Optical sound track playback apparatus and method

Abstract From the Canadian patent

The locally averaged or approximate D.C. value of a variable area optical sound track on a film is first determined, and this information is then used to control the width of a scanning slit at a playback station. The scanning slit width is reduced for small signals and increased for large signals, thereby reducing the amount of non-information bearing area on the film which is included in the scanned area. This in turn reduces the amount of noise from dirt and debris in the non-information bearing area and by reducing, the total amount of light transmitted through the film also lowers the level of quantum noise from the photodetector employed at the playback station.