Sound Motion Picture Technology

The cameras supplied for this purpose are portable and are driven by a spring motor (see Pl. XIII, fig. i ). The projectors also are motor driven and convenient in use (Pl. XIII, fig. 3), so that amateur cinematography is increasing very greatly in its use and applications. In addition to its employment for personal pic tures and for the recording of travel, it is much used for scientific, industrial, educational, and medical work, special apparatus being available for motion picture photomicrography and for high speed photography. Copies of some theatrical films are available from libraries. Sound reproducing equipments, both film and disk, are available for the i6mm. film. Sound-on-film is generally used and large libraries are available.

Synchronism with Sound.

All motion pictures now shown are accompanied by sound. In fact, the development of the mo tion picture itself by Thomas A. Edison was due largely to his de sire to have pictures to accompany his newly invented phono graph. The practical realization of the synchronized sound and picture records was made possible, however, only by the rapid progress in electrical methods for the recording, reproduction, and amplification of sound. Thus far, two general methods for obtain ing the desired results have been developed. One of these em bodies the synchronization of the motion picture with the sound record recorded in the form of a groove on a composition disk. This was used successfully first in 1926, by Warner Bros. Pictures, Inc., under the name of "Vitaphone." The other general method involves the recording of sound along the edge of the film by photographic means, or the printing along the edge of the motion picture film by photographic means from a sound negative which has been recorded separately. The pic ture area is diminished slightly and a narrow strip approximately I/I oin. wide is devoted to the production of a sound record. Records made by the photographic method may be divided into two classes: (a) variable density records, and (b) variable width records. In making the sound record, the film is drawn at a uniform linear speed past a very narrow "slit" which may be either an actual physical slit placed very close to the emulsion surface of the film, or an optical image of a physical slit or narrow light source placed at some distance from the film surface. In the case of the variable density method, the intensity of the light passing through the "slit" is so modulated that the record consists of a series of striations extending laterally across the sound track. This type of record is illustrated in Pl. XIII, fig. 8.

The sound is picked up by means of a microphone similar in appearance to those used in radio broadcasting stations. The electrical currents are then amplified and used to modulate the light incident on the film. The modulation of the light is gen erally accomplished by either of two methods : changing the brightness of a lamp of the gas-filled, glow-tube type, or by chang ing the width of a variable slit. The records shown at a, b, c, and d, in this figure represent pure sine wave tracks having frequen cies of i oo, 200, 400, and 7oo cycles per second, respectively.

In the case of the variable width type of record, a beam of light from a constant intensity source is reflected from the mirror of a recording galvanometer and permitted to illuminate the slit, either in the form of a rectangular area, whose motion during recording (caused by displacement of the galvanometer mirror) is parallel to the major axis of the slit, or in the form of triangular area whose base is parallel to the major axis of the slit while its motion during recording is perpendicular to this axis. In either case, the motion of the beam and hence the illu minated area is controlled by the amplified electrical currents coming from the microphone. This results in records similar to those shown at e and f in fig. 8, for the two types of areas of illumination.

The manner in which the sound is reproduced in the motion pic ture projector is illustrated schematically in fig. 4. The fila ment of the lamp is imaged by the condenser on a narrow slit as indicated. The objective re-images this slit on the sound track carried by the film. Light transmitted through the film then falls upon a photoelectric cell in which it produces a current whose value depends upon the instantaneous intensity of the light. (See PHOTOELECTRICITY.) As the film travels at a uniform linear speed, the amount of light passing through the film and falling on the photoelectric cell is controlled by the variation in density or by the variation in width of the sound track image. The vari able current flowing through the photoelectric cell is then amplified by the usual vacuum tubes and finally reproduced as sound by means of a loud speaker. In some cases, the objective shown in