By H. A. C H IN N ,f D. K. GAN NETT, and R. M . M O R RIS J In recent years it has become increasingly difficult to correlate readings of volume level made by various groups because of differences in the characteristics and calibrations of the volume indicators used. This paper describes a joint development by the considerations on which was based the choice of the characteristics of the new volume indicator and the other features of the new
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S T A N D A R D V O L U M E I N D I C A T O R A N D R E F E R E N C E L E V E L 101 n o t fa st enough to in d ica te th e in sta n ta n e o u s p eaks of speech or p ro g ram w aves b u t ten d to av erag e or in te g ra te a n u m b e r of p eaks of th e w ave.
A fe a tu re of th e usual p eak -read in g in s tru m e n t w hich from th e a n a ly tic a l s ta n d p o in t is of seco n d ary im p o rta n c e , is t h a t it is u su ally given a c h a ra c teristic of v e ry slow d e cay as well as ra p id response.
T h is is u su ally accom plished b y a circ u it such as illu stra te d in F ig. 2, w hich show s th e principle of th e ex p erim en tal in s tru m e n t used in th e te sts described la te r. T h e 0.01-m f. condenser is charged th ro u g h a full w ave v a cu u m tu b e rectifier, th e ra te s of charge an d discharge being d eterm in ed b y th e resistances. T h e d.-c. am plifier a n d d.-c.
m illiam m eter in d icate th e charge on th e condenser. T h e a d v a n ta g e of m aking th e discharge ra te of th e condenser v e ry slow is t h a t th e d.-c.
CONTROLS SPEED R2 OF RESPONSE CONTROLS RATE
OF DECAY
M ILLIA M M E TE R
Fig. 2—Schematic diagram of experimental peak reading volume indicator.
m illiam m eter need n o t th e n be p a rtic u la rly fa st an d , m oreover, th e ease of reading th e in s tru m e n t is g re a tly increased.
F ro m th e above analysis i t is seen t h a t th e r-m -s a n d th e peak- read in g in stru m e n ts are essen tially sim ilar a n d differ p rin cip ally in degree. B o th in d icate p eaks w hose d u ra tio n s exceed som e value critical to th e in s tru m e n t an d b o th av erage or in te g ra te over a n u m b er of peaks th e sh o rte r, m ore ra p id p eaks encountered in speech or p ro g ram w aves. E ith e r m a y h av e a lin ear o r a sq u are law d e te cto r, or one of som e in te rm e d ia te c h a racteristic. T h e im p o rta n t difference betw een th e tw o ty p es lies in th e speed of response as m easured b y th e len g th of im pulses to w hich th e y will fully respond, t h a t is, in th e tim e over w hich th e com plex w ave is in te g ra te d .
A general purpose volum e in d ic ato r m ay be called upon to serve a n u m b e r of uses, such as:
(a) In d ic a tio n of a su itab le level for a speech or program w ave to av o id audible d isto rtio n w hen tra n s m itte d th ro u g h an am plifier, p ro g ram circu it, rad io tra n s m itte r or th e like.
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S T A N D A R D V O LU M E I N D I C A T O R A N D R E F E R E N C E L E V E L 103 be a p h o n o g rap h p ickup, a d ire c t m icrophone pickup, or a pro g ram circu it, is conn ected th ro u g h co n tro l circu its to th e am plifier w hich is to be o verloaded, an d th en ce th ro u g h ad d itio n a l circu its to a loud sp eak er. T h e loud sp e ak er em ployed in th e te sts re p o rte d here was a special high q u a lity tw o u n it loud sp eak er h av in g a response w hich is su b sta n tia lly flat from 40 to 15,000 cycles per second.2 In clu d in g th e pow er am plifier used w ith it, th e overall response of th e sy stem w as su b s ta n tia lly uniform from 40 to 11,000 cycles.
T h e a rra n g e m e n t of th e c ircu it is such t h a t th e volum e level a t th e o u tp u t of th e te s t am plifier m ay be raised or low ered w hile keeping th e overall gain of th e sy stem c o n sta n t. T w o controls are p rovided for th is purpose. O ne, o p e ra te d b y a key, tra n sfe rs a 15 d b loss from ah ead to beh in d th e te s t am plifier. T h is p erm its co m p arin g a te s t
POWER
2-P O S IT IO N KEY CONDITIONS A AN D B
Fig. 3—Arrangem ents for determining volume level a t which overload of amplifiers is audible.
condition w ith a reference condition in w hich th e load on th e am plifier is 15 d b lower, while th e loudness w ith w hich th e program is heard rem ains th e sam e for e ith e r condition. T h e o th e r control, rep resen ted in Fig. 3 b y th e coupled a tte n u a to rs , p erm its th e load on th e am plifier for th e te s t condition to be v aried , also w ith o u t changing th e loudness.
