Radio Broadcast (May 1928-Apr 1929)

TECHNICAL DATA SOUND MOTION PICTURES BY CARL DREHER Sound Attachments for Standard Picture Projectors IN THE March issue of this department the general construction and operating principles of standard motion-picture projectors were described. Nothing was said in that article about the motive power for the machine. This is normally an electric motor drive through a system of gears on the left side of the head, a flywheel being provided to steady the speed. The projector may also be operated by means of a hand-crank from the right side, as shown in the accompanying cut (Fig. 1) of a Simplex (International Projector Corporation) machine, where the crank is seen with the handle over the lower magazine. This is feasible only with silent pictures, and then the crank is used only in the rare event of failure of the driving motor. When sound pictures are projected the speed must be kept constant at 90 feet per minute (24 pictures per second) with a regulation of about 0.2 per cent, in order to hold the pitch. Unless an a.c. source with reliable frequency control is available this requires special speed stabilizing circuits of the type described by H. M. Stoller, "Synchronization and Speed Control of Synchronized Sound Pictures," in the Transactions of the Society of Motion Picture Engineers, Vol. XII, No. 35. A synchronous motor operating on a constant frequency a.c. supply is the simplest means of securing the proper speed, and can generally be used in large cities. In projecting silent films a variable speed control is desirable, which means the addition of another motor if a synchronous motor is used for the soundpicture drive. The lower sprocket of the projector, in the absence of a sound attachment such as is described below, delivers the film to the lower magazine, where it is wound up at a constant rate, as the diameter of the roll increases, by a device known as the "take-up." This is usually an arrangement utilizing a split pulley and tension spring to allow loss of speed as the reel is filled up with the film, with a constant speed drive. The sound-head attachment shown in Fig. 2 below the picture head is that of the Movietone type, which has been widely illustrated. The optical principles have been described in October and November, 1928, Radio Broadcast. The principal problem in the sound head is that of preventing the intermittent movement above from influencing the continuous motion of the film past the point where the sound is taken off. This is accomplished in the mechanism shown, by means of a sound-head sprocket which revolves at constant speed (special mechanical filters to smooth out pulsations usually being applied) and drags the film through a gate similar to the picture gate, containing a spring tension pad which holds the film firmly as it slides through. This sound gate presents intricate problems to the designer. The tension pad must not scratch the film or take off the emulsion; on the other hand, it must not allow the film to vibrate or buckle in the sound gate, even when it has been subjected to the heat of a high intensity arc in the picture gate a few inches above. As soon as the film is allowed to move out of the plane of focus in the sound gate, the output quality deteriorates — high frequencies drop out, "fuzz" and extraneous tremolos come in, etc. This is because the sound track, instead of passing through a sharply defined rectangle of intense fight, the dimension of which in the direction of film motion is less than the wavelength, on the film, of the highest frequency to be picked off, runs instead through a relatively wide spot with irregular edges. It must be remembered that at the standard sound film speed of 18 inches per second, a 6000-cycle note, for example, is recorded in a space of 0.003 inch for each oscillation, and it does not take much to spread the light beam, which is designed to cover 0.001 inch, so that it will overlap more than one peak or line of the record. Much, therefore, depends on the construction of the sound gate. As shown in the figure, idler rollers are provided above and below to further control the motion of the film. Synchronism is maintained by setting up the film with the proper loops, so that when a given picture is at the picture aperture the appropriate portion of the sound track will be at the light aperture in the sound reproducing section of the projector. An error of one or two frames is allowable; beyond this the defect in synchronism becomes noticeable to observers. The proper separation of the picture and sound elements is taken care of in the printing of the film, the sound preceding the picture (since the sound head is below and a given point on the film reaches it after it has passed the picture aperture) by such an interval (19 picture frames, or about 14.5 inches) that scene and sound are projected simultaneously. The Technique of Wax Recording HALSEY A. FREDERICK'S paper on "Recent Advances in Wax Recording," printed in the Transactions of the Society of Motion Picture Engineers, Vol. XII, No. 35, 1928, contains material of much interest, not only to the sound-motion picture specialist, but to students of applied acoustics in general. Mr. Frederick is an engineer of the Bell Telephone Laboratories. His paper is concerned with lateral cut records, in which the groove is of constant depth and undulates about a regular spiral on a flat disc. After some preliminary discussion, the author refers to a curve, here reproduced in Fig. 3, which shows a typical frequency characteristic for a commercial electromagnetic recorder. The response is allowed to fall off below 250 cycles as a commercial compromise between quality of reproduction, the necessity for getting a certain amount of music onto a disc of reasonable size, and the amount of energy in the form of sound oscillations which it is desirable to get off the record when it is played. The intensity of the sound, in playing a lateral cut record, is a function of the velocity imparted to the needle, and that is the product of the amplitude of the oscillation and the frequency. With the characteristic shown in Fig. 3 constant velocity is secured from about 250 to 5500 cycles, with constant amplitude below 250 cycles. Were the amplitude to be increased below 250 cycles to keep the output constant, the cutter would break Close-up view of the picture head of a Simplex motion-picture projector • april, 1929 . . . page 385 •