The Exhibitor (Nov 1938-May 1939)

by supplying equipment or facilities for testing that equipment — was prepared for the guidance of the engineering departments of the producing companies participating in the Research Council cooperative technical program, and the engineering departments of the associated companies affiliated with these producing companies, in purchasing new sound projection equipment. Accordingly, the report represents theatre equipment studied from the standpoint of its ability to take from a celluloid sound track that which Hollywood technicians and artists have placed on it and to reproduce it in its fullrounded naturalness. Any manufacturer’s equipment which can fulfill the recommendations set forth in the report should, where a theatre has been acoustically studied and treated, give satisfactory performance. AM PAS RECOMMENDATIONS IT HAS BEEN FOUND in general that the currently manufactured speaker systems, consisting of a true multi-cellular high-frequency horn and a dynamic low-frequency unit, with a substantial air column, give a considerable improvement in the quality of reproduced sound over that secured in the past. However, certain differences exist between the present systems which make it difficult for the recording studios accurately to determine the average characteristic of theatre reproducing systems. While these differences may not seem to the casual observer to be of great magnitude, it is felt that the existing differences in studio product cannot be materially reduced until such time as the loud speakers appear more acoustically alike to the trained observer. The information below has been compiled by the Research Council of the Academy of Motion Picture Arts and Sciences, Gordon S. Mitchell, manager. The report, containing the following general specifications, has been divided for convenience into five parts. Sound Head and Associated Equipment THE SOUND HEAD shall be so designed that the noise level, due to vibration of gear and drive equipment, will be sufficiently low that the overall system, of which the sound head is a part, will meet the hum level, described in a later specification. Combining the photoelectric cell and optical system on a sin BETTER MANAGEMENT gle vibration-proof mounting plate, mounting the drive motors in a shockproof assembly, the use of adequate shielding of the photoelectric cell transformer, and other electrical equipment in the sound head, and the use of a static shield to reduce static pick-up by the film and drum are examples of good design to minimize hum and electrical noises in the sound head. The illumination in a lateral direction across the scanning slit shall be such that the maximum variation in the output of the photoelectric cell will be less than plus or minus 1 /z decibels when measured with a track consisting of nonoverlapping increments, each of not more than seven mils width. Regulation equipment shall be available so that when the proper power line voltage changes 10 percent, the overall gain of the system — including the exciter lamp, photoelectric cell, amplifiers, and horn fields — shall not vary more than 2 decibels as measured in terms of the acoustical output from the speakers. The weave in the sound head shall be reduced to the minimum. THE DESIGN of the head shall be such as to avoid all possibility of damage to the film, in both picture and sound track areas. At no point in the travel of the film through the picture and sound heads should be sound track area, either emulsion or celluloid side, come in contact with any part of the equipment. All idler rollers should turn freely and the shoulders on both sides of all rollers and sprockets should be relieved. The rollers and sprockets of the projection head and the rollers of the fire trap should also be relieved in the sound track area. Consideration should be given the possibility of guiding the film through the fire trap to avoid scratching the film. In the design of the sound head, consideration should be given the location of the electrical and optical equipment to minimize difficulties due to oil leakage from sound and picture heads. The use of oil-proof wiring throughout the sound head has been found to be very desirable. The design shall include provision for an easy method of obtaining a maximum push-pull cancellation. The balance should be extended as far as possible over the entire frequency range, from 5 0 to 8,000 cycles, with a maximum cancellation of 2 5 decibels below 1,000 cycles a second. The track location and scanning width shall be set according to the Academy specifications. Fly wheels, or equivalent device, shall be used to limit starting acceleration. The total flutter (the sound produced when film passes the scanning light at other than the constant rate of 90 feet a minute) shall be less than 0.15 of one percent, as measured by the ERPI flut BM-5 ter-measuring instrument, or less than 0.30 of one percent as measured by the RCA flutter measuring device. Volume Control Equipment ADJUSTABLE GAIN CONTROL of at least 5 0 decibels, in two-decibel steps, shall be provided, of which at least 16 decibels is available at the changeover position on the wall. The wall attenuator is required at the change over position, because of the great difference in level between news reels and "Hi-Range” prints. The additional volume control shall be incorporated in the main amplifier, to be used to compensate for any abnormal conditions which may rise. Balancing facilities shall be provided to allow the output of each machine to be balanced within one decibel. Amplifiers and Filters THE OVER-ALL GAIN of the system shall be sufficient to provide 20 decibels in excess of normal requirement. The complete electrical system shall be capable of a frequency response varying not more than plus or minus one decibel in the range from 40 to 10,000 cycles a second. At its rated output, the amplifier shall not generate more than two percent total harmonic in the frequency range from 5 0 to 5,000 cycles a second. Amplifier output is the average power into the specified resistance load when the amplifier is excited with sinusoidal input signal. The harmonic content is defined in terms of a ratio of currents between the root mean square sum of all harmonic components and the fundamental. THE LOW-PASS FILTER shall be designed as to obtain the standard electrical characteristic as specified by the Research Council. It is anticipated that changes in the electrical characteristic will be necessary from time to time, and for this reason the low-pass filter shall be adjustable over a wide range. The low-pass filter shall be inserted in a position in the circuit such that it is not subject to extraneous pickup and will be capable of filtering out hiss and high-frequency disturbances generated in the early stages of the amplifier. Under operating conditions the residual hum due to frequencies below approximately 3 00 cycles a second shall not be greater than minus 3 5 decibels/0.006 watts and the high frequency noises shall be sufficiently below this value so that the machine, running without film, will have an output noise which is principally hum and no greater than minus 3 5 decibels. This specification has been so set up that no hum will be audible in the front row of seats in the theatre auditorium under normal operating conditions. r lu December 14, 193 8 TRI