T h e volum e in d icato rs to be com pared are connected for convenience, to a p o in t w here th e volum e level is u naffected b y th e controls. T h eir readings are corrected for each te s t b y th e m easured loss or gain b e
tw een th e p o in t w here th e y are situ a te d a n d th e o u tp u t of th e te s t am plifier, so as to express th e levels w hich w ould be read a t th e am plifier o u tp u t.
T w o tech n iq u es w ere em ployed for conducting te sts w ith this eq u ip m en t. In one, th e ind iv id u al m eth o d , a single observer a t a tim e 2 “ A uditory Perspective— Loud Speakers and M icrophones,” E. C. W ente and A. L. T huras, Electrical Engineering, January 1934.
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a re show n in F ig. 5. F o r convenience an d ease of in te rp re ta tio n , th ese cu rv es h a v e been p lo tte d on “ p r o b a b ility ” r a th e r th a n re c ta n g u la r co o rd in ate s, as p ro b a b ility c o o rd in ates h a v e th e p ro p e rty of m ak in g d a ta w hose d is trib u tio n follows a n o rm al law 4 form a s tra ig h t line.
I t will be n o te d t h a t th e e x p e rim e n ta lly d e te rm in e d p o in ts a c tu a lly fall so n e a rly on s tr a ig h t lines, t h a t it is reaso n ab le to assu m e s tra ig h t lines to re p re se n t th e m . I t is lik ely t h a t w ith a g re a te r v o lu m e of d a ta , still g re a te r co n fo rm ity to th e s tr a ig h t lines d ra w n , w ould be o b ta in e d .
In o rd er to superpose th e cu rv es for th e tw o v o lu m e in d ic a to rs, th e levels a re p lo tte d in decibels w ith re sp e c t to th e a v erag e o v erlo ad level
- 8 - 7 - 6 - 5 - 4 - 3 - 2 - I 0 1 2 3 4 5 6 7 8 VOLUME LE V E L IN DECIBELS ( 0 = MEAN OVERLOAD P O IN T )
Fig. 4— D istribution of overload points.
d e te rm in e d from th e te sts. W h en c a lib ra te d to re ad a lik e on th e sam e sine-w ave pow er, th e ex p e rim e n ta l p e a k -read in g in s tru m e n t (w ith th e a d ju s tm e n ts described above) read s on th e a v erag e 7.4 decibels hig h er on a c tu a l p ro g ram s th a n th e r-m -s in s tru m e n t used in th e te sts.
N ow le t it be im ag in ed t h a t th e te s t am plifier is th e one critical lin k in a b ro a d c a s t n e tw o rk a n d t h a t a n o p e ra to r is given th e d u ty of sa tisfa c to rily a d ju s tin g th e v olum e levels th ro u g h th e am plifier using e ith e r of th e tw o v olum e in d ic a to rs te ste d . If he le ts th e lo u d er p o rtio n s of th e p ro g ram s ju s t reach th e v olum e level m a rk e d “ 0 d b ” on th e curves, it will m ak e no difference w hich v o lu m e in d ic a to r he
4 The “ no rm al” law has the form y = A
S T A N D A R D V OL U M E I N D I C A T O R A N D R E F E R E N C E L E V E L 107 uses. In e ith e r case, on th e average, h alf of th e listeners will h ear d isto rtio n w hen th e p ro g ram is loudest. H ow ever, th is re su lt w ould p ro b ab ly be considered too poor, so suppose th e m axim um level is low ered 3.5 decibels. R eferring to th e curves, it is seen t h a t if th e
Fig. 5—Comparison of peak vs. r-m-s volume indicators as overload indicators (using W .E. 94B amplifier).
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Fig. 6— Comparison of peak vs. r-m-s volume indicators as overload indicators (using W .E . 14B program amplifier).
to in d icate th e a c tu a l in sta n ta n e o u s peaks. T o check th is p o in t, some te sts sim ilar to those described ab o v e w ere m ade, using a gas tu b e trig g e r circ u it cap ab le of m easu rin g th e tru e in sta n ta n e o u s peaks.
T h e re su lts of th ese te sts, using th e 94-B am plifier, are show n in F ig. 8.
