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KINESCOPE
AUX. LENS
SEMI -SILVERED MIRRQR —
MIRROR I
MAIN LENS
MIRROR 2
PHOTOCELL
Fig. 16. 30-Frame alternate path projector.
cx
FILM TRANSPORT
Moreover, the performance can be further improved by the application of proper edge lighting and back lighting as well as circuitry to provide an essentially constant background signal level under wide changes in scene brightness.
The flying spot scanner does not make use of the storage principle and, therefore, requires the scanning spot to be maintained in proper register with the film at all times. The two types of projectors which have been suggested to accomplish this are either the fast pull-down projector, in which the film is pulled down in the television blanking interval, or the continuous projector, in which the film moves at a constant rate and proper registration is maintained by either optical or electrical displacement of the image of the scanning spot.
Several experimental models of fast pull-down projectors have been built. These devices have all been designed for 16mm film. Obviously the problems associated with minimizing the wear on both film and mechanical parts due to the high accelerations involved will require extensive life tests before the practicability of any design of a film transport mechanism can be evaluated. The ability of the mechanism to provide accurate registration in such a short pull-down time also presents mechanical problems. The results obtained on one model have been highly encouraging. After more than 400 passes through this model, no damage to the film sprocket holes was observed. Moreover, no per
ceptible increase in jump was observable with the SMPTE test film after more than 50 hours of operation. While these results must be confirmed by further operation over an extended period of time, it would seem that flying spot scanner operation with this type of projector is within the realm of possibility.
Continuous projectors are of two fundamental types: (1) those that allow the film motion to accomplish a portion of the vertical scanning and employ optical or electrical means to deflect the scanning raster to the proper position at the beginning of each scan; and (2) those that use continuously varying optical means to maintain registration of the raster and the film as the film moves.
The first type has been used in Europe where the 50-field television standard permits a relatively simple alternate positioning of the raster by running the film at a rate of 25 frames. For U.S. television standards, this would require a special 30-frame film. One version of such a projector is illustrated in Fig. 16. Since there are only two television fields for each film frame, only two positions of the scanning raster are required. The system shown employs mirrors to deflect the beam and provide the two alternate paths. Since the mirrors can be relatively large, a projection lens of high speed can be used. A projector of this type was built and tested and found to operate extremely well. Care must be taken to maintain
Herbst, Drew and Brumbaugh: Quality of Kinerecording
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