WHEN IT was decided to round out the Bell Telephone Laboratories' investigations in the stereophonic field by studying its peculiar problems of recording, motion-picture film was chosen as the recording medium. A 2000-foot reel will hold twenty-two minutes of recording, enough for nearly any single piece of music. Records on film, as on other media, have a limited range of volume. Steps therefore had to be taken to reduce the louder sounds before recording and to increase those sounds by a like amount in reproduction.

Sound current from each of three microphones is amplified and recorded by a light valve as a track of variable width on a moving film. Before it reaches the light valve the sound current is measured automatically and instantaneously, and whenever it exceeds a predetermined amount, loss is inserted so that the recording current never exceeds a certain level. One product of the measurement is an alternating current whose amount is proportional to the inserted loss. The three such currents resulting from the three sound sources are recorded as a separate and additional track on the same film along with the tracks representing the three sound sources. This fourth track is known as the "control track."

In reproduction, the film passes through light-beams, one for each track. Modulated by the sound record, each beam passes into its own photoelectric cell, from which an electric current, which now represents the original sound source, flows through amplifiers to the loud speakers. Simultaneously the re-created control currents are used to vary the amplification of these sound currents so that the sound regains its original volume.