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

Den Mon, 4 Sep 2017 19:19:55 +0200 skrev Gerrit Heitsch
<gerrit@laosinh.s.bawue.de>:
> 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
> >> <gerrit@laosinh.s.bawue.de>:
> >>> 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 
> > https://en.wikipedia.org/wiki/Dot_crawl implies that dot crawl can
> > occur on NTSC as well.

Now I remember. The difference between NTSC and PAL should be that both
could have dot crawl but PAL is supposed to make the crawl look more
"random" so it doesn't look like something is moving, more like some
random noise to an uninformed viewer.

> > 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.

Probably both.

> 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.

I'm not sure about the NTSC video output of A1000, and not sure about
the built in stuff in A600 and A1200, but every other Amiga doesen't
have any color clock at all as it only outputs RGB (and in some cases
luminance+sync on a RCA jack).


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