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so produce velocity modulation. The complete gate assembly can be withdrawn for cleaning.
It is hoped that the new film-traction mechanism will produce velocity stability adequate to obtain correct interlacing on the film.
(3.2) Intermittent-Motion Types
(3.2.7) Fast Pull-Down in Frame-Suppression Period: This is the simplest and most attractive method, but it involves a pull-down and registration period of about 12° on present B.B.C. standards, and, so far as is known, this figure has never been achieved.
(3.2.2) Cameras Employing Two Films: The proposal has been made to use a double optical system in conjunction with two gates which would have film movements 180° out of phase. The first television picture would be recorded in gate 1 , and then, while the film in this gate was being advanced, the next television frame would be recorded in gate 2. Each film, after processing, would have half the television pictures on it, and a step printing process would be required to produce a 25-frames/sec film.
The principal technical disadvantage of any two-film system is the difficulty of ensuring identical processing. Even when the two films are joined together and passed through the same processing machine, variations along the length are difficult to eliminate with the required degree of accuracy.
Obviously, the cost of any two-film system is much higher than that of a single-film system. The time required to produce a finished print is also considerably greater.
(3.2.3) Cameras Involving Memory Systems: If it were possible to delay every other television field by 0.02 sec, then two consecutive fields could be recorded simultaneously on the film and so the period of one field would be available for pull-down.
Fig. 3. Film-traction mechanism for the latest B.B.C. 35mm continuous-motion recording system. (Ml, M2, etc. — exposure period.)
A suggestion4 involving the use of a fluorescent screen for retaining the first field has been made. The objection to this method is the characteristic required of the fluorescent material. To avoid frame shading it would have to possess a decay time long enough to allow negligible falling-off in brightness during the time that information was being recorded. At the same time the brightness should have fallen to a negligible value after the next exposure period. The exponential decay characteristic associated with normal phosphors appears unsuitable for this purpose.
There has not yet been a practical development of a camera working on this principle.
(3.2.4) Use of Image Converter for Change of Standards: It has been suggested5 that the television signals to be recorded should be applied to an image converter which would change the scanning standards and give a greater frame-suppression period to allow an adequate pull-down period of, say, 30° or more.
An image converter would enable the incoming interlaced picture to be changed
W. D. Kemp: Television Recording — Abstract
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