An unwanted effect which can occur when two or more (sound) signals present in an amplifier modulate one another, giving additional outputs which are the sum and difference between the signals. In particular, a type of distortion peculiar to variable area optical recording.
When the modulated light beam from the recording galvanometer is reflected onto the film emulsion, the image of each wave is not confined to the area exactly under the beam because the crystalline structure of the emulsion allows the light to scatter and the image spreads slightly (hence the phenomena has come to be termed ‘image spread’).
This raises the transmission of the negative, and produces lower-frequency components corresponding to the envelopes of groups of waves. The result is a very unpleasant type of distortion especially noticeable in sibilants, which sound as if two pieces of sandpaper were being scraped together.
1969, The Focal Encyclopedia of Film and Television Techniques, Focal Press, London, New York