International photographer (Jan-Dec 1934)

Six The INTERNATIONAL PHOTOGRAPHER December, 1934 5 fyfSJ" , ' The "light valve,'' that delicate device that makes possible film recording by the Western Electric System. Courtesy Paramount Productions, Inc. iga JSJl-I^ tne preceding chapter of this series, the con^3-1/*'' 3 structional features of the lit/lit valve — the deli^S^sl cate device that makes possible the recording of I'^y.. .'cSai soun{} on f]im by tne Western Electric sound recording method — were discussed in considerable detail. The related operations of stringing and tuning the light valve were also described. The present chapter continues with the description of the light valve tuning panel, that complicated piece of apparatus to which light valves are connected to permit raising the resonant frequency of the metallic ribbon with which they are strung to 8500 cycles per second. That operation was described in Chapter XIV; but the appearance of the tuning panel and the use of the various control on it were not considered in very great detail. The Light Valve Tuning Panel Besides the volume indicator meter that was mentioned last month, there is a potentiometer in the volume indicator circuit that likewise is mounted on the tuning panel. This potentiometer allows adjustment of the grid bias voltage applied to the volume indicator tube to the value that provides a five-division reading on the meter scale when there is no a-c. excitation. The a-c. excitation that causes a higher reading of the meter is furnished by the audio oscillator ; and it is greatest when the oscillator is tuned to the frequency at which the light valve in place on the tuning panel resonates. This preparatory adjustment of the meter pointer to the five-division mark must always be made before a valve is tuned. There are a number of other meters and controls mounted on the light valve tuning panel, including rheostats and ammeters in series with the oscillator tube filaments, the PEC amplifier tube filaments, and the sound lamp filament. This sound lamp provides the light of constant intensity that shines between the ribbons of the light valve onto the sensitive surface of the photo-electric cell on the tuning panel. There are also the two switches that turn on and off the battery supply and the input to the light valve strings. The dial of the frequency control is marked directly in frequencies of cycles per second ; and, as was mentioned last month, just below the dial there is a three-position key switch that provides a selector for the three settings that extend the range of the oscillator well above and Motion Picture Sound Recording Chapter XV below the 8500-cycle frequency to which light valves are tuned. Inspection of Valves Necessary After it has been tuned, it is necessary to examine the light valve under a microscope again to determine if there has been any variation throughout the length of the slot in the one mil spacing of the ribbons. If there has been any divergence from this value, it is necessary to readjust the spacing of the ribbons by means of the pincers on the valve, and then to re-check the valve frequency on the tuning panel. The light valve must also be examined while it is under the microscope for nicks or imperfections in the edges of the ribbons where they form the sides of the slit and to make sure that there are no bits of fuzz or particles of dirt in the slot ; for any of those things would cause streaks in the sound track produced by the valve. Specks of dust and dirt can be removed by a fine camel's hair brush ; but the only thing that can be done with a valve that has a nicked or irregular ribbon is to break and restring it with new ribbon. The slot in the core of the electromagnet must likewise be examined for particles of dust. This is accomplished by placing a small light inside the exposure chamber and observing it through the slot by means of a powerful magnifying glass. The eyepiece of the microscope is usually used for this purpose. The recording lamp has to be removed from its socket to make room for the head of the observer for this examination. All of the operations described in this section have to be carried out each time a light valve is to be placed in service on a recording machine. If a poor grade of ribbon is encountered when stringing light valves, it is sometimes necessary to re-string and re-tune a light valve as many as a half dozen times before it is satisfactory. Stringing valves is hardly a task for a man lacking in patience. Theory of the Light Valve The principle on which the light valve operates is the same principle as that which causes an electro-dynamic (or "dynamic," as it is often called) loud speaker to function ; but in the light valve, the loop of duralumin ribbon takes the place of the speech coil and diaphragm of the dynamic speaker. The speech current from the recording amplifiers is fed to the loop of ribbon in the light valve through a repeat coil, which is necessary to match the 500-ohm output impedance of the bridging amplifier to the four-ohm impedance of the strings in the light valve. The position of the repeat coil (which is nothing more than a stepdown audio-frequency transformer, comprising primary and secondary windings of wire on an iron core) with respect to the film recording bridging amplifier and the light valve will be apparent from Figure 1. When the light valve is fastened in position by the two knurled screws to the electromagnet core of the recording machine, the ribbons of the valve are suspended in a plane that makes a right angle with the plane of the Transmission. Li f Kim ouTpur of 500 <_o— . ^A-'-ilak+tolverfbto Liqht Valve switch. britfai'n<] AmblihVr j gj ■( 0 ? (+o protW Valve dun'noreliersjls) Figure I. Repeat coil and light valve. Please mention The International Photographer when corresponding with advertisers.