13 Oct
2006
13 Oct
'06
2:45 p.m.
On Fri, 13 Oct 2006 13:43:42 +0000, Jules Richardson wrote:
There wasn't. In fact I've never seen a PAL version of any classic
IBM-compatible video card.
It was possible to reprogram the 6845 registers on a CGA card to give
European TV timing, albeit still with NTSC-like colour output on the
composite connector. I have -- somewhere -- a box that plugs into the DE9
(TTL RGV output) connector on a CGA card and which encodes said signals
to PAL. The result (after repogramming the 6845) is a PC that can display
on a European TV set.
That's interesting, though. IBM must have
believed that the 5150 would be a
global product and I'd expect that was the plan all along - so why put
circuitry on there for one market only?
I do not think there was ever was a PAL version of the CGA card. As far as I know the CGA card was the only one
dependant on the base clock freq being right on.
A few other cards use the OSC signal from the ISA connector, but AFAIK
none of them depend on the exact frequency like the CGA card does.
or It would loose color burst. I always thought the
4.77mhz was chosen because it could also be used as the
clock ref for the video.
Absolutely. The mater clock is 14,xx MHz. That was chosen becasue it's 4
tiomes the NTSC subcarrier and the division by 4 gives the quadrature
signals needed to make a simple NTSC encoder. The CPU clock is the same
master clock divided by 3 (I have no idea why the Intel clock generator
has a /3 stage in it...)
-tony