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March, 1952
kotte: a family-projector programme
73
velvet tapes wear out and not the steel runner-plate itself.
Only when running older, somewhat torn film prints, is it better to use a plain steel runner-plate, since older prints might catch on the velvet.
The pressure of the skates is adjusted by means of one central screw, ensuring that the pressure is uniformly distributed over the full length of the runner-plate. The skates are made of Novotext. They are very light in weight — the specific gravity of Novotext is only 1.4 — and have a great resistance against wear-and-tear. They can be replaced without tools.
The lengths of both runner-plate and pressure skates are such that a well-made splice is just at the beginning or the end of the runner-plate at the moment the inter
Fig. 9. The soundhead.
mittent sprocket is at rest. Hence the velocity at which the splices enter or leave the gate is virtually zero, greatly reducing the danger of film rupture. Moreover, splices pass absolutely noiselessly and picture jump is precluded, since the skates are shifted gradually over one extra thickness of film.
The pressure skates are spring-loaded in the casting which forms the lens holder. On pressing a button the casting glides on two rods, liberating the runner-plate for threading the film. In its closed position it is pushed against a stop by a strong spring, ensuring correct focusing of the lens.
The pad roller of the intermittent sprocket is closed automatically when the lens holder
is pushed back into position ; it can also be closed independently of the lens holder.
The Picture Aperture
The FP.7 projector is provided with an adjustable roller which allows the length of film between the picture aperture and the sound scanning spot to be shortened from 20 to 17 frames. This is of great advantage for longer halls in which the sound takes more time to reach the audience in the back rows. By means of the adjustable roller a compromise can easily be found. After having passed the silencer rollers the film is pushed
PRESSURE ROUSR
(a)
(o)
Fig. 10
by a spring-loaded steel roller on to the rotating sound drum (Fig. 9).
The problem of ensuring a constant film speed at the sound-scanning spot has been a source of difficulty for designers since the earliest days of the sound film. It is true that the introduction of the rotary sound drum brought a great improvement, but it did not solve the problem completely and even now the most diverse constructions are used for eliminating vibrations in the direction of film travel, caused by the projector mechanism
.