Full CGA graphics on MDA monitors
A few weeks ago, I talked about my 286 computer with an MDA monitor and ATI Graphics Solution card. That was my first experience with Hercules compatible modes and I could finally try all the games the way that was typical for entry-level PCs from the 80s and early 90s (at least in our post-socialistic Czechoslovakia).
There were plenty of games with Hercules support, but in most cases, the result was worse than in any other graphics modes - including CGA. You could use utilities providing emulated CGA support by copying data to a different memory location and doing operations like scaling and dithering, but that was slow and with the same or worse picture quality.
However, even this was addressed by certain cards, including the first ATI graphics chip (ATI Graphics Solution rev3). A special program could modify the chip's registers to do hardware emulation of CGA. I'm not talking about just a simple translation of graphics data to the typical B&W Hercules mode. This was much better. It used two interesting hacks to make the result almost like on a real CGA card.
Synchronization signals were modified to change a resolution from 720x348 to 640x400. A standard MDA monitor could not handle 400 rows of pixels so the output was interlaced. Thanks to long persistence of a typical MDA screen, this was not an issue.
A standard MDA/Hercules chip produced TTL mono and intensity signals that were typically translated into only three levels of brightness (black, standard, intense), which was not enough for CGA. However, many MDA monitors read the mono signal the way that different voltage levels were displayed as different levels of brightness. The ATI chip could produce up to 16 shades of "gray" using this trick. All the CGA games I tried worked well with the CGA emulation. There was no speed penalty or graphics glitches.
ATI pushed MDA monitors to their maximum. It allowed to use the same monitor for MDA text, high-resolution Hercules graphics, all CGA modes (including 160x100 with 16 colors) and also a special text-mode with 132x44 characters. Therefore even MDA monitors were usable for gaming back then.
Photos show the difference between the native Hercules mode 720x348@1bpp and emulated CGA 320x200@2bpp (except for the last two).
