Richardson's handbook of projection (1930)

996 HANDBOOK OF PROJECTION FOR In recording the motion pictures a standard motion picture camera is employed, differing in no detail from the cameras used for silent pictures. In making the sound record a device called the "sound recorder'' is used, which is in no way connected with the camera, save only that both are driven at precisely the same speed, by means which will be hereinafter explained. The sound recorder uses not only a different film, but a different kind of film as well, it and the motion picture negative being afterward combined into one positive in the printing machine, as will be hereinafter set forth. I have said a different kind of film is used for sound recording. That is because, while the usual negative is of course used in the camera, it was found that for sound recording a positive film gave best results. It therefore is used. I understand the positive film produces a record less marred by extraneous noises, commonly termed "ground noise" or "surface noise." Under the conditions described it is obviously imperative that both the motion picture camera and the sound recorder be driven at precisely the same speed — the speed which will cause each to pass exactly ninety feet of film per minute. In the very nature of things no variation in the speed between the sound recorder and the camera can be tolerated. Absolute duplication of speed in both is secured in a very simple manner. As you doubtless all know, if two synchronous motors, which we will assume to be sixty cycle motors, be connected to the same 60 cycle power supply, they must and will run at exactly the same speed. It is electrically impossible for them to get "out of step" with each other. Any variation in speed caused by vari