straight-line portion of the negative H & D curve, the print The negative track is recorded on a high gamma positive being made on the straight-line portion of the print H & D type in contrast to a low negative gamma film for the curve. Light valve recording followed the basic principles variable-density method. Since the major studios had al- of rendering tone values laid down by L. A. Jones of ready installed the ERPI variable-density system, RCA Eastman Co. [5]. An excellent analysis of straight bought up RKO Pictures and installed its variable-area line and toe records was made by Mackenzie [6] in 1931 system, a refinement of 's earlier efforts and shown in Fig. 4. [7]. Studioslater adopted the RCA system. While the variable-density systems were being intro- After that, variable density dominated the industry until the duced into the motion picture industry, another type of mid-Forties, when the variable-ama method became more track, known as variable area, was being developed. In popular. Almost every motion picture studio now uses variable area recording. I \ ' _ I ',, I i , I PRINTER H &D CURVES _ I OVERALL HLD CURVES /3 _Z_,_o - '--i _i_<_ _%%,,o Noise Reduction _" --_ ;2 _]"'"'.,._'4 I , The first technical improvement in sound recording was \ \ _--- i I--_' - This was done on variable-density tracks by superimposing [X.,*x_--_---_--_- a biasing current on the ribbons, reducing the spacing from ] _ ..... i i__--__:/:!L-2_- the ihtroduction of ground noise reduction techniques [8]. --'i (_-.,_i--[--i [.... 7_ '( "1",',,_ the usual one mil tod fraction of a mil. As the signal o o_1[ oI_ _ ira le o 04 o$ t2 le %20

._oDc.s...... _,I I c,I/I .° ][ !/ ! currentscelled out. increased This resulted in strength, in a low-density the biasing area current on the was nega- can- ¢o.vc._,o...._ _. tive, and therefore a high-density area on the print, during I_ // _I,,'"_°._o.c_v_ quiet periods. Since ground noise is a function of film

' -_/___ ...... ___ --____ylf' variable-amasystem,thedensity, this method reduced track itby is narrowedduringquiet about 10dB. lnthe

/ ..... i-J""_ l'- periodsrises (Fig. and is 5). restored In 1932, to normal Bell width Telephone as the sound Laboratories volume Fig. 4. Analysis of straight line and toe records adopted a new vertical-cut disc system in which pre- and post- was used to reduce record noise. This the early 1920's, the General Electric Company was exper- technique was later adopted for studios' original variable- imenting with a rotating mirror type of galvanometer, call- density recordings. At the same time, a 200-mil push-pull ed the Pallaphotophone (or wobbling light phone). This track was used to improve signal-to-noise ratio and reduce produced what was then called a variable-width track. It is harmonic response [9]. essentially an oscillographic type of trace, the track consist- The graphical method of controlling the processing of ing of clear bordered by opaque areas. It is not a photo- variable-density records was replaced in 1939 by a dynamic graphic process per se, although it uses the photographic method known as intermodulation [10]. This consisted of medium. Typical variable-area tracks are shown in Fig. 6. recording a I kHz tone superimposed on a 60 Hz tone at

o o.6 o 7 08 10 (a) (b) (c) ,,,su,L _,,,,,.,roE,,s,,,. Fig. 5. Three variable area tracks, recorded by RCA gal- Fig. 6. Results of intermodulation method of controlling pro- vanometer; a) bilateral; b) Class B push-pull; c) Class Apush-pull cessing of variable density records (replaced by graphical method) JULY/AUGUST1976,VOLUME24,NUMBER6 513 " [ / I I In spiteof theseimprovements,signal-to-noiseratioin photographic sound tracks stayed around 50 dBfor newly-

