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 ><firstname.lastname@example.org>: >> 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. Maybe the UltiMax was developed mostly independently of the Vic-20, and later on this single-crystal trick was copied from the Vic-20 design to the Commodore 64? Did anyone try to run a C64 with an independently clocked dot clock? How did it look like? Marko Message was sent through the cbm-hackers mailing listReceived on 2017-09-04 18:00:03
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