How to add sound to amateur films (1954)

and produce electrical impulses. These in turn are fed into an amplifier and loudspeaker system. The change-over from recording to reproduction is generally just a matter of turning a switch. Synchronisation Even an apparently simple commentary must be synchronised to the picture with an accuracy of better than 10 seconds. In fact it is sometimes difficult to accept a lack of synchronisation exceeding two seconds. You will, therefore, need some device to relate the speed of projection accurately to the speed of the tape. Unfortunately, this is not a simple matter. Unlike film, the tape has no perforations. The smooth capstan provides only a friction drive, subject to slippage and creep which vary with the tensions of the feed and take-up reels. Moreover, the tape stretches and shrinks with variations in tension, temperature and humidity. Although these variations are very small — usually well under 0-5 per cent. — they soon add up. Even this small amount of stretch or slip will produce a timing error of two seconds in less than seven minutes. Recording on Wire Although magnetic tape is by far the most popular medium, two others must be mentioned — wire and stripe. In a wire recorder, a thin steel wire replaces the magnetically coated tape. Also the take-up reel often runs at a uniform speed, 78 r.p.m., and so serves as a capstan. In other respects, the design of a wire recorder differs little from that of a machine using tape. For a straightforward commentary, a wire recorder is admirable. But for more complicated sound recordings it is less convenient than a tape recorder. Wire is not conveniently marked for reference. Nor can you cut it and splice it as you can magnetic tape. If it spills off the reel, wire can get into an awful tangle. On the other hand it is 48