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6 ENGSTROM, BEERS, AND BEDFORD [J. S. M. P. E.
through a suitable gear, thus pulling the film down 24 times per second, since the cam has two "throws" instead of the customary one "throw." In order to pull the film at unequal intervals as required, the "throws" are located 144 degrees and 216 degrees apart, respectively. The film picture in the gate is projected upon the small photoemissive mosaic of the Iconoscope by a standard projection lens. The light is chopped 60 times per second by a large rotating shutter, located near the lens. The shutter is accurately timed relative to the intermittent cam so that the film is always stationary when the light flashes occur.
The generator of synchronizing signals for the television deflecting system is synchronously controlled by the same 60-cycle power supply that drives the projector synchronous motor. The phase of this signal generator is adjustable so that the operator can make the short-duration light flashes fall safely within the Veoo-second intervals
JULL-OOWN *ULL-DOWM fULLDOWN
FIG. 3. Idealized sequence of events in film transmission by Iconoscope.
between the vertical scanning periods with some tolerance on each side for slight phase displacements such as are caused by small changes in the mechanical load on the projector or by voltage variations. This adjustment is very important, as any abrupt change in the illumination of the mosaic during the picture signal transmission time produces a spurious light streak across the received picture. An ordinary 3600-rpm synchronous motor has two identical pole structures which can assume either polarity, and hence such a motor can lock into synchronism in either of two phase positions, depending fortuitously upon starting conditions. Two such lock-in positions are apart in time by one-half of a cycle of the power-supply frequency, which for a 60-cycle power system is l/m second. Inspection of the diagram of Fig. 2 shows that displacing the light flashes Vi2o second with respect to the scanning periods would cause them to occur during instead of between the scanning periods. The abrupt change in mosaic lighting caused by a flash during the scanning period would produce a serious streak across the middle of the picture as mentioned