Quote:
Originally Posted by robinsonb5
That's absolutely right, but doesn't explain why the jailbars appear in areas of solid colour. If the Amiga's video output was clean there'd be no variation in signal level over the duration of a pixel, and thus nothing to cause interference patterns.
How can you make the Amiga's video output cleaner? The single most effective way is to tie off that open unterminated clock signal on the video port. I did this on my A500's DB23-VGA adapter a few weeks ago and the jailbars are quite simply not there any more (at least in solid areas - obviously you can't eliminate pixel clock interference patterns unless your monitor's phase/pixel-clock settings are flexible enough to achieve a perfect sync.)
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I have been trying your solution this weekend and it didn't work for me. So I did some investigation on the matter: It is indeed the CCK clock. But it only affects the red and green channels, not the blue channel. And, on the scope you can clearly see the interference as up and down going tiny wiggles. Up when CCK goes up, down when CCK goes down. Thus, right on the edges of the clock. This, combined with the observation that it only happens on red and green channels makes me believe it is capacitive crosstalk. Probably inside the connector as it only affects the pins that are adjacent to pin 15 (red and green).
I tried slowing the edges of the CCK down by adding some capacitance to the pin. Although it did help reducing the amplitude of the crosstalk, it also made the crosstalk a little wider e.g. lower in frequency. Thus instead of having few strong jailbars I ended up with many less strong jailbars. Also, the EMC filter on pin15 also slows the signal down a little as it contains a capacitor already.
The best solution is thus probably a series terminator resistor right after the '245 driver (u41). To be continued...
