- prefer to hear broadcast music and tions such as this can be avoided by speech at about the same peak levels utilizing a circuit similar to Fig. 6 ex- ar read on a standard volume ilzdicator. cept that the step-up ratio of the trans- Furthermore, listeners like to hear former is made only great enough to overcome its own losses and that which broadcast music and speech at the same results from the existence of lower relative levels, regardless of the absolute voltage (for a given volume level) on sound level that is individually pre- a circuit of lower impedance. In the ferred. particular instance cited a step-up ratio The Columbia Broadcasting System of slightly more than 1:2 would be is following this practice with consider- necessary if the transformer were with- able success. The measure of success in out losses and slightly greater than this this instance being the almost complete in a practical case. The input impedance of the volume absence of listener complaints concern- indicator (7500 ohms in the basic cir- ing the relative of speech and cuit) will be reduced under these cir- music. A few complaints continue to be cumstances by a factor equal to the received of course, but when specific square of the turns ratio. This is not cases are investigated it is almost al- a serious disadvantage, however, since ways found that either (a) the recom- the impedance of the circuit being mended transmission practices were bridged has also been reduced by es- violated'because of some special circum- sentially this same factor. Therefore the ratio of the circuit impedance to the stance or (b) that the program origin- bridging impedance remains approxi- ated on another network. mately the same. For the particular ex- ample cited, namely a 150-ohm circuit, practical volume indicator transformers, when terminated in a standard volume indicator, have input impedances of about 1700 ohms.

Transmission Level Practices The volume levels that are used for the transmission of speech and program waves may be determined upon in a number of ways depending upon the communications service involved. For example, in a system employed for voice communication, where loss of natural- ness of the speaker's voice is not a factor, the relative audio levels might be maintained as high as possible with- out loss of intelligence. Another cri- terion might be the transmission of voice and music at the same loudness. On this basis, using the standard volume indicator, voice would be peaked 2 or 3 db below music.l Still another possible basis for determining relative trans- mission level is the appearance of aural distortion caused by overloading of the particular facility involved. Determina- tions of this kind are usually made on an A-B test basis1 and the results will depend to a large extent upon the shape of the overloading characteristic of the system involved. Transmission practices based upon this criterion ensure the maximum use of the facility concerned while avoiding detectable aural distor- tion. For this reassn it has great appeal, from a purely technical standpoint, for high-fidelity broadcasting and sound recording applications. However, it does not take into consideration the listener's preferences. Program transmission practices, where listening for pleasure is con- cerned, may well be determined on the basis of the average listener's wishes. A study7. made with this criterion de- Chinn, Gannett and Morris; PTOC. I.R.E., Vol. 28, No. 1, p. 9, Jan. 1940. 1Loc. cit, pg. 4. 7 Chinn and Eisenberg, Proc. I.R.E., Vol. 35, No. 12, p. 1547 (Dec. 1947). AUDIO ENGINEERING OCTOBER, 1951