On a sunny day (Fri, 8 Feb 2013 06:47:57 -0800) it happened "William
Sommerwerck" wrote in
:

In PAL & NTSC the colour carrier was a multiple of the line rate.

Not quite.

In PAL, the subcarrier is 4.433618 MHz and the line rate is 15625 Hz,
the ratio is 283.75512, not an exact multiple.

When I studied the thing in the 60's, the explanation was to find
a frequency at as non-integer rate as possible, to get rid of moire
effects.

This is not a correct explanation -- and I'm certain your numbers are wrong
(you've rounded off the line rate).

The subcarrier HAS to be a multiple of the line rate -- specifically, an odd
multiple of half the line rate -- or the sidebands of the color signal will
not properly interleave with the sidebands of the luminance signal.

By the way, moire is not capitalized. It is not a person's name.

In PAL 25 Hz is added to the color subcarrier to force the interference pattern
caused by chroma in the BW picture to move, so it becomes less visible.
From:
http://en.wikipedia.org/wiki/PAL:
quote
The 4.43361875 MHz frequency of the colour carrier is a result of 283.75 colour clock cycles per line plus a 25 Hz offset to avoid interferences.
Since the line frequency (number of lines per second) is 15625 Hz (625 lines × 50 Hz ÷ 2),
the colour carrier frequency calculates as follows: 4.43361875 MHz = 283.75 × 15625 Hz + 25 Hz.
end quote