International photographer (Jan-Dec 1934)

Twenty 1 I, I N TERNATIONAL PHOTOGRAPHER September, 1934 MOTION PICTURE SOUND RECORDING Chapter XII By CHARLES FELSTEAD, Associate Editor HE main recording and monitoring amplifier system in the motion picture sound recording studio that employs Western Electric recording equipment was described in the three preceding chapters of this series. The first of those chapters dealt with the theoretical phases of audio-frequency amplification, the second of those chapters discussed the direct recording amplifiers that are installed in the amplifier room, and the chapter just preceding this one considered the monitoring amplifiers and the monitoring circuit arrangement. The discussion has advanced to the point where we are prepared to consider the actual recording device. This is the apparatus that converts the electrical energy in the speech current to mechanical energy so that an impression may be made by it on a sensitive medium. In recording systems that employ the AEOlight, or some equivalent form of flickering light, including the Fox Movietone recording system, the electrical energy in the speech current is converted directly to variations in intensity of the brilliancy of a light source without the intervention of mechanical motion. Development of Film Recording Great advances were made in the development of the microphone and of audio-frequency amplifiers long before motion picture sound recording was made possible. But after the development of those two pieces of apparatus, the engineers who were working on the problem of finding a method for recording sound to accompany the motion picture were faced only by the difficulty of creating a device that would transform the electrical copy of the sound waves into some other form of energy that could be recorded on a suitable sensitive medium. The amount of audio amplification they had available assured the engineers that the electric speech current would be of sufficient magnitude to operate the device after it had been perfected. They knew, too, that the fidelity of amplification of the equipment at their disposal was high enough to produce an electric current having practically the same wave form and proportionate amplitudes as the sound-pressure waves they wished to record. This development work progressed along several lines, as is demonstrated by the fact that there are a number of distinctly different sound recording systems in use in motion picture production work at the present time. In some of these recording systems, the electrical energy in the speech current is caused to operate a device that modulates a beam of light, either at the source of current that lights the lamp or after the light has been generated. In another method of recording, the electrical energy is transformed directly into suitable mechanical energy. In the first case, the modulated beam of light from the lamp is permitted to fall on a moving strip of unexposed motion picture film, thereby creating a photographic image of its variations as a narrow sound track near one edge of the film. In the other method of recording, the mechanical energy derived from the electrical energy in the speech current records itself as a varying trace on a soft wax record. These are rather round about methods of recording sound, but they are the best that is available as yet. It is logical to assume that eventually the speech current, as this varying electric current that represents the sound wave is called, will be recorded directly on some medium without recourse to the expedient of first changing it into some other form of energy that is more easily recorded. But that development cannot be evolved until some practical method for recording electrical energy directly has been devised. The Two Methods of Film Recording The two methods that are generally employed for recording sound on motion picture film are known as the variable area-fixed density and the variable density-fixed area methods. Since the space is not available to permit entering into a detailed discussion of these two types and systems of film recording, and since this series is concerned primarily with the Western Electric recording system, we will consider only the variable densityfixed area method of film recording in this chapter. There are two distinctly different recording systems that employ this method of recording sound on film. In the Fox Movietone recording system, this type of sound track (the variable density-fixed area) is produced by the flickering light from a lamp that is connected directly to the output of the recording amplifiers. In the Western Electric system of recording, the modulation of the light beam is accomplished by a device called a "light valve," which is a slit of variable width interposed between a light source of constant intensity and the moving film. Only the latter system will be discussed here. In order to avoid reviewing the complicated electrical equipment that is connected between the microphones on the sound stage and the recording machine in which the film is exposed to the modulated light beam (all of which has been described in preceding chapters), we will assume that we have at the recording machine a speech current that is of the proper amplitude and Left — The main amplifying equipment at Universal Pictures Studios. Charles Felstead, author of this series, illustrates how "patching" is done. Right — A film recording machine. Courtesy Paramount Productions.