8' DSDD disk

[Bjørn Vermo]

On 7 Jun, 2005, at 00:28, Tony Duell wrote:

>>> 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?
>
> 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!

>
> 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