Atari ST SYNC loss on demos

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  • March 17, 2022 at 4:35 PM #52490
    Total Eclipse
    Participant

      I have a PAL Atari ST connected via a good quality RGB SCART cable to the OSSC v1.7

      For most screens I get a great picture, detected as a 313p signal, however there are a few demo screens, that usually open at least one border, where the OSSC fails to sync.

      Examples –
      The Union Demo main screen. Opens both side borders, fails to sync, yet all other screens display correctly.
      The Cuddly Demos. The Fullscreen demo loses sync, all other screens are fine.
      Dark Side Of The Spoon. All overscan screens are fine, with the exception of a screen by The Respectables, that loses sync about once a second.

      I’ve tested these same demos on another ST, to rule out issues with one particular computer, and the same issues are present.

      So ideally I’m after some pointers as to what tweaks might enable accurate syncing.

      March 20, 2022 at 11:43 AM #52537
      BuckoA51
      Keymaster

        Could be the ST demos are using some trickery that your display doesn’t like when line doubled or processed. Sync losses are to be expected on scenes where demos switch to other screen modes, for instance.

        I would try increasing H-PLL Pre-Coast H-PLL Post-Coast coast to 4 or 5 see if that makes a difference.

        March 24, 2022 at 5:25 PM #52612
        Total Eclipse
        Participant

          Adjusting the coast settings doesn’t make any difference.

          By way of an experiment, I’ve connected the ST to another LCD TV that has a direct RGB SCART input. For each of the problematic demos, the TV shows the full picture, however there are definite sync issues. The Cuddly Demos fullscreen displays sync tearing at two or three points on the screen, suggesting that for some scan lines the sync pulses are out of spec compared to the rest of the screen.

          Similarly the Union mainscreen is displayed correctly apart from the scanlines containing the full-width scroller, which again struggles to sync properly and has random horizontal distortion. When the OSSC tries to display this, you actually see about half the screen displayed just for a single frame, then sync is lost. I assume this is the OSSC being able to display the incoming field right up to the point of the crazy sync pulses for the scroller, at which point it can’t lock on any more.

          Incidentally, connecting via a RF modulator doesn’t have any screen distortions at all.

          So while it’s a little disappointing that the OSSC can’t sync in these instances, it’s not too surprising either. I’m not sure whether there are any other settings that might improve the chance of getting the OSSC to lock on to the signal? The wiki manual is great for explaining exactly what each setting does, but is there a more simplistic guide to tweaking?

          March 30, 2022 at 12:16 PM #52688
          BuckoA51
          Keymaster

            It does sound like as you say the demos are doing some trickery that a CRT would tolerate but a fixed resolution screen is struggling on. I don’t think there’s much else you can try.

            They /may/ work on a device with a frame buffer, like RetroTINK 5x, but then again it might not work at all as with the STs high res mode.

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