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A lth o u g h a sm aller n u m b e r of o b se rv a tio n s are in clu d ed in th e se d a ta , th e re su lts show conclu siv ely t h a t th e re is no s u b s ta n tia l difference b etw een th e e x p e rim e n ta l p e a k -re ad in g v o lu m e in d ic a to r a n d th e fa ste r trig g e r tu b e a rra n g e m e n t, in th e ir p e rfo rm a n c e on a c tu a l p ro g ram w aves.
OUTPUT L E V E L IN D E C IB E LS AB O V E ONE M ILL IW A T T
Fig. 7— Gain vs. load characteristics of amplifiers.
T h e d a ta from th e te s ts h a v e been p re se n te d a b o v e in th e form w hich m o st d ire c tly in d icates th e c o m p a ra tiv e p erfo rm an ce of th e tw o ty p e s of volum e in d ic ato rs. H ow ever, a b re a k d o w n of th e d a ta w ith resp ect to th e ty p e s of p ro g ra m m ay be of in te re s t a n d is show n in T a b le s 1
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Fig. 8— Comparison of experimental peak volume indicator with gas tube trigger device as overload indicators.
a n d I I for th e d a ta on th e 94B am plifier show n previously in Figs.
4 a n d 5.
I t will be observed in T a b le I th a t th e average overload p o in ts for th e d ifferen t ty p e s of p ro g ram s fall w ith in a range of a b o u t 2 db for
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S T A N D A R D V O L U M E I N D I C A T O R A N D R E F E R E N C E L E V E L 115 Fig. 9. T h e 50 p er c e n t p o in t on th e cu rv e is in te rp re te d as in d icatin g th e level of th e te s t pro g ram a t w hich it ap p e a rs to th e av erag e observer to h av e th e sam e loudness as th e reference p rogram . T h e te s t p ro gram is th e n se t a t th is “ equal lo u d n e ss’’ volum e level a n d th e levels of b o th te s t a n d reference p ro g ram s are read w ith each of th e ty p e s of
Fig. 9— Per cent of observers choosing symphony music a t indicated volume levels to be louder than th e male speech reference.
volum e in d icato rs of in te re st. In th is w ay, th e figures given in T ab le IV w ere d eterm in e d .
I t is e v id e n t from th e figures in th e tab le t h a t th ere is no significant a d v a n ta g e for e ith e r ty p e of volum e in d ic a to r w here loudness is th e c riterion.
T a b le IV show s t h a t w hen th e new volum e in d ic a to r is used th e m usical pro g ram s m u st be 2 to 3 d b higher th a n speech to sound eq u ally loud. I t is of in te re s t to n o te t h a t according to T a b le I th is sam e
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th is, th e opinions of technicians, accu sto m ed to read in g volum e in d i
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te n d e n c y to p a u se a t th e to p of th e sw ing before s ta r tin g d o w n w ard m ak es A e a sy to re a d , a n d th e failure to do so ex p lain s th e o b serv ed
“ j i t t e r y ” m o tio n of in s tru m e n ts such as B a n d C.
As a fu rth e r p a r t of th is s tu d y , high speed m oving p ic tu re s w ere ta k e n of th e a v a ila b le v o lu m e in d ic a to rs, show ing th e ir resp o n se to s u d d e n ly a p p lied sine-w aves. T h e p ic tu re s w ere ta k e n a t 400 fram es a second a n d in clu d ed on th e edge of each fram e w as a p h o to g ra p h of a clock d evice w hich in d ic a te d tim e in th o u s a n d th s of a second. F ro m
0 0.1 0.2 0-3 0 .4 0.5 0.6 0.7 0 .8 0.9 1.0 1.1 1.2 1.3
TIME C 1.0 = TIM E TO REACH 99 PERCENT F IN A L DEFLECTION)
Fig. 10— Effect of dam ping on instrum ent characteristics.
m ea su re m e n ts m ad e on th ese films, th e d a ta p lo tte d in Fig. 11 were o b ta in e d . I t is in te re s tin g to observe how lig h tly d a m p e d are th e o scillations of th e 203C v o lu m e in d ic a to r, w hich u n til th e a d v e n t of th e new in s tru m e n t h a s been in use in considerable n u m b ers. T h e cu rv e for th e p e a k v o lu m e in d ic a to r on F ig. 11 m u s t n o t be m ista k e n for th e tru e speed of response b u t is m erely th e speed w ith w h ic h th e in s tru m e n t re a d s th e ch arg e on th e co n d en ser (see Fig. 2). T h e ch arg e b u ild s u p q u ite ra p id ly , b u t th e in s tru m e n t follows in m ore leisu rely fashion a s show n. T h e in s tru m e n t, as n o te d earlier, will a c tu a lly give
S T A N D A R D V O L U M E I N D I C A T O R A N D R E F E R E N C E L E V E L 119 a read in g of 80 p e r c e n t on an im pulse of sine-w ave as sh o rt as .025 second.
