Re: clock unit

From: HÁRSFALVI Levente <>
Date: Tue, 14 May 2013 14:32:01 +0200
Message-ID: <>

On 05/14/13 13:36, Gerrit Heitsch wrote:

> ...
> Until we know the actual circuit we can only guess, but with 2 Crystals,
> the 2 frequencies are not in sync as they are with the old PLL circuit
> based on the 74LS629. This should result in a slightly unstable picture
> on the monitor, especially if you use FBAS/CVBS.
> ...

They usually aren't in sync in such scenario, though, there's still some
chance that they are.

A couple of years ago, I was "forced" to use an A520 RF Modulator to
connect an Amstrad CPC to a projector. (I was not sure whether the
projector was to support the standard analog RGB signal I had, thus the
ugly solution of hooking a pure analog RGB source to an RGB projector
using composite video). After receiving absolutely no composite color
first, I was very surprised to learn that the MC1377 in the A520 had a
slightly hacky color subcarrier oscillator circuit, which relied on the
presence of a master clock signal which is provided by the Amiga. This
master clock (7.7xMHz) is derived from the Amiga master clock, which is
(in turn) an even multiple of the dot clock of the Amiga. Thus, the
resulting color subcarrier clock is in sync with the machine's dot
clock, even though the color encoder has a separate color subcarrier
crystal and oscillator circuit.

It would be interesting to know whether the SX utilizes similar tricks
in order to provide sync'd dot and color clocks.


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Received on 2013-05-14 13:00:03

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