7 Jun
2005
7 Jun
'05
1:38 p.m.
On 7 Jun, 2005, at 00:28, Tony Duell wrote:
There's no such animal. All IBM PCs, XTs, ATs run at US video rates
(for
example, 60Hz vertical sync frequency). There never was a PAL version
of
the CGA card, etc.
I made a hacked CGA in order to get the colours from the original
Flight Simulator displayed on a CGA monitor, since NTSC monitors were
not generally available in Norway at that time. As far as I recall, I
found some spare gates which could be used to demultiplex the composite
signal into RGB. As you may remember, the FS used programming tricks to
make NTSC colour in the monochrome "high resolution" mode.
We used it in a running demo to show off the power of the PC!
Yes, that was the reason. They had a 4x colorburst
crystal
(14.31818 MHz).
They divided by three for the CPU clock, and by four for the
colorburst.
Does that mean that IBM machines running PAL display rates had a
slightly different CPU clock frequency as the main crystal would be
different?
It was possible to re-program the CGA card (there's a 6845 in the
middle
of it) to give 50Hz vertical, and there were 3rd party encoder boxes
that
plugged into the RGB output (the DE9 connector) and produced PAL video.
I also fried a monochrome display by trying to run it off a hacked CGA.
It was not designed to stand up to the frequency change, even though I
could adjust it so that it synched.
But they didn't use the motherboard clock for the
subcarrier.
-tony
--
-bv