Re: 6569 vs. 8565

From: Gerrit Heitsch <>
Date: Mon, 26 Nov 2012 19:22:09 +0100
Message-ID: <>
On 11/26/2012 04:17 PM, wrote:
> On 2012-11-24, at 20:25, Gerrit Heitsch wrote:
>> If you use a 6569R5 on a 250466 board and replace the modulator with a just a driver circuit for S-video, you get a very clear picture.
> Here is what I mean. Note the shift from lblue into reddish domain and the dark patches between some (mostly close to each other) pixels.
> Whenever I disconnect the chroma, luma exposes no inaccuracies.

Well, yes... I have seen this on all my C64, more or less clearly 
depening on individual VIC and monitor used. But remember how color is 
encoded in the signal. It's in the phase angle of the color clock signal.

The problem with the C64 and other computers that produce Y/C or CVBS is 
that the frequency of the pixel clock is higher than that of the color 
clock. That means it's impossible to have the color change every pixel 
(The reason why all horizontal parts of the characters are at least 2 
pixel wide). If you try it anyway, the color decoder in your monitor 
will produce unexpected results. (*)

Also, color clock and pixel clock on the C64 are derived from the same 
master clock, but pixel clock is not twice color clock. So when you 
change the pixel color, it depends on the position of the pixel on the 
screen where the color clock is in its period when it switches from one 
color to the other. That seems to introduce some problems depending on 
the color decoder in your monitor. NTSC or PAL was never meant for such 
a high color resolution.

Maybe the 6569 is bit more pronounced in that respect. but I have seen 
this on a 8565 as well. But even my 6569R5 on the testboard connected to 
a 1084 monitor is not as bad as your picture.

(*) Set up a picture of alternating white or grey and black pixels for 
example and get a surprise.


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Received on 2012-11-26 19:00:06

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