On 09/04/2017 07:03 PM, Marko MÃ¤kelÃ¤ 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 >> <email@example.com>: >>> 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. 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. Gerrit Message was sent through the cbm-hackers mailing listReceived on 2017-09-04 18:02:20
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