We use Optical Character Recognition (OCR) during our scanning and processing workflow to make the content of each page searchable. You can view the automatically generated text below as well as copy and paste individual pieces of text to quote in your own work.
Text recognition is never 100% accurate. Many parts of the scanned page may not be reflected in the OCR text output, including: images, page layout, certain fonts or handwriting.
374 PRINCIPLES OF CINEMATOGRAPHY
normally possible to obtain maximum efficiency at both speeds. If the silent projector shutter and claw mechanism shown at *C* and 'E\ Figure 160, is rotated at sound film speed the flicker frequency will rise well above the threshold of perception, in fact, it will rise to 72 interruptions per second. However, this high frequency is not necessary and is only obtained by employing a claw mechanism which completes the pull-down motion within 60° of the total cycle. Whilst this rapid pull-down is quite permissible when operating at 16-frames per second, the added strain on the film perforations, caused by operating at a speed higher than that for which the intermittent was designed, is not a desirable feature. On the other hand, if the sound film mechanism shown at 'D' and 'M', Figure 160, is operated at a speed of only 16-frames per second slower than that for which it was designed the flicker frequency will thereby be reduced to 32-cycles per second and will, of course, be below the optimum frequency. Any machine provided with speed changing devices whereby it may be operated at both silent and sound film speeds, but which is not provided with interchangeable parts, must obviously result in a compromise between the ideal conditions required at each speed. Since, if sound film speed is provided at all, it is more advisable to maintain the correct conditions when operating at 24-frames per second, it is usual to find screen flicker to become noticeable only when such machines are running at 16-frames per second.
Maltese Cross Intermittent Movements
At this point one may very well ask why the majority of 16-mm projectors are fitted with some form of claw intermittent mechanism whereas 35-mm projectors usually employ a maltese cross and an intermittent sprocket wheel to advance the film through the projector gate. One reason for this difference is that the claw mechanisms are relatively simple to manufacture and, of great importance, that they all repeat the entire cycle of movement during each pull-down. This point is of considerable importance since any slight errors which may exist in the claw mechanism will be repeated to an equal extent every time the film is moved forward from one frame to the next.
The greater accuracy with which a maltese cross mechanism must be manufactured will be gathered from Figure 161. The cross itself is shown at 'A' and is mounted upon a shaft 'F' which also carries a four-picture sprocket wheel with which the film is held in contact. A 'four-picture' sprocket is one of such a diameter that a section of film four pictures in length can be wrapped around