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1.0
Y (W)
u)0T =
RADIAN FREQUENCY Fig. 9. Amplitude response characteristic for very close-in echoes.
In following up the conclusion which was just stated, the coarse structure, or long sweep average, of the envelopedelay characteristic is plotted in the dashed lines. The peak-to-peak amplitude of this average is then the quantity upon which a tolerance can be specified.
On the other hand, in the phase plot, in the upper part of the figure, parallel curves are drawn to the long sweep average. They are drawn just far enough apart to touch the peaks of finestructure excursions. The tolerance can then be placed on the separation between the parallel curves.
Close-in Echoes, Merging Into Cutoff Shaping
The inspiration for the material in this section was drawn from the unpublished memorandum of S. Doba, which forms item (b) of the sources for the data of Fig. 7. The treatment given here, however, is simpler and rather less sophisticated than that used by Doba.
It was noted in Fig. 3. that the closer in the echo, the \\ider is the sweep of the scallops, along the frequency scale,
which are observed in the transmission characteristics. When the separation amounts to one picture element, only a half-cycle of the scallop is left, within the range of the nominal passband.
This situation is illustrated in Figs. 9 and 10. The co0r in these refers to the quantity used in the Appendix, equation (14), to measure the echo delay. When COOT= TT, the echo delay is one picture element. The curve Yfa) in Fig. 9 is illustrative only and indicates the relative amplitude response which corresponds to a signal element of the "raised cosine" type, where the nominal video passband ranges from 0 to w0 (in units of radian frequency).
From both Figs. 9 and 10 it is noted that when the echo delay is 0.5 picture element, the nominal passband ranges over only one quarter-cycle of the scallop. When the delay is 0.25 element, the range covers only one-eighth cycle; and when it is 0.125 element, the range is only one sixteenth-cycle. Thus for these very short echo delays, such a small portion of the scallop cycle is covered in the passband that the nature of the influence on the transmission characteristic is
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May 1953 Journal of the SMPTE Vol. 60