': V_ __ _[ __ _~_5r,,i_ , processedphonic soundvariable-areawas recordedprints.liveAfterin 1933,three-channelBell Telephonestereo-

xx_"i-X",[_>o,/, / __ Laboratories began working to put a similar effect on film _- i', [ _,_' I phonyorchestrawitha rangeof 80 dB on a mediumwhose i Lx_,, I_',, l_r_-t' I_._ ?-_- I range[12].Sincewas aboutthe engineers50 dB, theywantedhad totorecordrecordtheatoplarge30 dBsym-at constantmodulationofthefilm(Fig. 10).Inreproduction, 17 1 - o.,' P ...... ::A / It_ ' I tracksFig. 7. Intermodulation obtained from single and push-pull _ 20,o r/_...... _,c,_ ",, / __/ ,_D...... ,o,sE / t t LEVEL .___....i./ Oo/I0 I20 30 4f0 5D eO 70 -,eO 0 710 20 .30, 4O _.O eo 70 8o SOUND LEVEL IN DECIBELS AMPLIFI£R OUTPUT IN DECIBELS various expoures, developing to various gammas and print- (,) (b) ing at various exposures. Families of curves are shown in Fig. 10. Noise reduction in film, ca. 1933: relationship between Figs. 6 and 7, giving the lowest values ofthe 60 Hz tone and a) ga{n and light to sound level and b) indicating the correct processing parameters. About the photocell output and noise level to sound output in compandor same time, RCA developed the cross-modulation technique to indicate the correct processing of variable-area tracks for the gain of the amplifier system remains constant over the minimum rectification of zero-shift [11]. The curves in lower 50 dB, increasing 1 dB for each decibel of output up to the 80 dB level. The result is an effective 30 dB of noise reduction. In this compandor system, the amplified output

' I __ tone is modulated by the signal envelope, the output being o' &_ I[--I +1 _[____ [ offedeachintomicrophonethecontroliscircuitbridgedof theto acompressor.modulator inThewhichcom-a '_ i i--[-I I I--i ] I I I _ !_ bined output ofthe modulators of the three tones (250, 630 "";_'_,'____' _'V<.L'Y.3_'x"_x_j /,'/x¢_.;, /_/_ !' and 1600 Hz) is recorded on a fourth control track (Fig. 11). _'='- I I ?'d' I _ _( \_ilk/'× _,l 'r I

"-_-i-i-[-i-C_i_ i,o, _ _ , i

.... "2.... _,5,,,,';,~,,,] ...... Figs. 8, 9. Tests required for minimum cross-modulation prod- '"-ucts -i- i-i--i-C'-'VF;_C' _v4/ Z l- _[_L _!.i_! _[__2_ .... r-_-_1 _ l .E..... _.,_ F:;:_,.-rjF:_._,wq

W-' RECORDER

"'" [ Fig. 11. Stereophonic recording channel and resulting sound "° ._` /' __J,// _:/_ .I I I tracks

4e ' __ __ _: I i [ [ i--_i _- _ t _ restoresIn reproduction,the correct thelinearprocessvolumeis reversedrelationships.(Fig. 12).ThisPre- and .I l_ 1.0 1.2 1.4 I.t I.I 20 2.4 vtsuAt .... r _#$trr post-equalization networks were also effective in reducing high-frequency noise components. This system was dem- Figs. 8 and 9 graphically illustrate the tests required to onstratedat the PantagesTheatre, Hollywood, in 1940. But obtain minimum cross-modulation products. Another Em- after World War II, when magnetic recording came on the provement in track definition was the use of ultra-violet scene, no serious effort was made to commercialize the light in both recording and printing techniques, compandor stereo system. In passing, it might be noted that

514 JOURNAL OF THE AUDIO ENGINEERING SOCIETY Studios tended to use 35 mm full-coated tape: they were fully-equipped with expensive 35 mm sprocket-type record- ____ began to use the magneticmediumfor in-studio recording. - r-' ...... t' ' ...... " "1 '_ ers and reproducers, and synchronization of iA" tape with f ,E,,oDucE, } sprocket-type cameras and projectors was yet to come.

