To me the patent US4786893 appears to relate to possible IIGS video conversion from NTSC bit stream to RGB output and may help. The Super HIRES mode appears to be the same screen size as the older modes, therefore it needs to output 8 bits for every 7 of the old mode. If you look at patent US4823120, Fig 2 ref 39 it would appear to be related to 8MHz vs 7MHz for standard video modes. Geoff