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SPLICING IN VIDEO RECORDING
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vertical bars represent the horizontal scanning lines in the vicinity of the splice, the height of each bar indicating the relative exposure contributed by the line. The vertical scale is expanded to emphasize the differences that exist. The horizontal co-ordinate indicates the location of the line in the vertical dimension of the picture. It too is expanded so that individual lines may be shown.
The effective exposure for an extended area will be an average of the individual line exposures, this average being shown in the diagram by the two horizontal lines. They indicate the average exposure on the two sides of the picture splice to differ by some 6 per
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LINE POSITION
Fig. 5 — Relative line exposure in picture-splice area.
cent. Multiplying this difference by the photographic gamma indicates a brightness difference at the projection screen of 15 per cent, a value which will certainly be visible.
The defect thus calculated is so large that one would tend to doubt the validity of the procedure except for the fact that Pll tubes have been encountered which do exhibit a persistence defect of this magnitude. Other Pll tubes seem to be completely satisfactory.
A similar calculation has been carried out, starting with an assumed screen-brightness difference which is acceptable and then working backward to determine a corresponding phosphor decay curve. For this curve the exponent in the inverse power law has a value which is very close to 2. The existence of satisfactory Pll tubes may mean that some tubes actually follow an inverse square law, although substantiating data have not been found. It may alternatively mean