RGB-VGA Converter -- buy or build?

Tony Duell ard at p850ug1.demon.co.uk
Mon Mar 15 16:15:19 CDT 2010


> 
> On 3/14/2010 4:29 PM, Tony Duell wrote:
> >>
> > If this is a real CGA card, there is most likely an NTSC-compatible
> > composoite colour output available too. It may be on an RCA phono socket
> > on the bracket (IBM had that), a header inside, or sometimes on pin 7
> > (otherwise unused) of the DE9 socket.
> 
> The colors are not the same, which is why I can't use that technique.

A few suggestions : 

1) Have you tried adjusting the 14.3...MHz clock trimmer in the PC? The 
original IBM machines (including the 5161 expansion unit), and many older 
clones, have a trimmer across the crystal that's used for the 14.3...MHz 
oscillator (this is divided down to get the 3.58...MHx NTSC colour 
reference, of course). The IBM manuals recomend adjusting this if the 
colours are misisng/incorrect. I am not sure it will do any good, but it 
can't hurt.

2)If the colours are wrong but consistent, can't you modify them after 
digitising them?

3) Take a look at the CGA monitor scheamtic, particularly the part from 
the RGBI TTL inputs to the CRT cathodes. This will show you how the 4 TTL 
signals are ocmbined to produce analogue RGB signals.

-tony



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