International projectionist (Jan-Dec 1935)

The New RCA Sound Track By JAMES FRANK JR. THE process of recording sound-onfilm is a very simple one and in general is quite similar to that of reproducing sound from film. In order to photograph sound it is necessary, of course, to in some way transform the sound waves — which vary both as to frequency and amplitude — into directly proportional light waves, which may be directed at the negative film. It has been found most practical to accomplish this by first transforming the sound waves into electrical waves which can be easily amplified to a useful intensity and then transform the electrical waves into light waves. There are in general use two distinct methods of recording sound-on-film. One employs a galvanometer for transforming the electrical waves to light waves and produces a variable area sound track. In this case there is a single contrast of black and white with varying portions of the sound track being exposed. In the other method a light valve is employed, producing a variable density sound track. In this case the entire width of the sound track is always exposed, but with a varying density ranging from black through gray to white. Variable Area Advantage RCA High Fidelity sound-on-film recording employs the variable area method. There are several distinct advantages to this method of recording over the variable density method consisting essentially of the following: 1. In the use of variable area, variations in film processing and difficulties encountered in the care of the film will not affect the quality of reproduced sound to the same degree as it will in the case of variable density recording. 2. Variable area recording permits greater output level. 3. The apparatus required for recording sound-on-film by the variable area method is considerably simpler and easier to control. The RCA High Fidelity method of recording sound-on-film will be described in some detail herewith. Probably the most important piece of apparatus employed is the recorder device. The recorder consists of a machine designed for moving 35 mm. film past the light beam at an absolutely constant speed of ninety (90) feet per minute. RCA recording devices employ a novel device consisting of a magnetic drive for accomplishing this necessary constant motion of the film. The Optical System The optical system, which includes the galvanometer, consists of a constant source of light focussed through an equilateral triangular mask on the mirror of the galvanometer. The galvanometer consists of a rod with a coil wound around it mounted with its axis perpendicular to the surface of the film in a magnetic field with a very small mirror about one-eighth of an inch square mounted at its end nearest the film. The image of the light is reflected from the mirror through a slit 70 mils wide and *4 mil thick onto the portion of the negative film reserved for the sound track. When no current is passing through the coil on the rod of the galvanometer, the mirror is adjusted so that the reflected image exposes the center half, or 35 mils of the sound track. When the varying electrical waves from the microphone, having been amplified, pass through the coil of the galvanometer, a motor action occurs causing the rod to move in a vertical plane. A. B. GECOGO/A/q SLIT POS/T/OK OF T/?//WG(/i/te /M/iqe of eight roe O % MODUl/lT/OM o/eccr/o/v of f/lm m/tv£L past the im/ige OF 0ECO/?/)/A/G SL/T 0ECOPD//VG SUT MEAN POSIT/ON OF TP/AAJGl/LA/P /M/IG£ OF l/GHT FOG MOCl/LAT/OM qee/tree th/>n so % (d) SYMMETP/CAL YAP/ABLE AP£A SOWJ3 TP/ICH fOY£g O/VS CYCLE OF OPEPAT/OAf OF THE GPOCW0 /VO/SE PEDl/CT/ON SYSTEM) M SIAGPAM Of OPEPAT/ON OF qeouND HO/SE PEBUCT/ON SYSTEM The RCA Sound Track Since the current is an alternating one, the rod really swings with a pendulum motion upwards and downwards in direct proportion to the intensity of the electrical waves. The result is that the mirror, of course, moves up and down too, and the reflected light image exposes more or less of the slit in direct proportion to the electrical waves. Maximum motion in one direction exposes 70 mils, and in the other none at all. The result is that the sound track portion of the negative film is exposed by a line of light *4 mil thick and varying from 0 to 70 mils in length. Since the film is moving at a speed of 90 feet per minute during the process, the resultant negative sound track appears to be an exposed portion in the middle of the sound track, both edges of which consist of symmetrical peaks and valleys. The shape of these peaks and valleys depend upon the frequency and amplitude of the recorded sound. When the negative is printed the sound track on the positive film is just the reverse: that is, the center portion of the sound track is transparent and the outside portions of the track are opaque with symmetrical peaks and valleys upon the inner edge. Ground Noise Reduction In order that the exposed portion of the film be reduced to an absolute minimum, resulting in a minimum amount of transparent sound track on the positive film which would permit stray light to pass through it, or on which dirt might collect, affecting the light passing through it and thereby causing extraneous noise in the reproduced sound, a method known as "ground noise reduction" is employed. This is accomplished by taking a small portion of the electrical signal, amplifying and rectifying it to a direct current, and passing it through a second coil wound on the rod of the galvanometer. This system is adjusted so that when no signal is passing through the galvanometer, the mirror has been pulled down in a position where somewhat less than one-half, or 35 mils, of the track is exposed. Its effect, however, is varied in such a way that it decreases as the amount of signal or modulation increases. In other words, at 0% modulation its effect is greatest, but at 80% modulation its effect is eliminated. By so doing, the amount of track exposed is kept at a minimum. [21]