A photographic motion picture soundtrack in which the audio signal is represented by a line of varying width running the length of the film.
With a variable area sound track in its simplest form, i.e. where there is no sound (silent passages), one half of the track is opaque and the other half is transparent, with a straight line down the middle dividing the opaque and transparent parts. When sound is recorded there, the centre part moves from the centre, from side to side, producing a sawtooth type configuration, the shape of which is in accordance with the sound being recorded.
In the reproduction the sound track flows continuously past an illuminated slit which runs across the track from one side to the other and is typically 1/1000” high (0.025mm). As the sawtooth configuration passes this slit with the light shining through, at the point where there is no sound recorded, the light is blocked from one half of the slit but goes through the other half. When the sawtooth configuration comes up there will be points at which nearly all the slit will be obscured and other points where nearly all the slit will be clear.
As the film flows past the slit there is a constant change of light passing through the slit. The light falls onto a photo-electric cell which reacts to the changes in light by producing a constantly changing electric current. This current is amplified through an amplifier and then reproduced as sound in loudspeakers.