How to add sound to amateur films (1954)

the bars appear only one-third of their true width, but spaced three times as closely. Sound film projectors generally have shutters operating twice per frame. They will therefore produce a similar effect when running at 24 f.p.s. This stroboscopic system produces more reliable results than the others described above. But like them, it calls for a careful and constant watch on the bars and frequent correction of the projector speed. For professional purposes, several schemes are in use to eliminate this tedious job. In one, a photo-cell "watches" the printed bars and is made to adjust the motor speed accordingly. In another arrangement, no visible marks appear on the tape. Instead, tone pulses are recorded magnetically alongside the sound record. Generally, one pulse is recorded for every frame on the film. For playback or re-recording, an additional playback head is used to detect the pulses and these are kept in step with the film. At present, the cost and elaboration of equipment for operating such systems place them beyond reach of the amateur. On the other hand, they hold three great attractions. They combine the low running costs of magnetic tape, the accuracy of synchronisation found in sound-onfilm, and the advantages of a sound track which can be edited independently of the film. If ever it becomes available, a simple and cheap version of the professional equipment will be a highly attractive proposition. 96