Boxoffice (Apr-Jun 1939)

PRACTICAL DISCUSSIONS ON THE MECHANICS OF PROJECTION AND SOUND A Mechanical Detective on the for many months yet all have an important bearing on flutter. Since flutter is easily the most elusive, deceptive and confusing of the enemies of sound quality, its control is possible only through a measuring device which responds only to flutter, which is sensitive enough to evaluate the minute magnitudes involved in the performance of high quality equipment, and which gives numerical data of good accuracy regardless of which of the many types of flutter predominates in the individual case. Trail ol Flutter By E. S. Seeley* MNQUIRIES received since the announcement of Altec’s portable flutter bridge indicate a widespread interest in the manner in which the diagnosis and measurement of flutter, that most elusive enemy of good sound reproduction, is being handled by the use of the new flutter bridge. The elusiveness of flutter stems from the fact that it is caused by extraordinarily minute irregularities in the film speed resulting from defects or wear in the driving mechanism. As a result, flutter has often been an unsuspected cause of bad sound that seemed to point to other adverse conditions in the equipment. Let us make a medical analogy. A patient complains to his physician that his eyes hurt him. The physician examines the patient’s eyes, finds a diseased condition, and prescribes a course of treatment. The patient’s eyes, however, refuse to respond to the treatment, whereupon the physician, by further diagnosis, discovers that the eye condition was caused *The author here personally demonstrates the use of the new portable flutter bridge as it is actually used in the projection room. Mr. Seeley, whose contributions to the solution of electrical problems in sound reproduction are well known, is on the engineering staff of Altec Service Corporation. wholly, or in part, by an apparently unrelated diseased condition in the teeth or tonsils. Correspondingly, the problem of diagnosing bad sound in a theatre lies in ascertaining what proportion of the poor quality is the result of flutter, and wnat proportion is caused by other factors. In other words, flutter must be measured. Pussy Magnitudes There is nothing strange about the fact that flutter measurement is only now becoming a part of the engineering lore or the service technician. We are dealing here with fussy magnitudes. A nutter amount of about two parts in one tnousand represents excellent performance, three to four parts constitutes acceptable performance, and an excess of five parts in a thousand is intolerable — provided the system is otherwise capable of good quality. Flutter of many times these magnitudes produces no preceptible effect on instruments used to measure other factors affecting sound quality. And this diabolical agency usually appears in so effective a disguise as to be mistaken frequently for harmonic distortion, recording irreguiari.y or just “the house.” Flutter stands squarely in the way when efforts are made to improve quamy by correcting the frequency response of a system to that required for best results for tne speaker equipment and the particular auditorium. While it may have gone unnoticed before the other improvements were made, excessive flutter will usually nullify completely any benefits accruing from the improved characteristic. Control of flutter, then, is important in two general respects. When attempting ic modernize sound quality from equipment of earlier design, it is imperative that ihj first step be a reduction of flutter to or below the acceptable amount. After an older equipment has been correctly treated to produce sound of good quality, or when a modern high quality sound system is involved, flutter must be given periodic attention as this factor — of all those affecting quality — most readily degenerates. Condition of sprockets, pressure pads, bearings of film driven rollers or flywheels, film shrinkage, methods of threading film — none of these remains constant New Angle of Approach To bring flutter under control, in the past, leading manufacturers developed and installed what are known as flutter bridges. During 1937, members of the present engineering department of Altec decided to attack the flutter bridge problem from an entirely new angle. Selecting a new principle of flutter measurement capable cf greater sensitivity, then increasing sensitivity by design methods to the highest point consistent with measurement precision, it was shown mathematically that a measuring instrument could be built without the use of vacuum tubes, heterodyning circuits, demodulators, amplitude limiters and other complex and temperamental circuit structures which made the existing flutter bridges unsuited to field use. When the new flutter bridges were made, they entirely vindicated these principles. They weigh less than nine pounds, and are dimensioned approximately to the size of a portable typewriter case. The sensitivity is adequate to accommodate any reproducing system, and the accuracy is better than required. Containing no delicate parts, and ruggedly built, the ability of the new flutter bridges to stand the gaff in actual dayto-day use was demonstrated by a preliminary field trial in which an instrument in constant use traveled over 20,000 miles in six months in an engineer’s car. Retest of the instrument after this trial showed no change in calibration or serviceability. Flutter Factor Measured The special films to be used with these flutter bridges, produced by M-G-M at Culver City and by Fox Movietone and Eastern-Studios in New York, presented a serious problem because of the very severe flutter-free requirements placed on them. Not only was it necessary that this (Continued on page 72) Have You a Problem? Conductors of the CINE' CLINIC will be glad to answer questions relating to problems of projection and sound operation and practices through these columns. State your problem clearly, detailing symptoms of trouble. Answers to questions of general interest will be published in a later issue, identified by inquirer’s initials only. Should you desire private information or advice, enclose stamp for reply. Address The MODERN THEATRE, Room 334, 332 S. Michigan Ave., Chicago, 111. The MODERN THEATRE SECTION