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CONTROL TRACKS AND STEREOPHONIC SOUND 579
that the output of the ordinary soundtrack is divided between two hanks of amplifiers, which in turn drive three banks of loudspeakers. One amplifier drives the
center bank of speakers; the other amplifier drives the right and left wing .speakers. The rectified 96-cycle output of the control track photocell is applied equally to both banks of amplifiers; but the grid bias of the tube to which that output is applied is not the same in both amplifiers. It is so adjusted that when the control voltage is at minimum (maximum transparency between sprocket holes) there is no effect at all. Under these conditions the wing speakers are silent. As the control voltage increases (decreasing transparency between sprocket holes) the wing speakers begin to play; at maximum control voltage all three sets of speakers are working at their maximum volume.
(10) The stereophonic effect of this system is limited to an apparent increase in the width of the source of sound; but even this small difference is very impressive when a full orchestra replaces a soloist on the screen, and in similar conditions. The sound acquires a marked increase in depth and "presence," and the increase in volume range over that of ordinary reproduction adds to the dramatic value of the results.
(11) Fig. 217 presents a more elaborate system. The control track is located in the sprocket hole region, but in this case it consists of a variable density recording of three different control frequencies. In reproduction the 96-cycle frequency unavoidably produced by the sprocket holes is filtered off and discarded, and the three control frequencies are further separated from each other by frequency filters. Each then is separately amplified and rectified to d.c. ; and each controls the amplification of one of three separate sets of sound amplifiers.
(12) There are two soundtracks (each half the normal width) which occupy the usual soundtrack position on the film. Each plays into its own half of a dual photocell. Thus there are in this system, in effect, three photocells ; one for the control track, and one double cell for the two soundtracks. One of the soundtracks actuates the