I.q5'4' REI_ .007 _Et_ i z I I I I

Fig. 12. Schematic diagram of stereophonic reproducing chan- I I --i I o_z ·o5o _[_.zot,Ii. . .//o rvP. 6 rt_cKS _ _ '4_ nel -- _.4-g4

this experimental work by the Bell Telephone Laboratories Fig. 13. Todd-AU 70 film coating, track layout was chiefly an attempt to achieve true acoustic perspective in reproducing what a listener would hear from the stage. , the brainchild of 20th Century-Fox, was the first serious attempt to introduce stereo into ordinary release prints. It was combined with an anamorphic system " While Bell was working on compandor stereo, RCA of lenses on camera and projector to give the effect of a wide developed Fantasound, the system used in , for screen. There were three stereo tracks and a narrower Walt Disney Studios [13]. The original recordings were control track (Fig. 14). The magnetic tracks were super- made with 8 push-pull variable-area channels. Six separate ss,_ channelswere used for closeups of variousorchestral groups. A seventh channel recorded a combination of all f '_ six, while an eighth recorded a distant pickup of the or- reAcgs reAcKs

chestra. The number was later recorded with I 1. 4 three channels. In re-recording, the original tracks were .o,o ./_o .e?8 _.,_ .o,o transferred to three channels. These, plus a control track, _ J_f---__ 'l_ i -' I' were made on a separate 35 mm film. The control track I lll contained frequencies of 250, 630 and 1600 Hz, which GUIDED were used to enhance the limited volume range of the sound Eo_ tracks. Fantasound differed from the Bell Telephone H serveLaboratoriesthe acousticalsystem perspectivein that the latterof thewasoriginaldesignedorchestra,to pre- . coM_fr__ c:_:cl!_l_ _--_ - whereas Fantasia set out mainly to create effects that '_,i' , i , _mr reflected the mood of the animated pictures on the screen. It i i''°°_ . _7_, .o_e _1_.II_._.___._ i' [ .o,az was the first film that used a surround system to enhance the .o_&__z__._ _ r" overall audience reaction. Fig. 14. Cinemascope:threestereotracksandcontroltrack While the Bell compandor stereo and Fantasound sys- tems aroused interest in the motion picture industry, there imposed on the 35 mm film by a striping process. Cinema- was no immediate follow-up. The first commercial use of Scope was never very popular. For one thing, striping each stereo in the theatre was the system. This used a individual release print was much more expensive than separate 35 mm magnetic-coated film carrying the stereo printing a photographic track. Magnetic tracks, either stan- and surround tracks. It was followed by the successful dard or CinemaScope have, for all practical purposes, dis- Todd-AU 70 film carrying the picture and the coated appeared from the screen. magnetic tracks. This system employed five independent Since their demise, the industry has been working on a stereo tracks plus a control track, and used five compatible stereo system that uses the photographic systems behind the wide screen (Fig. 13). medium. As early as 1955, Frayne [14] demonstrated a - After World War II, and the success of the two-channel photo stereo release print which carried two , developments, the motion picture industry separate tracks in the standard sound track position (Fig.

JULY/AUGUST 1976, VOLUME 24, NUMBER 6 515 the use of a three-speaker system for reproducing music and .076 sound effects. An excellent paper on the Dolby system as applied to - ,?_43 _ tageopticalofsoundlocalizingtracksdialogappearedat centerin thescreenSeptember,while197permitting5Jour- nal of the SMPTE [17]. The author says that a flat recording

and reproducing characteristic using the Dolby noise reduc- improved high-frequency response with lower distortion. -i tionsystemgivesanimprovedsignal-to-noiseresponse,an He also claims that the print is compatible with unconverted

t theatreThe photographicplayback equipment.medium is still a highly satisfactory

_ andAdaptingeconomicalstereo towaythis tomediumrecordwillsoundprobablyon releaseguaranteeprints.its _-- J continuedusein thefuture.

