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In 16-mm. one set of perforations is omitted to make space for the optical sound track. The film on the left has a variable density track, that on the right a variable area track. In either case the track is produced by modulating the width or the intensity of the recording light by means of the impulses generated by the sound to be recorded.
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as the picture and subsequently it cannot be lost, muddled or played out of step with the picture.
The original sound record is made by shining light through a narrow slit on to a photographic film. Variations in electrical pressure delivered by a microphone are applied to a light valve. This controls the amount of light exposing a narrow band on the photographic emulsion as the film moves past the slit. After development, the film carries a sound track letting more or less light through at different points along the track.
In a variable density recording, the width of the track is constant, but the opacity or density varies from point to point.
A variable area recording, on the other hand, produces a transparent track of varying width, bordered by two opaque bands, straight on the outsides.
Both types of recording, variable area and variable density, can be reproduced in the same way. A narrow slit of light again shines on the film and the proportion passing through depends on the density or width of the track. This transmitted light falls on a photo-cell which passes an electrical current proportional to the light falling on it. So as the film passes rapidly past the slit of light scanning it,
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