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246 GILLETTE September
the exposure interval, film motion during any part of the exposure will serve to displace certain lines with respect to the remainder. Incorrect camera phasing may allow the exposure interval to extend into the pulldown interval resulting in gross displacement of lines at the splice. Line pairing over a considerable area of the picture may be caused by a minute amount of film creep. With some movements such creep might be introduced by the action of a register pin which is in motion during the exposure. In any case, very careful design is required to avoid any trace of film motion.
BRIGHTNESS DEFECTS
Brightness defects can also serve to make the splice evident. An abrupt brightness discontinuity will be observable simply by comparison of adjacent areas. A more gradual transition will be noticed primarily as flicker. The fact that the picture splice occurs in a given location only on alternate frames means that any brightness defect associated with the splice will give rise to 12-cycle flicker. In either case the brightness defect need not be large to be annoying to a critical observer.
Screen brightness is, of course, related to the exposure received by the film, the exact relationship being determined by the intervening photographic processing. When the recording is a direct positive, the photographic step will have a gamma of about 2.5. This means that a small exposure difference will give rise to a screen-brightness difference which is two and a half times larger. Obviously very close control of the exposure is required.
If the exposure interval determined by the camera is not exactly equal to the period of a television frame an exposure defect will be introduced. With electronic-shutter equipment the effect is concentrated in the lines right at the splice, either as doubly exposed lines or as lines which receive no exposure. This would be a serious defect, but its prevention is not difficult. Since each television frame contains exactly 525 horizontal scanning lines it is possible to use pulse-counting circuits to insure that each exposure interval contains the correct number of lines.
Incidentally, counting pulses to determine the exposure provides a system which is quite tolerant to variation in frame rate of either television or camera. In fact, the two systems need not operate at the same frame rate; a point of possible advantage if signals originating at a distant point are to be recorded.