T h e abo v e ch a ra c teristic s w ere decided upon only a fte r m a n y te sts co rro b o rated b y field tria ls u n d e r a c tu a l w orking co nditions. T h e v a lid ity of th e conclusions reached in th e te sts of earlier r-m -s volum e in d icato rs w as checked w ith resp ect to th e new in s tru m e n t b y fu rth e r tests.
0 O.l 0 .2 0.3 0 .4 0 .5 Q.6 0.7 0.8 0 .9 1.0
TIM E IN SECONDS
Fig. 11— Deflection of volume indicators to suddenly applied sine-wave.
T h e questio n of w h e th e r th e rectifier should be half-w ave or full-w ave needs little discussion. T h e oscillogram of th e speech w ave show n in Fig. 1 show s a v e ry m ark ed lack of sy m m etry . E v id e n tly if a volum e in d ic a to r is to give th e sam e reading no m a tte r w hich w ay its in p u t is poled, a b a lan ce d full-w ave rectifier is required.
T h ro u g h o u t th is pap er, th e te rm “ r-m -s ” has been used loosely to describe th e general ty p e of in s tru m e n t u n d er consideration. Some te s ts w ere m ade to d ete rm in e how closely th e new volum e in d icato r a p p ro x im a tes th is c h a racteristic .
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I t is e v id e n t t h a t th e in s tru m e n t scale should be easy to read in order t h a t th e p eak reached b y th e needle u n d e r th e im p e tu s of a given im pulse m a y be a c c u ra te ly d e term in ed . T h e in s tru m e n t scale, th e re fore, should be as large as p ra c tic a l since in th e case of th e b ro a d c a st a n d m o tio n p ictu re a p p lica tio n s, a tte n tio n is d ivided betw een th e actio n in th e stu d io a n d th e volum e in d ica to r.
T h e in s tru m e n t scale g ra d u a tio n s should convey a m eaning, if possible, even to those n o t te ch n ically inclined b u t who are, n e v e rth e less, concerned w ith th e p ro d u ctio n of th e p ro g ra m m aterial.
F in ally , th e scale m u s t be p ro p erly illu m in ated so t h a t th e relativ e lig h t in te n s ity on th e face of th e in s tru m e n t is co m p arab le to t h a t on th e sound stag e. U nless th is co n d itio n prevails, th e eye will h av e difficulty in acco m m o d atin g itself w ith sufficient ra p id ity to th e changes in illu m in atio n as th e te ch n ic ian glances b a c k a n d fo rth from th e stu d io to th e v o lu m e-in d icato r in stru m e n t.
E x istin g Scales
T h e v o lu m e-in d icato r scales m o st com m only em ployed in th e p a s t are show n in Figs. 12, 13, 14 a n d 15. I t is e v id e n t t h a t all th ese scales differ from each o th e r in one or m ore respects.
Fig. 12—Scale on 203C volume indicator.
T h e color co m b in atio n s em ployed for th e scale show n in Fig. 12 an d th e sim p licity of its m ark in g s are o u tsta n d in g v irtu e s. T h e division m ark in g s a n d th e n u m erals of th e m ain scale are black on a yellow
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b a c k g ro u n d . The decibel divisions a n d asso ciated n u m e ra ls are in red a n d co n sid era b ly less conspicuous th a n th e m ain scale.
H ow ever, th e 0 to 60 scale, w hich is used on b o th of th e in stru m e n ts show n in F igs. 12 an d 13, is an a r b itr a r y one b e a rin g no sim ple relatio n to th e electrical q u a n tity being m easu red . B ecause of th is, som e of th e n o n -tech n ic al persons con cern ed w ith p ro g ra m p ro d u c tio n are p ro n e to re q u e st t h a t a c e rta in “ e ffe c t” w hich th e y desire to tr a n s m it a t a lo u d e r-th a n -n o rm al level be p e rm itte d to sw ing th e in d ic a tin g
Fig. 13—Scale on 21 ty p e volume indicator.
needle b ey o n d th e n o rm a l reference p o in t of “ 3 0 ” on th e scale. I t is n o t e v id e n t to th e m from th e in s tru m e n t scale t h a t th e n o rm a l read in g of “ 3 0 ” corresponds to m ax im u m “ u n d is to r te d ” o u tp u t of th e sy stem .
