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LOUIS DUNOYER
[J. S. M. P. E.
produces an image on the film of a width which varies according to the amplitude and the frequency of the sound vibrations. Fig. 1 shows an example of constant density recording.
In the variable density process the photographic density of the sound band is the same at all points of its width but this density varies in the direction of the length of the band. The recording system which is used most at the moment consists in letting the film slide by a very fine slit illuminated by a flashing lamp. This lamp contains a gas under a low pressure illuminated by the discharge; the voltage at which this discharge is produced is modulated by the current from the recording microphone, conveniently amplified. The luminescence of the gas follows these modulations which are the more intense the higher the voltage. These modulations, therefore, are transferred to the sound band by corresponding modulations of the density in the direction of the length of the band. Figs. 2 and 3 show two examples of variable density recording. Fig. 2 refers to an invariable musical note (approximately 440 vibrations per second).
In order to reproduce the sounds the entire width of the sound band must be illuminated, but only for a width equal to that of the spot or the slit which illuminated it during the recording; then having passed through the film the light is received by a photoelectric cell. If the photographic
opacities are proportional to the luminous fluxes received during the recording, and if the photoelectric currents are proportional to the luminous fluxes received by the cell, these currents finally will be proportional to the currents of the recording oscillograph or to the brilliancy of the flashing lamp, according to the recording process employed. Then they are amplified and sent into a loud speaker. The distortion of the sound can be due only to the microphone circuit or the circuit of the loud speaker.
Since the light received by the cell should be only the light which has passed through the film in a rectangle 3 millimeters long and a few hundredths of a millimeter high (0.05 mm. at most) much light
FIG. 1. An example of constant density recording.