Projection engineering (Sept 1929-Nov 1930)

Projection Engineering, September, 1929 Page 9 How and Why the Talkies A Resume of the Scientific Principles Underlying the Art of Sound Pictures By Horatio W. Lamson* THE keen interest shown in the synchronism of sound with motion pictures has led us to believe that our readers \vuuld welcome a resume of the scientific principles underlying this new art. The great majority of us recall, no doubt, how, in the early days of broadcasting, we were frequently entertained by "canned" music, that is, phonograph records broadcast over the air. During those years the electric phonograph and the modern publicaddress systems were reaching an advanced state of perfection. It was only natural, then, that our motion picture producers, always keen for a new appeal to the public, should visualize the combination of an electric phonograph with the public-address system to give a synchronized sound accompaniment to feature pictures in their theatres. All this appears simple, but in reality the present day achievements have been attained only by painstaking and costly experimentation. A considerable amount of credit for this is due to the researches of the Bell Telephone Laboratories and associated concerns. * Engineering Dept., General Radio Company. The Western Electric film recording machine which employs the light valve. Two Fundamental Systems There are two fundamental systems of synchronizing sound : first, that which employs a disc record similar to the familiar phonograph disc ; and, second, that which utilizes an optical record imprinted either along the edge of the motion picture film or on a separate film run in synchronism with the picture. A long series of factors enter into the satisfactory recording and reproduction of sound picture entertainment by the disc process. Taken in sequence these are, in the studio : (1) A skillful rendition of the program by instrumental or vocal artists. (2) A correct treatment of the acoustical features of the studio. (3) The proper location and fidelity of the microphones. (4) A skillful operation of the mixing controls. (5) A high degree of fidelity in the recording amplifiers. (6) The perfection of the electromechanical recording device. (7) Care and attention in preparing the wax record before and during recording. (8) Exact synchronization of recording with photography. (9) The skillful reproduction of playing records from the original "wax." In the theatre : (10) The fidelity of the pickup or electrical reproducer, is a critical problem. (11) Perfect synchronization must be maintained between the reproducer and the picture film projector. (12) Proper use of (he fader is essential. This device serves to shift from one record to the other and likewise functions as a volume control. (13) A high degree of fidelity is required in the reproducing amplifier system. (14) The output control panel must, be properly adjusted. (15) The loud-speaking equipment must li" suitable for the purpose. (16) A study must be made to give the proper acoustical treatment of the individual theatre. Frequency Band In order to obtain a realistic and pleasing reproduction throughout the whole theatre, it is essential that all these factors function to give a true rendering over a wide scale of frequencies and a large range of intensity levels. Good reproduction de Recording equipment for soundon-film system employed in RCA Photophone studios. The microphone at the right feeds into the microphone amplifier, in the black box, which in turn feeds into another amplifier operating the recorder on the table. mands impartiality to frequencies from 50 to 5000 cycles per second. Decreasing the lower limit to 30 or 25 cycles gives a noticeable improvement to certain types of music, while. if the upper limit be Increased to 8000 cycles, an improvement in naturalness and articulation is attained. ''(insider, for example, the specific problem in which the producer desires to furnish his audiences with :i sound record of a large orchestra to accompany a motion picture of Ibis orchestra in action. The members of the Orchestra would be seated in the customary manner in semicircular tiers. Two or more cameras would be