A band at one side of a cine film on which
the accompanying sound is recorded. Usually
it takes the form of an optical track (a
pattern of light and shade). The pattern is
produced on the film when signals from the
recording microphone are made to vary the
intensity of a light beam. During playback, a
light is shone through the track onto a
photocell, which converts the pattern of
light falling on it into appropriate
electrical signals. These signals are then
fed to loudspeakers to recreate the original
sounds.