How to add sound to amateur films (1954)

As with any stroboscope system, continuous observation and speed correction is a tedious business. Electromechanical systems are therefore available for controlling the projector speed automatically from the capstan speed. Usually they employ mechanical coupling from projector and recorder to a unit which switches a resistor in and out of the projector motor circuit. This controls the projector speed without affecting the speed of the capstan. Synchronisation from Tape Although synchronisation from the capstan can give good results, it cannot be perfect. There is always some degree of slip or creep between tape and capstan. This would be immaterial if it were constant. Unfortunately, anything which affects the tension on the tape will change the amount of slip. Moreover, the tape itself stretches and shrinks with changes in tension, temperature and humidity. If the tape carried perforations, like the cine film, these factors could be eliminated by using a sprocket in place of a capstan. Some professional film studios do in fact use recorders of this type. For domestic purposes, however, unperforated tape is invariably used because it is cheaper, lighter and easier to handle than the perforated type. Even with unperforated tape, however, you can get perfectly accurate synchronisation provided the tape carries regular marks which can be related to the frames on your film. One make of a paper-based tape carries printed stroboscope marks on the back. It is intended for use at 1\ inches per second and so 16 black bars are printed in every 1\ inches. Thus one bar moves by for every frame of a film running at 16 f.p.s. In use, you illuminate the tape from the projector beam in the same manner as a capstan stroboscope (p. 94). The pulsating light transmitted by the projector shutter should then appear to arrest the motion of the marks on the tape. Usually a silent projector has a shutter operating three times for every frame projected. This makes 95