Exhibitors Herald and Moving Picture World (Apr-Jun 1930)

MOTION PICTURE SOUND RECORDING June 7, 1930 EXHIBITORS HERALD-WORLD 93 By Western Electric Method DR. DONALD MACKENZIE* THE object of all recording is to furnish a sound which would be indistinguishable irom the sound one would get from the real source if it were there. At best, it will be no better than direct transmission from the microphone which picked it up in the set, out to the horn which reproduces it in the theatre. From the point at which the recording device comes in, to the point where the photocell furnishes current for the amplifier, we have done nothing but introduce a delay circuit to stop the currents coming from the microphones and store them up until we want them to actuate a loud speaker. You will see that the permanent record will be nothing more than a delay circuit. The effort to give a complete illusion, then, is dependent upon the success of the transmission line and it is affected with all of the disadvantages of listening with one ear (one microphone) whereas you have two ears. The acoustical conditions which are favorable and give a fair illusion are discussed by Mr. Maxfield. The recording method I wish to describe is used in the Western Electric system, and k [depends upon the light valve to affect modulation of the light on the sound negative. The Photophone method described by Mr. IjTownsend is a variable area method. The method Mr. Hansen described is a variable density method, and I am about to discuss another variable density method. In Mr. Hansen’s device we have a light source, whose D C C' FIGURE 1 intensity is varied, shining on the film through a slit of fixed width. The factors of intensity and time constitute the exposure and one or the other is varied. In the Fox device, the intensity is varied and the time of exposure is constant. In the light valve shown in Fig. 1, you have a shutter opening and closing. That shutter is focussed on the film to form a line J4 mil wide when undisturbed and varying from zero to twice its normal width. The intensity of the light is unchanged. A fixed source of light shines upon a loop, the sides of which open and close and the width of the image as it varies from zero to one mil varies the time it takes for the film to pass the exposure point. Fig. 1 shows a photograph of the light valve, invented in 1922 by E. C. Wente of the Bell Telephone Laboratories. Essentially, it consists of a loop of duralumin tape suspended in a plane at right angles to a magnetic field. The tape, 6 mils wide and 0.5 mil thick, is secured to windlasses A and A1 and stretched tight by the spring held pulley B. At points C and C1 insulated pincers confine the central portions of the tape between windlasses and pulley to form a slit 1 mil wide. Supporting this loop and adjusting devices is a slab of metal with central elevation D, which constitutes the armature of an electromagnet. The central portions of the loop are supported on insulating bridges to lie 3 mils above the face of D ; here the sides of the loop are cen *Technical Service Engineer, Electrical Research Products, Inc. Fourteenth paper of Technical Digest. FIGURE 3 tered over a tapered slot, 8 mils wide by 256 mils long in this plane, opening to 204 mils by 256 mils at the outside face of the armature. Viewed against the light, the valve appears as a slit 1 mil by 256 mils. The electromagnet core has a similar elevation opposing D across an air gap of 8 mils which closes to 7 mils when the magnet is energized from a 12 volt battery. A tapered slot in the magnet core begins 8 mils wide by 256 mils long and opens with the same taper as the slot in the armature. When the assembly of magnet and armature is complete, the valve constitutes a slit 1 mil by 256 mils, its sides lying in a plane at right angles to the lines of force and approximately centered in the air gap. The windlasses A and A1, one of which is grounded, are connected to the output terminals of the recording amplifier. If the magnet is energized and the amplifier supplies current from an oscillator, the duralumin loop opens and closes in accordance with the current alterations. Length and tension of the vibrating part are so chosen that its resonance is at 8500 cycles which puts it out of range of the conspicuous cycles in speech and music. A If this appliance is interposed between a light source and a photographic film we have a camera shutter of unconventional design. Fig. 2 shows a diagram of the optical system for studio recording. At the left is a light source, a ribbon filament 18 ampere projection lamp, which is focussed on the plane of the valve. The light passed by the valve is then focussed with a 2 to 1 reduction on the photographic film at the right. A simple achromat is used to form the image of the filament at the valve plane, but a more complicated lens, designed to exacting specifications by Bausch and Lomb, is required for focussing the valve on the film. The undisturbed valve opening appears on the film as a line J4 mil by 12S mils, its length at right angles to the direction of film travel. The width of this line varies with the sound currents supplied to the valve, so that the film receives a varying exposure: light of fixed specific intensity through a varying slight. (See Fig. 6.) Both the aeo light and the light-valve result in variable density records, and the transmission of the positive print at every point should be proportional to the exposure of the negative at the corresponding point. If that can be accomplished, then we deliver to the photoelectric cell a light the same as it would re ceive had there been no record interposed. Fig. 3 shows a studio recording machine with the door of the exposure chamber open. In this machine the film travels at 90 feet per minute, and the sound track is made at the edge away from the observer. The line of light, the image of the valve, overruns the perforations by 6 mils, extending toward the center of the film 122 mils inside of the perforation line. The right-hand sprocket serves to draw film from the feed magazine above and to feed it to the take-up magazine below; this sprocket is driven from the motor shaft through a worm and worm-wheel. The lefthand sprocket engages 20 perforations and is driven through a mechanical filter from a worm and worm-wheel similar to that driving the feed sprocket. The mechanical filter enforces uniform angular velocity of the lefthand sprocket which carries the film past the line of exposure : the focussed image of the valve. Balancing of the flywheel which forms part of this mechanical filter holds the angular velocity constant to one-tenth of one per cent, despite the imperfections of the driving gears. In Fig. 3 the photograph shows a photoelectric cell mounted inside the left-hand sprocket, which carries the film past the line of exposure. Fresh film transmits some 4 per cent of the light falling on it, and modulation of this light during the record is appreciated by the cell inside the sprocket. This cell is connected to a preliminary amplifier PLANE Of RIBBONS (O.OOI 'X 0., k SUT I (0-0005X0120*) 0 Cens system FIGURE 2 mounted below the exposure chamber, and with suitable further amplification the operator may hear from the loud speaker the record as it is actually being shot on the film. Full modulation of the valve implies complete closing of the slit by one side of the wave of current; this modulation should not be exceeded or photographic overload will abound. Fig. 4 is a skeleton diagram of the studio recording channel, showing the recording amplifiers and the direct and photocell monitoring circuits. It is my purpose here to describe the procedure necessary to render the film as nearly perfect as possible, and produce a satisfactory delay circuit. We ask of the film or any other recording device that it should take the current fed to it and reproduce that without distortion. By that is meant that all of the cur V O C A IL I TE Sonimd Screen BRILLIANT PICTURES — PERFECT SOUND Clear, realistic pictures — Freedom from eye-strain — Natural tone quality — All these are essential to hold your patronage and are assured by Vocalite Sound Screen. THE BEST BY SCIENTIFIC TEST Descriptive Literature on Application BEADED SCREEN CORP 448 WEST 37T" STRftT NY. 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