T h e scale show n in F ig. 14, on th e o th e r h a n d , w as p rim a rily in te n d e d for s te a d y -s ta te a n d n o t v o lu m e level m e a su re m e n t purposes.
C o n seq u en tly , th is scale h as little , if a n y th in g , to co m m en d i t for p ro g ra m m o n ito rin g use. N ev erth eless, th e sim p lic ity a n d th e fine electrical fe a tu re s of th is ty p e of in s tru m e n t, to g e th e r w ith its re la tiv e ly re aso n ab le cost, h a v e re su lte d in its g en eral a p p lic a tio n to v o lu m e
S T A N D A R D V O L U M E I N D I C A T O R A N D R E F E R E N C E L E V E L 123 in d ica to r service. I t is ev id en t, how ever, t h a t th e scale card , w hich contains all k inds of id en tificatio n d a ta , is e n tire ly too confusing for quick, acc u ra te o b se rv atio n s as th e needle swings rap id ly b ac k and forth across th e scale.
T h e scale show n in Fig. 15 h as th e m erit of sim plicity a n d easy read ab ility . I t is, how ever, so m ew h at lim ited in th e decibel range ap p earin g on th e scale.
Fig. 14—Scale on type 586 power level indicator.
N ew Scale
B o th v u 5 m ark in g s an d m ark in g s p ro p o rtio n al to voltage are in c o rp o ra ted in th e new in s tru m e n t scale. T h e need for th e form er is obvious, b u t th e philosophy w hich leads to th e inclusion of th e la tte r requires a n ex p lan a tio n .
I t is e v id en t, assu m in g a linear sy stem , t h a t th e v o ltag e scale is d ire c tly p ro p o rtio n a l to p e rcen tag e m o d u latio n of a radio tra n s m itte r u pon w hich th e pro g ram is finally im pressed. If th e system is a d ju ste d for com plete m o dulation for a deflection to th e 100 per ce n t
6 Defined later.
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m a rk , th e n su b se q u e n t in d ic a tio n s show th e degree of m o d u latio n u n d e r a c tu a l o p e ra tin g co n d itio n s. In th e in te re sts of b e st o p e ratio n , i t m a y be d esirab le, of course, to a d ju s t th e sy stem for so m ew h at less th a n co m p lete m o d u la tio n w hen th e 100 p er ce n t in d ic a tio n is reached.
In a n y e v e n t, th e in d ic a tio n s on th e v o ltag e scale alw ay s show th e percentage utiliza tio n o f the channel. T h is is a d ecided a d v a n ta g e b e
cause ev ery o n e in v o lv ed h a s a clear co n cep tio n of a p erc e n ta g e in d ic a tio n . F u rth e rm o re , since th e scale does n o t ex te n d b ey o n d th e 100
Fig. 15— Type of scale used on 1G and 700A volum e indicators.
p er c e n t m a rk (except in th e form of a red w arn in g b a n d ) a n d since it is im possible to o b ta in m ore th a n 100 p er c e n t u tiliz a tio n of th e facilities, th e re is no in c e n tiv e on th e p a r t of n o n -tech n ical people conn ected w ith p ro g ra m o rig in atio n to re q u e st an e x tra loud “ e ffe c t”
on special occasions.
A c tu ally , tw o scales, each co n ta in in g b o th v u a n d v o ltag e m arkings, h av e been devised. O ne of these, know n as th e ty p e A scale, F ig. 16, em phasizes th e v u m a rk in g s a n d h as an in conspicuous v o lta g e scale.
T h e second, know n as th e ty p e B, F ig. 17, reverses th e e m p h asis on
S T A N D A R D V O L U ME I N D I C A T O R A N D R E F E R E N C E L E V E L 125 th e tw o scales. T h is a rra n g e m e n t p e rm its th e in sta lla tio n of th e in stru m e n t w hich featu re s th e scale t h a t is m o st im p o rta n t to th e user, while re ta in in g th e a lte rn a te scale for co rrelatio n purposes.
T h e new scale re ta in s th e sim p licity a n d th e general color schem e of th e form er Fig. 12 scale. T h e m ain division m ark in g s a n d th e asso
ciated num erals are, in each case, in black. T h e seco n d ary d a ta are
ciated num erals are, in each case, in black. T h e seco n d ary d a ta are