"Peter L. Peres" <plp@ACTCOM.CO.IL> wrote:
> incorrect for Australia and places which have PAL because the color
> decoder crystal is phase modulated at +/-90 degrees every other line, and
> this phase modulation is a pain to remove for other uses.

On alternate lines the phase of the burst relative to the color subcarrier
+/- 135 degrees.  The color subcarrier is continuous, and can be recovered
from the burst using suitable circuitry.  This would be a pain if you had
to do it in a discrete implementation, but there are plenty of chips
available that will do it for you.

In NTSC, the subcarrier frequency is 455/2 times the line rate, so the
subcarrier phase differs by 180 degress on successive lines.  Since there
are 525 lines in a complete frame, this also means that alternate frames
have alternate subcarrier phase, for a two-frame (four-field) color frame
sequence.

In PAL, the subcarrier frequency is 1135/4 of the line rate, plus an
additional 25 Hz.  This results in the subcarrier phase advancing 90.576
degrees per line.  Since the 25 Hz offset adds up to a complete subcarrier
cycle per frame, the total subcarrier advance per frame is 90 degrees
((90.576 * 625) modulo 360), so there are four frames (8 fields) per
color-frame sequence.

Reference: _A Technical Introduction to Digital Video_, by Charles Poynton