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Figure 1
between the ribbons and this density was caused to vary with the voice currents but maintained always its constant average.
Under the new system of recording an auxiliary electrical circuit is associated with the lightvalve, so that when the sound currents are small and the ribbons need vibrate over but a very small amplitude they are brought close together and this small vibration almost entirely tills the space between them. Then as the sound increases in loudness, so that the ribbons are required to vibrate with a greater amplitude, the spacing is automatically increased by the electrical circuit, so that it is always just a little more than sufficient to permit this vibration of the ribbons.
This is equivalent to altering the average spacing of the ribbons, so that it is at all times proportioned to the envelope of the sound currents. Now, if we regard the amount of light which passes through the average spacing of the ribbons to the film, we find that this light is considerably reduced during moments of silence or of low sounds, which results in a dark sound print.
No Volume Distortiion
As the ribbons open up for increased sound currents the amount of light correspondingly increases and a lighter sound print results. Since the actual vibration of the ribbons under
Figure 2
<he action of the sound currents has been undisturbed in this process, the amount of change of light which reaches the film and in turn the reproducing photoelectric cell has been unaltered even though the total amount of light has been decreased.
Since the amount of change of light i ■ unaffec ed, there is no volume distortion on reproduced sound as a result of this method of recording.
The extent to which the light-valve ribbons may be closed during quiet intervals is necessarily limited. They must not be completely closed, because it is not possible to construct a device which can instantaneously sample the amplitude of the sound currents and set the ribbons to their proper spacing without introducing expensive delay circuits as auxiliary equipment.
Technique Unchanged
Therefore, in setting up the device, the spacing of the ribbons is reduced to something considerably less than their normal spacing but not as far as complete closure. Furthermore, the latitude of the photographic emulsion is not infinite and also limits the extent to which the closure of the ribbons may be effected without exceeding the straight line part of the emulsion characteristic.
Since this new method contemplates recording over the same part of the film characteristic, and within
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