[Coco] CoCo 2 Factory Composite Video RE'd

[John W. Linville]

On Mon, Dec 15, 2014 at 04:37:46PM -0600, Zippster wrote:
> Except that people with PAL machines are likely to have PAL displays, aren’t they?
> 
> The composite output from the MC6847 is NTSC.  I wonder if the PAL conversion
> is done in the modulator box?

Modulation is relatively simple if you already have the video baseband
signal.  Converting from NTSC <-> PAL is a bit more intricate!

The color signals coming from the 6847 are actually fine -- there are
chips that can generate PAL from those signals that are analagous to
the MC1372.  I forget the designation -- LM18something...

The main problem is the number of lines (i.e. horizontal syncs) in
the display and the vertical sync rate.  My understanding is that the
solution to this problem involves adding circuitry around the 6847
to actually stop it once (or twice?) per frame and to add the extra
lines for a PAL display.  This also slows down the vertical sync from
60Hz to 50Hz at the same time.

Anyway...once all that is done then the result is a baseband composite
video signal in PAL format.  This signal can then be fed to an RF
modulator that (like the one in the CoCo1) only serves to modulate
the existing composite video signal onto the RF carrier.

Hth!

John
-- 
John W. Linville		Someday the world will need a hero, and you
linville at tuxdriver.com			might be all we have.  Be ready.