On Mon, 13 Feb 2012, Daniel Kruszyna wrote:
Geoff Body <gbody4@bigpond.com> wrote:To me the patent US4786893 appears to relate to possible IIGS video conversion from NTSC bit stream to RGB output and may help.Thanks, Geoff, this is it! I'll have to study it more tomorrow, but the gist is that there is a 4 bit sliding window with some extra circuitry to check when the 4 bit pattern changes.
Yep, there was non-linearity now that this patent is mentioned and my memory is refreshed. With a very simple circuit they handle passably most of the cases.
We have discussed this somewhere in the past, distant enough that I couldn't remember it quickly. I even had provisions for this mode (IIgs RGB) in my RA2 code. IIRC (obviously not to be trusted), IIgs RGB mode is much better than simple 4-bit sliding window, which in turn is better than simple 4-bit non-sliding window. The last one, again IIRC, was used in our Pravetz-8C color output, but it was normally not populated on the motherboard out of the factory.