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KINESCOPE RECORDING 127 situation will improve. Another requirement is that the magazine be capable of holding enough film for a half-hour show. Most of the cameras used for 16 mm recording hold 1200 feet of film which is sufficient for thirty-three minutes of continuous running. In the event of a show lasting more than this time, it is a simple matter to arrange to switch to another machine at the end of the first reel in the same way that a motion picture projector is switched during projection. But even now it is too soon to discuss the methods used, for there are two more small matters to consider. First, due to the small amount of time available for film pull-down, it is necessary for it to be moved very rapidly. This means that the wear on the film is higher, and it also calls for greater precision in the mechanical de- tails of the movement. Secondly, if 1200 feet of film are used for one take rewinding it presents a slight headache. The size of a roll of this length is about ten and one half inches in diameter. Yet when rewinding commences, the core or the spool center is only approxi- mately two inches. It is at once obvious that the peripheral speed will increase as the film winds onto the take-up due to the unavoid- able increase in diameter. Some form of slipping clutch is therefore required to provide a varying speed from start to end. Practical Conversion Since a field is 1/60 of a second, it fol- lows that half a frame is 1/120, or 72° if expressed in terms of shutter action or blanking. A shutter which is open for 288° or 1/30 of a second is not hard to design; this leaves half a field, or 72°, for closed time. If the sequence commences with the shutter open and an odd field is followed by an even, both will be recorded on the first film frame. Then the shutter closes for 1/120 of a second and the first half of the next odd field is lost, the whole of the even field is recorded and also half the following odd field (two film frames). The shutter then removes half of this odd field and records a full even field plus half an odd field before it closes again (three film frames). The fourth film frame records in this order half an even field, a whole odd field, and half an even field—the second half being lost under the shutter. Now the whole cycle repeats itself.