Sound Motion Picture Technology

SOUND MOTION PICTURE TECHNOLOGY As the general principles which cover the production of photo graphic images also apply to sound motion picture photography, the article on PHOTOGRAPHY should be consulted. The manufac ture of the film is dealt with under PHOTOGRAPHY.

The Film.

Negative film used in the cameras is supplied in rolls 35mm. (1.38in.) wide and 200 to I ,000ft. long. For special services, where portability and weight of equipment are a large factor, the film is supplied in 200-ft. lengths, but in the case of studio sound motion picture production, the film is supplied in rolls approximately i,000ft. long.

The film used for picture negatives is practically all panchro matic; i.e., sensitized to all colours by means of dyes. The film used for sound track negatives is coated with a positive type of emulsion capable of sharply resolving the high-frequency images present in music and dialogue. The picture and sound are usually recorded on separate films for convenience in developing. In many cases, the sound camera is separated from the picture camera by distances up to half a mile, the picture camera being at the point of action to be photographed, with the sound camera at a point remote from the action, in a central recording building. Both cameras are electrically interlocked to provide synchronism between the picture and its associated sound.

In the case of special services, such as newsreels, the picture and sound are generally recorded on the same negative film using a combination picture and sound camera.

The picture and sound recorded in the studios on separate nega tives are printed on separate positives for convenience in editing, but when the completed sound motion picture is exhibited in thea tres, the sound and the picture are printed on the same film for ease in handling and to assure synchronism. The sound is printed on the film 194 frames (approximately 'din.) ahead of the pic ture to provide smooth and even sound reproduction.

The photographic image of the sound is translated this distance from the picture because the sound must be projected at a point where the film is in continuous movement. Were it projected di rectly opposite the picture, it would be interrupted 1,440 times per minute, which would result in very annoying and unintelligible dialogue and music. The positive film, on which the prints are

made, is supplied in rolls Boo to i,000ft. in length and may be secured in lengths up to 2,000 feet. On the edge of the negative and positive films are perforations by means of which the films are moved through the cameras and projectors. The films are per forated in accordance with rigid standards. Figure 1 shows the standards which have been adopted for international use for posi tive films which are distributed to theatres.

The Camera.

The motion picture camera (see fig. 2) is ar ranged to move the negative film step by step through a film gate and past an aperture, where it is exposed to the light coming through the lens. As the film must necessarily be stationary during the exposure of each in dividual picture or frame, an in termittent movement is installed in the camera. The most common form of intermittent movement is a triangular cam known as "the Lumiere cam," because of its introduction by A. and L. Lumiere, of Lyons, France. The intermittent movement, clearly indicated in fig. 2, is usually ac complished by means of a claw, which is operated by a cam.

The standard speed for taking sound motion pictures is 24 pictures per second, corresponding to a film speed of 9oft. per minute. If the shutter is set at 180°, the exposure of each frame (or picture) is 1/48 of a second. One thousand four hundred and forty pictures are taken each minute and 9oft. of sound negative is exposed and recorded each minute to accompany the 1,440 pictures.

By means of a sprocket, the unexposed film is fed continuously from the magazine in which it is held, to the claw, and after ex posure in the aperture and passage through the gate, it is wound into another magazine by the same or a second sprocket. A rotat ing shutter is fitted between the lens and aperture, and it is so driven as to protect the film from light coming through the lens while the film is being moved by the intermittent mechanism, and to expose the film while it is stationary in the aperture. Practi cally all cameras are driven by electric motors to insure the con stant speed necessary for even exposure and synchronization with the sound record.