Projection engineering (Sept 1929-Nov 1930)

Page 24 Projection Engineering, April, 1930 Sound Motion Pictures* The Electrical Engineering of Control Motors and of Synchronism. By Jean V. Kresser IN the days of the silent picture, the production of a motion picture did not depend much upon electrical engineering. The electrical engineer did not have the important position in theatre work which the coming of the sound picture has given him. The talking motion picture owes its success and development mainly to the help it has received from electrical engineering. It is therefore fitting for us, as engineers, to make a brief survey of the different processes involved in the production of sound pictures, from the electrical engineering standpoint. In what follows, I have in mind principally the Western Electric system of recording and reproduction. As you know, the talking motion picture is a development of the electrical, mechanical and chemical sciences. It is a combination of the silent picture and the electrical pickup, amplification, transmission, recording and reproduction of sound. The main requirements of production are the recording of sound in synchronism with a motion picture and the reproduction of the sound in synchronism with the projected picture, so that the actors in the picture appear to be actually talking. The essential parts of a sound recording system are the microphones on the stage, the mixer and volume control in the monitor room, amplifiers, recording machines and a motor system which runs the cameras and recording machines in synchronism and at constant speed. Where the silent picture could be filmed almost anywhere "on location," the talking picture requires, for the most part, the use of specially constructed sets called sound stages. The sound stage is made soundproof by means of double or triple wall construction with intervening air spaces and a covering of a sound absorbing material such as rock wool. The monitor room is made not only soundproof but with its acoustical conditions similar to those of the average theatre. * Paper delivered 'before M.I.T. Branch, A.I.E.E., December, 1929. In this room are the microphone controls, the sound mixer, amplifiers and signal lights. The monitor plays an important part in the production of the picture. It is his duty to place the microphones for the best results and to control the output of these microphones during the action, maintaining best volume balance and over-all volume. He must sometimes show considerable ingenuity in placing the microphones so as to have them in the best places for good pickup and yet keep them and the associated wiring out of the field of view of the cameras. The cameras are installed in pairs in soundproof booths, one camera being used for "close-ups" and the other for "long shots." Several of these booths are placed on the stage, so that the picture is obtained from several different angles. These cameras are all motor-driven in synchronism with the sound recorders. In this way, the other unknown but important factor of production, the film cutter, can use any combination of different angles, long shots and close-ups to enhance the beauty and dramatic effect of the picture without destroying synchronism. Condenser Transmitter The microphone used is of the condenser transmitter type. It is essentially a condenser in which one of the plates is a very thin stretched piece of duralumin which is set in vibration by the sound waves striking it. The capacitance of the condenser is thereby varied and a varying electromotive force is set up in the circuit to which the microphone is connected. The outputs of the different microphones are combined in a sound mixer and from there the varying current goes through several stages of audio amplification before it is received by the recording machine, which may be of the film or disc type. The film recorder consists of a lamp and lens assembly, a light valve and a film drive mechanism. The light valve DISTRIBUTOR FIG.1 Synchronism by electrical interlocking. is an electromechanical shutter and is made of a loop of duralumin placed in front of a slit and at right angle to the field of an electromagnet. This loop opens and closes, the opening being proportional to the intensity of the current flowing in the loop. This controls the amount of light, from a source of constant intensity, which can get through the slit and be projected on a film. The variation in density of the light striking the film is therefore proportional to the variation of the sound current. The image of the light valve aperture appears on the film, in an unmodulated state, as a line 0.12S"x0.001". The disc recorder is somewhat similar to a phonographic recorder. A sapphire or ruby stylus, actuated by the amplified sound currents, is moved radially across a wax disc revolving at a constant speed of 33% r.p.m. If a picture is to be released with the sound recorded on the film, two film recorders are used for the permanent record and one wax disc recorder for playback purposes. The cameras and the recorders do not run only in synchronism and at constant speed, but they also all have the same number of revolutions from start to stop. This is an important feature since it permits the marking of the picture films and sound films for synchronization while stationary and making synchronization possible throughout, even if many stops are made. Constant speed is important since the musical pitch of sound varies directly with the frequency and therefore, in order to have the reproduced sound the same pitch as the original and undistorted, the film both in the recording and in the reproduction must run at an assigned constant speed. The speed chosen is ninety feet per minute at the* sound recorder, corresponding to a motor speed of 1200 r.p.m. The principle used for synchronism of all motors is that of electrical interlocking, first discovered by Michalke in 1901. Three-phase, wound rotors, induction motors of similar impedance characteristics are used. The stators of these motors (Fig. 1) are connected in parallel to a 220-volt, 3-phase supply and the rotors are all connected in parallel by short-circuiting leads. The rotor of one of these motors (called the distributor) is mechanically driven by a d-c. compound motor at a constant speed of 1200 r.p.m. The rotors of all the other motors will run at this same speed due to the strong