[Coco] CoCo 2 Factory Composite Video RE'd

[Zippster]

Ah,  so the conversion of the 6847 signals to a PAL compatible
sync and number of lines would be done on the motherboard
before being sent to the RF modulator.  Perhaps the modulation
chip (1372 PAL equivalent) is then still in the RF box similar to
the NTSC CoCo2’s.

This circuit might then be adaptable to a PAL machine depending on
how the PAL chip analogous to a 1372 is designed. But lacking
a PAL machine and display unfortunately I won’t be able to work on this.

- Ed


> On Dec 16, 2014, at 9:47 AM, John W. Linville <linville at tuxdriver.com> wrote:
> 
> 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.
> 
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