SOUND MOTION PICTURES. Since the first days of motion pictures efforts have been made to provide the addition of an appropriate sound accompaniment or speech, accurately syn chronized with the motion picture, which would add realism and artistic value to the performance. The requirements of such a system are that the picture shall be as clear as heretofore, that the sound produced shall be appropriate and reasonably natural in quality and that the synchronism, or exact time relation, between the pictures and the sound shall be maintained, preferably by automatic means. (See MOTION PICTURES.) The reproduction of sound may be accomplished either by means of a phono graph record or from a sound record on the motion picture film itself that may be made in several ways; e.g., it may be a long narrow band or "sound track" of variable density of developed silver deposit, the density variations corresponding to the wave outlines of the sound to be reproduced ; or it may be in the form of a long narrow band, the width of which at any point varies as the wave form of the sound to be reproduced. It has also been suggested that the sound record on the film might be a mechanical indentation or wavy line impressed into the celluloid or the emulsion, in which groove a needle will rest as in a phonograph (q.v.). The use of a photographic sound track on the film is now nearly universal.
The initial stages in recording on a film are similar to the first stages in electrical phonograph recording or in radio broad cast transmission. The performers appear before a microphone of high quality, the output of which is amplified by a vacuum tube amplifying system. The output of the amplifier is used to control the recording device. In some cases the intensity of a light source, e.g., a neon, or other gas-filled glow-tube, is sys tematically controlled, and this variable light is caused to shine on a narrow slit, or its optical equivalent, placed over the rapidly moving negative film. In other cases the light from a constant source passes through a light valve or an aperture of variable width or a Kerr cell or similar device controlled by the ampli fier output, and then falls on the stationary narrow slit placed over the moving negative film. In each of the two preceding methods a variable density record is produced. In another method of recording, a sharply limited beam of light from a constant source is reflected from an oscillograph mirror and focused upon a narrow slit or its optical equivalent, placed over the moving film. In this case a variable width record is produced. Various
technical expedients have been made available whereby back ground noise is greatly reduced during both sound recording and reproduction. Multiple sound tracks, and sound tracks with non useful portions automatically blocked out are used for this pur pose. Fidelity of reproduction has been greatly increased by use of acoustically correct studios and theatres, improved microphones and amplifiers, and advanced optical systems for sound recording and reproduction on photographic film. Sound recording on colour motion pictures has been satisfactorily developed.
The process of reproducing variable density, or variable width film sound records, consists in throwing on the continuously mov ing film a narrow line of light which optically takes the place of the phonograph needle. The light line, which is perpendicular to the direction of motion of the film and perpendicular to the sound track, on passing through the film is caught by a photo electric cell, the varying output of which is suitably amplified electrically by vacuum tubes and used to actuate the loud speak ers. The applications of sound motion pictures are diverse. Films have been orchestrated, i.e., provided with a suitable orchestral accompaniment permanently available to any theatre having the necessary reproducing equipment. News reel subjects have em bodied in them the synchronized sound of speeches, street noises, aeroplane motors in action, and the like. Sound is recorded simul taneously with the picture on the same negative film in this case. Reproduction of plays with the speech of the actors recorded as well as their appearance has been developed, giving the illusion of an actual occurrence or of a theatre performance. In this instance, the sound is recorded on a separate film from the picture. After the "cutting" or editing of the picture, the sound and the picture are combined in synchronous relation on the same negative. Other sound effects may be added or "dubbed in" at this point, or the sound quality may be modified. Re-recording and dubbing have proved artistically and economically valuable. See MOTION PIC