Ken Cameron, Sound and the Documentary Film, Sir Isaac Pitman & Sons, 1947


The variable-area system of recording sound on film depends for its operation upon the amplified speech signals from the microphone actuating an electromagnetic device which effectively causes a line of light to vary in length, while it remains constant in intensity, according as the signals themselves vary. This line is focused on to the moving film within the recorder, and correspondingly photographs upon that film a trace which varies in width with the varying sound impulses.