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DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

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1135<br />

4<br />

1<br />

+ =<br />

625<br />

709379<br />

2500<br />

= 283. 7516<br />

There are 709379 samples in<br />

a composite 4f SC PAL frame.<br />

576i PAL<br />

composite video 44<br />

This chapter details the formation of 576i PAL-B/G/H/I<br />

composite video, and S-video-625. I assume that you<br />

are familiar with 576i component video on page 519.<br />

I describe 576i PAL by explaining its differences from<br />

480i NTSC, so I assume that you are quite familiar with<br />

NTSC composite video, described on page 511.<br />

Subcarrier<br />

Synchronous with the 576i raster is a pair of continuous-wave<br />

subcarriers having exactly 283.7516 cycles<br />

per total raster line: a sine-wave (hereafter referred to<br />

as subcarrier) whose zero-crossing is coincident ±20°<br />

with 0V , and a cosine-wave in quadrature (at 90°).<br />

Derived color subcarrier frequency is 4.43361875 MHz<br />

±4 ppm. Subcarrier drift should not exceed ± 1 ⁄10 Hz per<br />

second. Subcarrier jitter should not exceed ±0.5 ns over<br />

one line time.<br />

PAL four-frame sequence<br />

In PAL, the total number of subcarrier cycles per frame<br />

is an odd multiple of one-quarter. This causes subcarrier<br />

to fall in one of four relationships with the start of<br />

a frame. Where necessary, colorframes I, II, III, and IV<br />

are distinguished by the phase of subcarrier at 0V at the<br />

start of the frame. The four-frame sequence is due to<br />

the 1135 ⁄4 fraction, which relates subcarrier to line rate;<br />

the fraction 1 ⁄625 contributes precisely one cycle per<br />

frame, so it has no effect on the four-frame sequence.<br />

(Some people call this an 8-field sequence.) Starting<br />

with 1135⁄ 4 fH (i.e., 283 3⁄ 4 fH ), the 1⁄ 625 fH term is equivalent<br />

to a +25 Hz frequency offset; see page 355.<br />

529

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