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DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

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Figure 45.11 Modulated 12.5T<br />

pulse waveform for 480i systems<br />

1 __ ⁄3.579545 MHz<br />

≈ 279 ns<br />

Modulated 12.5T, 20T pulses<br />

HAD 1.5625 µs<br />

In Phase response (group delay), on page 159,<br />

I explained that the time delay through a filter can<br />

change as a function of frequency. This phenomenon<br />

can be a problem in a video system, which can be<br />

considered to be a long cascade of filters. This condition<br />

is tested through use of a signal having two widely<br />

spaced frequency components.<br />

The modulated 12.5T pulse used to test group delay is<br />

shown in Figure 45.11 above. The signal comprises<br />

a low-frequency raised cosine of half amplitude duration<br />

12.5T (1.5625 µs), upon which is superimposed<br />

a subcarrier-frequency cosine wave that has been<br />

amplitude modulated by the low-frequency raised<br />

cosine. In an ideal system the resulting signal has an<br />

envelope that matches the raised cosine above and sits<br />

precisely at the baseline. Should the subcarrier<br />

frequency suffer delay, the envelope exhibits distortion;<br />

this is particularly evident along the baseline. Sometimes<br />

a 20T pulse is used instead of 12.5T. Note that T<br />

denotes the HAD of the envelope, not of the highfrequency<br />

waves.<br />

Figures 45.12 through 45.15 overleaf show several standard<br />

test signals that combine various elements<br />

discussed in this chapter. These signals may be broadcast<br />

as vertical interval test signals (VITS).<br />

544 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES

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