Motion Picture Herald (Jan-Feb 1937)

MODERN PROJECTION PROJECTION * SOUND REPRODUCTION . ACOUSTICS Maintenance of the Projection System to Insure Good Sound • Examining the relationship between the sound system and visual projection elements and how faults in the latter affect the quality of sound By AARON NADELL with new developments in sound recording and reproduction making a finer motion picture available to the public, and with widespread replacement of older models of reproduction equipment rapidly taking place as a result of these developments, it is timely to consider — in some respects, yet again — the practical operating relationship of the projection mechanism proper, to the attainment and consistent maintenance of sound quality. One of the ways in which projection equipment influences sound quality is in its effect upon the steadiness of motion of the film, as it passes the photelectric cell. The importance of smooth motion of the film at that point in its path can readily be understood by looking at any sound track. One type of track consists of a series of changes in the width of the clear area. The other consists of a series of changes in density. In any good sound equipment the motion of the diaphragm of the loudspeaker keeps step, to the minutest degree, with the progress of these changes in sound track appearance as they pass the photoelectric cell. FILM MOVEMENT If any condition in the projection room causes the film, even for a moment, to move more rapidly than it should, the diaphragm of the loudspeaker will pump more rapidly than it should. Having moved more rapidly than it should, the film, obedient in the last analysis to a constant-speed driving motor, must in turn move more slowly than it should, since the motor will hold it back, and the loudspeaker behind the screen now compensates for a sound too high in pitch by emitting one too low in pitch. At some tones, especially at high ones, the original note may be lost entirely, and replaced by series of other notes higher and lower than the original. At other tones or frequencies, the original note remains in the sound, but is reduced in volume and accompanied by other, spurious tones higher and lower in pitch, which were never present in the original, and which constitute a form of sound distortion peculiarly displeasing to the human ear. This type of sound distortion can be divided into two general classes, which differ from each other in accordance with the rapidity with which film speed changes. One of the most common rates of speed variation is, as will be seen below, 96 per second. On the other hand, film speed may slowly increase over a period of a second or so, slowly decrease over a corresponding period, introducing a different and more recognizable, but equally disagreeable distortion into the sound. To check upon some of the many ways in which projection equipment, as distinct from sound equipment, can cause these forms of distortion, it is only necessary to consider the course of the film as it passes from the upper magazine to the lower The author has been identified with sound and projection for many years. He is the author of a book on sound, and also of the chapters on sound in F. H. Richardson's Bluebook of Projection. — The Editor. one. In any standard theatre projector there are three sprockets — upper, intermittent and lower. The upper sprocket draws the film from the upper magazine. The reel in that magazine rotates slowly at first ; more rapidly as the amount of film remaining decreases, but the upper sprocket rotates steadily, and film passing it theoretically moves at constant speed at all times. SPROCKETS The intermittent sprocket of course does not move at constant speed, but starts and stops 24 times a second. In this portion of its path the film also moves and stops 24 times per second. The lower sprocket again moves steadily and restores substantially constant motion to the film. The loops are left in the film between the intermittent sprocket and the two steadily moving sprockets, theoretically absorb all the intermittent motion. From the lower sprocket the film passes into the soundhead, and thence to the lower magazine. Now in the case of the upper magazine, the film is drawn downward by the upper sprocket, and the reel rotates merely because of the pull on the film, the speed varying with the amount of film. THE TAKE-UP The lower magazine reel must also change its speed of rotation in accordance with the amount of film already reeled up. At the beginning of the run, the diameter of the film and hub may mount to 6 inches, and the lower reel revolves approximately once a second. At the end of the run the diameter of film and hub may be perhaps 12 inches, and the reel revolves approximately once in two seconds. This gradual decline in speed is, of course, effected by means of a friction clutch, which drives the reel as fast as the film, coming down from the soundhead, will allow it to turn. Naturally, any roughness, wear, or defect of lubrication in that friction clutch will make the pull of the lower magazine jerky instead of steady. In examining in detail how this assemblage of mechanism can cause unsteady sound, it is necessary to remember that despite the weight and bulk of the apparatus, it constitutes a machine of extreme delicacy. February 6, 1937 33