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1949 SPLICING IN VIDEO RECORDING 245
There are many sources of line displacement, some of which are not at all connected with the splice or the video recording equipment, but even these often serve to betray the location of the splice. If one recalls that a frame bearing a splice in the middle of the picture is assembled from three television fields, the reason for this effect may be seen. The bottom section of the picture is a combination of fields 1 and 2 while the top section is a combination of fields 2 and 3. A vertical displacement which occurs only in field 1 will produce visible line structure only in the bottom half of the picture leaving the top half smooth. The transition from structure to no structure is a very effective eyecatcher.
An example of line displacement, which is characteristic of television in general and not specifically of video recording, is that which is caused by variation in the repetition frequency of the horizontal synchronizing pulses. The frequency is controlled at the transmitter by tracking the 60-cycle power-line voltage. Since the power line does not usually constitute a completely stable reference frequency, any frequency which is controlled by the power-line frequency will show a certain amount of random deviation. Such deviations are generally small, but the deviation required to produce a significant vertical displacement of the scanning lines is also small. Consequently, it is currently believed that frequency deviation of the horizontal pulses is a fundamental source of picture defect, which cannot be eliminated by any practical modification of the video recording equipment.
.Other sources of structural defect are found in the video recording equipment. Of these several might be mentioned. Clearly any variation in picture size may make accurate superposition of two successive fields impossible. For this reason it is highly desirable to regulate all supply voltages which have any effect on deflection amplitude. This presents no difficulty except for the accelerating potential which is usually of the order of 30 kilovolts. Commercial regulators are available for this range, but they cannot cope with the type of current variation introduced by the gating action of electronic-shutter equipment. Consequently it is common practice to shunt the output of the supply with a large capacitor, a practice which makes operating personnel very respectful toward this part of the equipment.
Film motion may also create a structural defect. Since each line of the picture is registered on the film during a particular portion of