Richardson's handbook of projection (1930)

1000 HANDBOOK OF PROJECTION FOR higher the frequency it represents ; conversely the thicker the "peak" the lower the frequency. That is very simple when we come to consider it. Since the film moves at steady speed, if the sound frequency be such that the mirror is vibrated with great rapidity, naturally there is less time per vibration, hence the film will move a less distance during its continuance ; the "peak" will therefore be of less depth — less broad at its base, than would be the case did the frequency be lower, hence the film able to move a greater distance during the continuance of each vibration. And since the amount of movement of the mirror per vibration is controlled by the volume of sound (the greater the volume the greater the movement), it follows that as volume is increased, the height of the "peaks" is also increased because the light line will be moved or swung farther across the sound track. There, I think that is as clear as it is possible to paint the thing with words. The method of establishing perfect register in synchronism as between the motion picture and sound film in the printer is to place registering marks on each film. This may be done in any one of several ways, one of which is to have a person stand before the camera both before and after each shot and bring two sticks together with a sharp impact. Thus each film has the imprint, one in the form of a photograph ; the other in sound. Another method employs magnetically operated marking lights placed in both the camera and sound recorder. The camera has a small mazda lamp, so placed that when a shutter is operated a thin line of light strikes the edge of the film outside the sprocket holes. Upon develop