The traditional medium for carrying a film's soundtrack. The sound was converted to markings which were developed onto the edge of the film. The majority of optical soundtracks were analogue (in the form of squiggly lines, like the track on an LP). However Dolby Digital is also carried on the film in optical form. Rather than replacing the analogue soundtrack (which is left on for broad compatibility), the Dolby Digital data is optically recorded between the sprocket holes on the edge of the film.