Fwd: questions about Sanyo MBC 55x collectors

[Tony Duell]

> > Almost all the pc's used a crystal that was some
> >multiple of color burst frequency. Did the Sanyo
> >actually use a 3.58mhz crystal? The PC had a crystal
> >that was 4x that, and divided it by 3 to get 4.77mhz.
> 
> the IBM PC used the 8284A clock generator to pump the 
> 8088 and that chip generates the 30% duty cycle required
> by dividing a higher frequency (14.31818mhz) to 4.77mhz.

And if you divide that 14.3...MHz by 4, you get something close to 
3.58MHz -- the NTSC colourburst frequency.

> That higher frequency was high enough for Video clock 
> generation as well.
> 
> The IBM PC also another osc for the baud rate clocks 
> and the Video had it's own clock.

The MDA card has its own clock oscillator (one of those 4-pin oscillator 
cans), the IBM CGA card does not. It uses the 14.3xx MHz motherboard 
clock for the video timing chain and the NTSC encoder. 

In fact the PC/AT motherboard has a 14.3MHz clock circuit on it, fed to 
the expansion slots for those cards, like the CGA card, that require it. 

IIRC, there's a trimmer capacitor tied to that crystal (on the PC, XT, AT 
boards) that you can tweak to get the colourburst frequency right so that 
an NTSC monitor or TV will display in colour. 

-tony