[Coco] Documentation of "complex" artifacting (i.e. more than black/white/blue/red)?

[john dumas]

  On 9/25/2010 5:16 PM, Robert Gault wrote:
> John W. Linville wrote:
>> On Fri, May 21, 2010 at 05:05:41PM -0400, John W. Linville wrote:
>>> Are there any articles in Rainbow or similar CoCo-oriented publications
>>> that documented use of "complex" artifacting for producing additional
>>> colors (e.g. purple, green, etc) with a CoCo?  Can anyone refer me
>>> to one?
>>
>> I suppose it is bad form to reply to oneself, but at least if I
>> reply in this thread then I am more likely to be able to find this
>> information when I need it again. :-)
>>
>
> You need to understand how the NTSC system works to understand color 
> artifacts. A good place to start is here:
> http://en.wikipedia.org/wiki/Color_Graphics_Adapter
Excellent explanation.  It was so long ago that memory is dim, but I
think the none of us were aware of the possibility of artifact color
when the VDG was designed. Certainly I was not. Terry might have
been when he worked on the spec since he knew more about NTSC
than others in the design group. But I suspect not.

As I recall, we were all surprised to see the artifacts with the first
silicon. But, some 20/20 hindsight pointed to the single pixel patterns
being seen by the receiver as chroma data and compared to the color
burst, producing and artifact color. Two colors actually since there
was a flip-flop uninitialized at reset. [That's why I think Terry didn't
suspect the "problem" - he's too swift to let something go by if he
knows there's a problem.]

As far as John's question about multi-bit patterns, I can only guess.
[Been way, way, way too long ago!] My guess would be that those
patterns have enough energy in the first harmonic for - some -
receivers to treat the harmonic as chroma data. The resultant
differences in the pattern, compared to "real" data might have
a different phase relationship to the color burst and explain
the different color(s).

If any of this is actually true, I would expect different results from
different receivers - particularly old vs. new ones...

Perhaps someone who has current in depth knowledge of NTSC
could say if that's correct or false................

cheers,
johnd