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244 GILLETTE September
exposure coincide in vertical position on the cathode-ray tube, and that three television fields are involved in the exposure of each frame of film. Of the first and third of these fields only such segments are included as will add up to one field. The region on the film where these two picture segments join is called the ''picture splice" in this paper.
The two exposures shown in the figure indicate that the picture splice will occur at two different positions. In following frames the splice will alternate between these two positions which differ in phase by one half of a television field. This is a significant point. It means that no matter how the camera is phased with respect to the television system at least one of the two splices will lie in the picture area.
It is for this reason that the quality of the picture splice is of such concern. Since the splice cannot be hidden by means of phasing the only alternative is to make it invisible. This requires that there be no change in picture brightness or structure above, below, or coincident with the splice. The factors which oppose this result are the subject of the present paper.
STRUCTURAL DEFECTS
Changes in picture structure have been mentioned as a source of visible splice defects. Some elaboration of this point is in order. The scanning mechanism of television breaks the picture down into a number of discrete horizontal lines, a total of 525 per frame, with some 500 of the lines in the useful picture area. Presenting these lines so that they are clearly resolved will add nothing to the clarity of the picture so that one normally would adjust the resolution of the photographic and projection equipment to conceal the line structure by merging each line with its neighbor. If overdone this action will lose detail in the picture so that one would attempt to hold the resolution of this part of the system somewhere between 400 and 500 lines, using here the television method of counting lines.
The success of this step depends upon the accuracy and stability with which each horizontal line is registered in its correct position on the film. If the lines of one field in a particular frame are displaced vertically with respect to the other field of that frame, the picture is no longer correctly interlaced, and if the displacement is an appreciable fraction of the space between lines, the lines become paired. In this case the picture contains but 250 lines, and with the system adjusted as suggested above, 250 lines will be resolved with ease.