[Coco] RGB video on CoCo3

[jdaggett at gate.net]

On 15 Feb 2009 at 15:04, Phill Harvey-Smith wrote:

> Success !
> 
> I have now got RGB working, and it provides a much better picture, for
> the first time I can actually use NitroOS9 in 80 column mode and read
> the text !
> 
> My problem was that I was not feeding +5V to Pin 16 on the SCART
> connector, so the TV was interpreting my signals as Composite video,
> which is why it was way to bright. Ended up examining my BBC micro
> scart cable which I knew worked, and was a machine of a similar age.
> 
> Once I did this I could feed the signals direct from the CoCo to the
> TV and it produced a picture without problems, though a little bright,
> placing a set of 120R resistors inline with theRGB signals seems to
> have brought the brightness down, though it does seem a little more
> noisy, maybe due to impedence mis-match....analog electronics was
> never my strong point.
> 
> 
> One thing I did notice in ECB was that the CLS colours are slightly
> different :-
> 
> CLS	Composite/TV	RGB
> 0	Black		Black
> 1	Green		Green
> 2	Yellow		Blue
> 3	Blue		Orange
> 4	Red		Grey
> 5	White		White
> 6	Greenish	Yellow
> 7	Purple		Red
> 8	Pink		Light blue
> 9	Green/MS	Green/MS
> 
> Allowing for the fact that this is an NTSC CoCo3 on a PAL TV, though
> it is NTSC capable, and the fact that I'm slightly Red-Green
> colourblind, is this normal ?
> 
> Cheers,
> 
> Phill.

This is because the GIME handles the data in the palette registers 
differently. When told palette CMP then the six bits are treated as two bits 
of intensity (bit 5 and bit 4) and four bits of phase (bit 3 downto bit0). 
Composite NTSC works off of intensity levels and pahse to reproduce color 
and to aligh color with the chromanance section of a TV set. 

RGB is three separate colors in which the 6 bits describes the 2 bit level of 
each three components of color. So then the three phosphers of the 
monitor have their own intensity levels. Therefore the combination of the  
varations of the levels will determine color. 

james