[Coco] Composite Video

Gene Heskett gheskett at wdtv.com
Wed Aug 14 14:08:03 EDT 2013


On Wednesday 14 August 2013 13:52:30 billg999 at cs.uofs.edu did opine:

> > CoCo 1 & 2s need an adapter for composite output.  Their standard
> > output is
> > analog RF for TV channel 3 or 4 (or was that 2 and 3?)  I have not had
> > any issues with the CoCo 3 composite output into a composite monitor
> > of the same
> > vintage.
> 
> Of the same vintage, that is the problem.  I guess I made the mistake
> of assuming "NTSC composite video" actually was some kind of standard.
> Looks like it is back to the drawing board as I don't see many monitors
> "of the same vintage" being offered today.  Is there anywhere I can find
> out just what the COCO3 does different from other NTSC Composite Videos?
> Is it something as simple as levels? Or are we talking about wacked out
> waveforms and timing issues?  I was utterly amazed at how crisp and
> clean the Apple2 output is on a modern technology monitor.  Surely we
> can do the same.
> 
> bill
> 
There is a huge difference between Never Twice Same Color, and what the 
coco3 actually does.  

1) the coco's color subcarrier is not interlaced, so the color dots on a 
green screen will all line up vertically, and will be enhanced rather than 
canceled by a monitor with a "comb" filter, which most are, particularly if 
the filter is done on the cheap.

2) The NTSC synch signal is a complex beast, designed to maintain horzontal 
sync on the home tv by breaking up the vertical component such that each of 
its 6 pulses, has a leading (falling) edge that is in time with the H sync 
rate.  The coco doesn't to that either, sending a simple pulse 3 lines 
long, with no horizontal information at all.

3) the coco3's vertical sync is not 'interlaced' either, so instead of each 
field having 262.5 lines, which puts alternate scans in between each other, 
it uses 262 lines, the flicker reduction being the same because the refresh 
rate is still about 60hz, but the vertical resolution drops from 525 lines 
to 262 while maintaining the scan rates within 1% of the NTSC std.

Digital sync separators in modern monitors will generally go stark raving 
bonkers when encountering such a signal, where the analog sync circuits in 
a 25 YO CRT monitor simply didn't care and if you didn't mind a small 
amount of waving tree tops at the top of the screen while the H sync got 
back in step with the horizontal pulse when they resumed after the vertical 
had passed, then you were happy.
> 
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Cheers, Gene
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