Old NTSC tricks: 240p?

[Eric Smith]

Tor wrote:
> (PAL means Phase Alternating Line, except for the lines/frequency
> differences it is really just NTSC but with the phase of the colour
> subcarrier alternating every other line. Thus automatically cancelling
> out transmission phase errors which would otherwise change the colour.
> That's all really: NTSC with automatic colour phase error
> cancellation.)

NTSC does have alternating color carrier phase on consecutive lines. A
horizontal scan line is exactly 227.5 cycles of the color carrier, so
the color phase changes 180 degrees on each successive line.  In PAL,
the scan line is 283.75 cycles of the color carrier, plus a 25 Hz
offset so the color carrier phase changes by slightly less than 90
degrees on each successive line. The 25 Hz offset makes digital
generation of a correct PAL signal a bit more difficult than NTSC;
that is one of the places that a lot of cheap consumer electronics
cheats.

What PAL does significantly differently than NTSC is that the color
burst (in the back porch) doesn't have the same phase as the color
carrier. The burst phase is offset 45 degrees in opposite directions
on consecutive scan lines, so it alternates by 90 degrees. This is the
"alternating" referenced in the name.

Eric