Re: Questions about pal clocks

From: Marko Mäkelä (
Date: 2001-11-22 15:13:36

On Thu, 22 Nov 2001, Jozsef Laszlo wrote:

> 1. The source Quartz for the PAL system is 17.73442 Mhz. If I replace
> this quartz with an 18.000 MHz one, will it work?

Probably yes.  But you probably won't see any colours (or even any black
and white picture) on the TV output.

> 2. The VIC-II chip needs two input clocks: the color phase clock (this
> is the 17.7Mhz I mentioned above), and the DOT clock. The 8701
> generates both of them, the color clock is the same as the source
> clock, and the dot clock is the source*2/4.5 rate, which is approx
> 7.88 Mhz.

The dot clock is 8 times the bus clock.  Normally, the bus clock is one
18th of the 17.734472 MHz colour clock on PAL systems and one 14th of the
14.318181 MHz colour clock on NTSC systems.  So, the above is correct.

> I mean that if drive the VIC-II with two independent clock sources, it
> will work or not?

That's how the UltiMax and I think the SX-64 work.  On the UltiMax, the
dot clock is generated from an 8 MHz crystal, while the colour clock is
14.318181 MHz (as the machines were NTSC only).  I think that you can
replace the 8701 with two independent crystal oscillator circuits.  But
note that someone (maybe Christian Bauer or Andreas Boose) pointed out
that this will give a very horrible picture.  If I remember correctly, the
one who did the test replaced the dot clock with exactly 8 MHz and left
the colour clock at its original frequency.


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