Comparison of CGA card versions

Over at the Vintage Computer Forums I asked about the differences between CGA card versions.

The main change that occurred during the CGA's lifetime seems to be to do with composite output. In particular, the old CGA (part numbers 1804472 and 1501486) had the following formula for the composite output voltage: COMPOSITE = 0.72*CHROMA + 0.28*I. The new CGA (part numbers 1504910 and 1501981) has the formula COMPOSITE = 0.29*CHROMA + 0.1*R + 0.22*G + 0.07*B + 0.32*I. The consequences of this are more obvious on a monochrome monitor, since there CHROMA only makes 3 different shades of grey (0 for colours 0 and 8, 1 for colours 7 and 15 and 0.5 for all the others). So an old CGA will only yield 6 different shades of grey on a monochrome monitor, while on a new CGA the 16 different colours will yield 16 (theoretically) different shades of grey (though some of them may be very similar).

On a colour monitor, a new CGA will give a lower saturation then an old CGA, but the brightnesses of the different colours will seem more appropriate. On an old CGA, blue seems lighter than it should be while yellow seems darker - new CGA fixes that.

The other thing about new CGA is that its composite output is a better match to standard NTSC than old CGA's, which means that the results will be more consistent between different monitors. The old CGA's color burst has both too high of an amplitude and too high of a DC offset, which causes many NTSC output devices to reduce the gain, making the resulting image too dark (I have to turn the brightness and contrast right up to get a decent image from my 1501486 card on the TV I connect it to).

That's the theory - to check how well it works in practice, I'd like to do some side-by-side comparisons. Rather than trying to buy a new CGA card (which is likely to be expensive, might be unsuccessful and probably wouldn't work too well side-by-side with another CGA card in the same machine anyway), I want to make an add-on card for my old CGA card which adds a second composite output, with new-style colours. The differences between the two cards are localized to a small part of the circuit, so there isn't too much to duplicate.

One Response to “Comparison of CGA card versions”

  1. […] should also note here that there were two different versions of IBM CGA produced, which differ mainly in how composite colors are generated.  We had equal numbers of […]

Leave a Reply