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176 THREE-DIMENSIONAL MOTION PICTURES a standard system does not have all of the tone color and spirit of the original because the volume or dy namic range has to be compressed when the recording Is made. Physical limitations of the recording system make compression necessary. The smallest modulation that it is practical to record on a soundtrack is limited by the film background noise. The limit is reached when the ratio of signal to noise becomes so small that noise becomes perceptible in reproduction. The largest modulation that can be recorded is 100 % when the peaks of the recorded wave extend to the very edges of the soundtrack area. The dynamic or volume range between these two limits is approximately 35 decibels; whereas the actual range of a symphony or chestra is approximately 70 decibels. Thus a means must be provided for restoring the 35 decibels that were lost because it was necessary to compress the range in the recording process, If it were possible for the projectionist to twirl the volume control knob rapidly enough according to a complete set of cues, he could continually boost or re duce the gain by just the right amount at the right time and thus compensate for the compression intro duced during recording. However, because of the in numerable changes required, this is too tough an assign ment, hence some sort of automatic means must be used. In the Fantasound system this means is the con trol track and the variable-gain amplifiers which are inserted in each amplifier channel. The control track was recorded after the final re- recordings of the program tracks were made in ap proximately the following manner. The output of an oscillator generating a single artificial tone, let us say 250 cycles, was connected both to the input of the con trol track recorder and to a variable-gain amplifier in a standard reproducing system. As each recording was played through the reproducing system, a musician con trolled the output of the oscillator in accordance with