[1] For optical recording, the film only had a single row of perforations and the area where the other set of perforations would have been [citation needed] was exposed to a small bright lamp inside the camera controlled by an amplifier (usually in a separate box) that would vary the area of recording (RCA type), by means of shutters pulled back by variation in current, or variations in intensity (Western Electric type).
Editing involved painting or taping over the optical track for the distance the sound led the film through the camera and projector.
Auricon and Cinema Products Corporation were two popular manufacturers of this type of camera, which were made obsolete by the takeover of video for news shooting in the late 70s.
The main problem with placing a magnetic stripe on film is that the tape head cannot block the projector gate and must be slightly offset, which means that sound and light cannot be recorded on the exact same piece of film.
Editing such film by splicing it risks cutting off the current speaker's last syllable.