Re: Difference in luma-chroma delay of C64/C128 compared to standard S-video

From: Gerrit Heitsch <>
Date: Mon, 4 Sep 2017 19:19:55 +0200
Message-ID: <>
On 09/04/2017 07:03 PM, Marko Mäkelä wrote:
> On Sun, Sep 03, 2017 at 12:18:38AM +0200, Mia Magnusson wrote:
>> Den Sun, 3 Sep 2017 00:10:38 +0200 skrev Gerrit Heitsch
>> <>:
>>> On 09/02/2017 11:58 PM, Mia Magnusson wrote:
>>> >
>>> > But isn't this even how the clock in a C64 is already? We know that
>>> > the h-sync frequency is wrong to avoid some PAL dot crawl. (Dot
>>> > crawl is kind of a feature of PAL...).
>>> I thought dot crawl happens if pixel clock and color clock are not in
>>> sync? Like on a ZX Spectrum. With the discrete PLL in the C64 (and
>>> later the 8701), dot clock and color clock are in sync and that
>>> prevents dot crawl.
>> Yes, but AFAIK a correct PAL clock cannot be evenly divided to get a
>> correct PAL h-sync. (you need fractions in the divisor). Thus dot crawl
>> in a correct PAL signal.
>> 15625 * 284 = 4437500 = too high
>> 15625 * 284 = 4421875 = too low
> As far as I understand, the UltiMax attempted to get this right, by 
> having a separate 8 MHz dot clock crystal in addition to the 14.318181 
> MHz NTSC clock. The Wikipedia article 
> implies that dot crawl can occur 
> on NTSC as well. If I remember correctly, this issue (for the UltiMax) 
> was mentioned on this list several years ago.
> I wonder if the decision to derive the dot clock from the chroma clock 
> (14318181 Hz/14 or 17734472 Hz/18) was made in the name of reducing 
> costs, or in the name of making the video output look nicer.

I remember the reason was to eliminate dot crawl. And since they needed 
this disrecte PLL with the 74LS629 and the MC4044P for a while, it can't 
have been cost reduction. And I also remember one engineer saying that 
he now considers running the VIC with 2 different clock signals a 
mistake. They fixed it in the 264 series, there TED gets the 14.318 or 
17.734 MHz and does the rest internally. The Amiga also runs everything 
from a single clock source.

> Did anyone try to run a C64 with an independently clocked dot clock? How 
> did it look like?

That would be interesting to see... I wonder if modern TVs/monitors 
would be able to display an image with 8 MHz dot clock, after all, the 
rest of the timings change as well.


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Received on 2017-09-04 18:02:20

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