8' DSDD disk

6 Jun 2005

...
   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.

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. 
But they didn't use the motherboard clock for the subcarrier.

-tony

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8' DSDD disk