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TOLERANCE
EXCURSIONS
du>
f TOLERANCE
~ ON ENVELOPE
DELAY EXCURSIONS
Fig. 8. Illustration of the method of placing tolerances on fine-structure and large-scale excursions.
sensitive pictures group together reasonably well.
b. The data tend to follow a trend surmised on the basis of experience with telephotograph transmission. The prolongation of the curve, summarizing the Christopher data, toward the shorter echo delays is not too certain from these experiments. It is drawn from rather scattered data, particularly on "negative" echoes. (In these, black in the original appears as white in the echo, and vice versa).
c. There is some indication of a trend, with the passage of time, toward tightening of the tolerances. The CCIR recommendations represent the most severe limits to date. In part, of course, this trend comes from a general improvement in picture quality and fidelity of contrast rendering.
The course of the curves mentioned in conclusion b above shows that largeamplitude echoes are tolerated when the echo delay is short, i.e., toward the left of the plot. As the echo delay becomes greater the permissible amplitude becomes much smaller, and finally tends to approach a fixed value. The apparent explanation is that when the echo is spaced close to the picture, it is in part
masked by it. In this region, therefore, the tolerance varies in an inverse manner with the spacing. Echoes that are spaced far from the picture, however, lose the effect of this masking. The tolerance on them becomes independent of the spacing.
This quickly leads to the further conclusion that the tolerance on close-in echoes toward the left of Fig. 7 can best be specified in terms of the constant envelope-delay excursions illustrated in Fig. 6, for which the echo amplitude varies inversely with echo delay. The tolerance on the remote echoes toward the right of Fig. 7 can best be specified in terms of the constant-phase excursions illustrated in Fig. 5, for which the echo amplitude does not vary with echo delay.
What this conclusion means in terms of the phase characteristic is indicated in Fig. 8. In the upper line there is illustrated a very simple case of a phase characteristic, in which occur both fineand coarse-structure excursions. The derived envelope delay obtained from this characteristic is presented in the lower part of the figure. Here, as would be expected, the fine-structure excursions are relatively magnified, as compared to those in the phase shift.
Pierre Mertz: Television Transmission Echoes
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