The international photographer (Jan-Dec 1934)

Twenty-two Th INTERNATIONAL PHOTOGRAPHER November, 1934 MOTION PICTURE SOUND RECORDING (Continued irom Page 5) not sound so very difficult ; but if so, that is a wrong impression. It is a very exacting task that requires a high degree of skill on the part of the recording engineer. When it is explained that the duralumin ribbon is only one-half mil thick and six mils wide, something of the difficulty of handling it will be realized. When a valve is strung, the two ribbon edges that form the sides of the slot must be perfectly parallel and without flaw. Sometimes it is necessary to string a valve three or four times before a ribbon is found that is satisfactory and that will not break when the valve is tuned. Tweezers are used for handling the ribbon ; and it is picked up only by the ends to prevent nicking it. Tuning the Light Valve After it is strung, a light valve is tuned by the aid of a light valve tuning panel. This tuning panel comprises an adjustable audio-frequency oscillator that supplies a tone of predetermined frequency to the light valve, a photo-electric cell, a PEC amplifier, and a valve mounting assembly. The light valve is fastened to an electromagnet on the front of the tuning panel and the ends of the ribbon loop are connected to the output of the oscillator. This electromagnet closely resembles the magnetizing winding of a film recording machine. A light beam of constant intensity from a lamp in a circular lamp house shines through the light valve onto the sensitive surface of the photo-electric cell, which is mounted in a metal box that shields the cell from all other sources of light. The photo-electric cell is connected to the PEC amplifier, the output of which terminates in a volume indicator very much like the one used in the recording circuit. With the light valve in place on the tuning panel, the apparatus is switched on. That causes the electromagnet to be energized and a tone of fixed frequency to be generated by the audio oscillator and sent through the strings of the light valve. The audio-frequency tone may be varied over a wide range by means of a calibrated dial and a three-position key switch, the three frequency ranges being marked A, B, and C. Control of the level of the tone applied to the light valve is made possible by an attenuator marked "String Input." The frequency of the audio-frequency current passing through the light valve strings is adjusted to the value to which it is desired to tune the natural period of vibration of the valve ribbon. A frequency of 8,500 cycles per second is usually chosen for that value — although 7,000 cycles per second was formerly employed — because it places the resonant frequency of the light valve strings well out of the frequency band occupied by tones of the human voice. With the valve in place on the tuning panel and the oscillator generating a frequency of 8,500 c.p.s., the tension on the loop of ribbon in the light valve is slowly increased by turning the screw attached to the spring that holds the pulley in the center of the ribbon loop. When the tension on the ribbon is great enough to cause the frequency at which it resonates to be the same as the frequency generated by the audio oscillator, the amplitude of vibration of the ribbon will attain a maximum value and the meter of the volume indicator will indicate highest reading. An increase or decrease from this resonant value in the tension on the ribbon will cause the reading of the volume indicator meter to decrease. It is at this point of highest meter reading that the valve is properly tuned. The valve is removed from the tuning panel and is ready for use when it resonates at 8,500 cycles per second. But first the calibrated dial of the oscillator is swung over its three ranges to make sure that the valve does not resonate at any other frequency ; or if the valve is found to resonate at some other frequency, this check up is to make sure that the second peak is very much lower in amplitude than the 8,500 cycle peak. If the minor peak is much over ten per cent of the larger peak, the valve is worthless for recording and must be broken and strung again, unless jarring and retuning will eliminate the second peak. After the valve is thus tuned to 8,500 cycles per second, it must be examined again under the microscope to make sure that the one mil spacing of the ribbons over the slot has not varied. This spacing must be precise ; and the ribbons must have no nicks or imperfections on the edges that form the sides of the light aperture, with the two ribbon sides absolutely parallel. This is the most delicate portion of the sound recording equipment, and so it must be handled with the greatest of care. When not in use, the light valves are kept in a bell jar (known as a dessicator) that has a chemical in its lower section to absorb all moisture from the air in the jar. This is chiefly to protect the valves from dust in the air. They are always checked on the tuning panel for resonance at 8,500 cycles and for secondary resonant peaks before use ; and at the same time they are cleaned to remove any trace of dust that may have accumulated. The chapter next month will conclude this discussion of film recording. In it the description of the light valve tuning panel will be continued. The theory of the functioning of the light valve and the procedure followed by film recording engineers during shooting will be explained in detail. Kinetna Krafts Kompany calling AMATEURS and PROFESSIONALS Who want fresh 35mm. EASTMAN or DUPONT Gray Backed Negative. Price— $.02 1/2 per foot 6510 Selma Ave. Hollywood, Calif. GLadstone 0276 Max Factor's MAKE-UP A Easy to Apply --Easy to Remove ^ Stays on All Day Photographically Correct MAX FACTOR'S MAKE-UP STUDIO • HOLLYWOOD Please mention The International Photographer when corresponding with advertisers.