I REFERENCES ,/ [1] Tykocinski-Tykociner, J., "Photographic Record- ing and Photo-Electric Reproduction of Sound," Transac- .036 .036 tions of the SMPE, Vol. 6, p. 90 (May, 1923). - [2] De Forest, L., "The ," Transactions of the SMPE, Vol. 6, pp. 61-77 (May, 1923). o_9_a_ [3] Frederick, H. A., "Recent Advances in Wax Re-

Fig. 15. Two-channel photo stereo print demonstrated by au- cording," Transactions of the SMPE, Vol. 10, p. 294 thor (August,1927). [4] Mackenzie, D., "Sound Recording with the Light Valve," Transactions of the SMPE, Vol. 12, p. 730 (Sep- 1'5). Variable-area recording was employed, but the system tember, 1928). would work equally well with variable density. In this [5] Jones, L. A., "On the Theory of Tone Reproduc- demonstration, the original recordings were made with a tion," Jour. Opt. Soc. Am., Vol. 4, p. 420 (1920). two- channel system. A third channel was [6] Mackenzie, D., "Straight Line and Toe Records," created by mixing the two channel outputs into a center SMPE Journal, Vol. 17, p. 172 (August, 1931). channel. On reproduction, a third channel was similarly [7] Dimmick, G. L., "The RCA Recording System," created by combining outputs from the dual photocell. The SMPE Journal, Vol. 29, p. 28 (September, 1937). [8] Silent, H., and Frayne, J. G., " result was a three-loudspeaker stereo system (Fig. 16). Noiseless Recording," SMPE Journal, Vol. 18, p. 551 (May, 1932). g_,%}[° ...... [9] Silent, H., and Frayne, J. G., "Push-Pull Recording ,__ _,,;3_ Lo_, {}QL with the Light Valve," SMPE Journal, Vol. 31, p. 46 ...... (July,1938). ,_;l;%i,.°2, ' I [10] Frayne, J. G., and Scoville, R. R., "Analysis and _°[_°D_ I [.._; ,2_5:.o!__.?_ c Measurement of Distortion in Variable Density Record- II ;? -_-'t[L_j ' ing," SMPE Journal, Vol. 32, p. 648 (June, 1939). ,,I _!k : [11] Baker,J. O., andRobinson,D. H., "Modulated I_ _,_ o_,,_,_ [}(]_ High-Frequency Recording as a Means of Determining Conditions for Optimal Processing," SMPE Journal, Vol. 30, p. 3 (January, 1938). Fig. 16. Stereo reproducing channels for 2- and 2V2-channel [12] Steinberg, J. C., "The Stereophonic systems System,"SMPE Journal, Vol. 37, p. 36 (October, 1941). [13] Garity, William E., and Hawkins, J. . A., "Fan- tasound," SMPE Journal, Vol. 37, p. 127 (August, 1941). Revival [14] Frayne, J. G., "A Compatible This system remained dormant until 1973, when R.E. System,"SMPTE Journal, Vol. 64, p. 303 (June, 1955). Uhlig described the use of the Dolby noise reduction system [15] Uhlig, Ronald E., "Stereophonic Photographic to enlarge the signal-to-noise ratio and thus overcome the Sound Tracks," SMPTE Journal, Vol. 82, p. 292 (April, loss in volume caused by the reduction in track width that 1973). the Frayne proposal made necessary [15]. The Dolby noise [16] Uhlig, Ronald E., "Two and Three Channel Stereophonic Photographic Sound Tracks for Theatres and reduction system, originally applied to magnetic tape ," SMPTE Journal, Vol. 83, p. 729 (Sep- tracks, applies equally well to optical tracks. In a later tember, 1974). paper, Uhlig proposed a further improvement for the [17] Allen, I., "The Production of Wide-Range, photographic stereo system: a phantom third track was Low-Distortion Tracks Utilizing the Dolby incorporated by combining the output of the two tracks and Noise Reduction System," SMPTE Journal, Vol. 85, p. feeding it to a central loudspeaker [16]. This has the advan- 720 (September, 1975).

516 JOURNAL OF THE AUDIO ENGINEERING SOCIETY