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CONTROL TRACKS AND STEREOPHONIC SOUND 577
mechanism for the clumsy cue sheet. It effects automatic changes in the amplification of the theatre system, and thus achieves volume ranges of from 75 to 100 db although there is only a 25 db range in the soundtrack.
(5) The basis of such automatic volume control is a control track — a second soundtrack, recorded on film — which carries control signals, not sound. These control signals take the form of sound frequencies which operate through a second and independent photoelectric cell. The output of that second photoelectric cell is amplified separately from the sound. It is then rectified — converted to d.c The d.c. thus obtained is filtered and applied to the grid of one of the tubes of the sound amplifier, modifying the grid bias of that tube, and hence increasing or decreasing its amplification. Thus the control track governs the amount of amplification of the theatre's sound system, in accordance with the degree of volume desired by the studio that made the film.
(6) The control track also can be used to create the illusion that the source of sound is moving in accordance with the motion of the apparent source on the screen. For in control track-stereophonic sound systems there is more than one bank of speakers behind the screen, and there is more than one set of amplifiers. By changing the amplification in the different sets of amplifiers, the control track can cause the volume of sound to increase ' in the left-hand bank of speakers while it declines in the right-hand bank of speakers, thus creating the illusion that the source of sound has moved to the left.
(7) 'The control track may be located in the sprocket hole region of the film; different types of track have been designed for such location. It may be a very narrow track placed between the soundtrack proper and the picture. Or it may be located, along with the soundtracks, on another film altogether — a control film without pictures, which is operated through a separate soundhead in synchronism with the regular film.
(8) Fig. 216 depicts a sprocket-hole type of control track, and the sound system through which it operates. The track is shown at the right, but the drawing does