Some Working Characteristics of the Colortek System

The Audio Spectrum. Theoretically, it appears desirable to reproduce the entire audio spectrum, although in reality this will be limited for some time due to available equipment. Normal theater transducers are deficient at both ends of this spectrum. The new 288-G series Altec high-frequency drivers have corrected the deficiency in the top-end response. But there is no easy answer to the low-end problem, due to the design of the universally used bass cabinet (which cuts off steeply below 40 Hz), and the desirability persists to reproduce the low-end frequencies as well as the rest of the spectrum. The only dramatic use of low frequencies in recent times was made by the Sensurround system. As an integral part of the Colortek system, components have been developed specially for the better reproduction of low frequencies. They form a third speaker network in the range from 20 to 80 Hz to cross over with, and to be complementary to, the normal theater system. This component, called Veritone, remains in the circuit permanently, forming part of the overall loudspeaker system. Because these frequencies are non-directional, only one Veritone system is required regardless of the number of audio channels in the theater. It has been found to bring out the low end of older sound recordings, as well as to produce a sensation of "feeling" when the tracks are appropriately dubbed. When the earthquake sequences of San Francisco were played through a Veritone installation at the SMPTE Conference in the Academy of Motion Picture Arts and Sciences Theater, the auditorium literally shook.

Sound Recording Negative Materials. The sound recording negative in general use in the U.S.A. is the Eastman fine grain sound recording film 5375. Its high-frequency resolution is limited. Another black-and-white negative that has a re­solving power almost an order of magnitude greater is the Eastman high-contrast panchromatic film 5369. Doubts were ex­pressed concerning its capability of exhibiting satisfactory sound-recording image spread so as to yield a good print with low crossmodulation performance. Initial tests gave very encouraging results, and further thorough evaluation proved that 5369 yields a vastly superior high-frequency recording, Figure 23 shows comparative data for these two types of sound-recording films together with data for three others, Using a normal-speed contact printer, 20 kHz was well and consistently reproduced from a 5369 negative. As a rule of thumb, this material should be used when it is desired to have a good high-end response beyond 10 kHz and when the added negative cost can be justified. Measurements on a Westrex RA 1 251 reproducer gave a 6-dB improvement at 1 5 kHz with this material. It has a panchromatic emulsion and a clear base and re­quires handling in complete darkness. The clear base calls for an adjustment in the printing filter pack due to its increased transmittance over the normal gray base.

Conclusions
The Colortek optical stereophonic sound film system offers a novel approach to four-track variable-area recording and applies CCD video technology to the readout of the soundtrack, within a totally integrated sound system. Although a small modification of the projector sound head is required to realize the full potential of this system, the increase in sound quality and performance and the economics inherent in a single-print-type inventory will justify the change. The progress realized with this system should satisfy, it is hoped, sound reproduction requirements for years to come. The technique can be extended to the 16mm and super 8mm formats.

Acknowledgements
Many individuals and organizations, too numerous to be listed in full, have contributed by lending equipment and facilities and by their invaluable advice and comments to the development of this new sound reproduction technology. To all of them the authors wish to express their sincere thanks, especially to the Academy of Motion Picture Arts and Sciences, the Burbank Studios, the Christie Electric Company, Consolidated Film Industries. DeLuxe General Laboratories, Metro-Goldwyn-Mayer, 20th Century Fox. United Artists and Westrex.

1. John Mosely, "Quintaphonic Sound,"' SMPTEJ. 86, 20-29, Jan. 1977.
2. John G. Frayne, "A Compatible Photographic Stereophonic Sound System," J. SMPTE. 64, 303-308, June 1955.
3. "A Comparison of Tlhe Attack Behavior of Telcom, dbx, and Dolby A." Wermuth and Senf.AEG Telefunken. July 1976.