Old NTSC tricks: 240p?

On Mon, Mar 16, 2015 at 8:20 AM, John Foust <jfoust at threedee.com> wrote: There were several non-standard aspects of the Apple II "NTSC" video signal which made it not actually NTSC-compliant. It was mostly a matter of timing. The most significant deviations were lack of interlace (60 frames per second, rather than 30 frames per second each comprised of two interlaced fields), and that each horizontal scan line was 228 color carrier cycles long rather than 227.5, which was done so that the color carrier phase relative to horizontal timing was the same on all scan lines. Early revisions of the Apple II were also missing the serrations in the vertical interval. Some people claim that the way color was generated in the Apple II was somehow "fake" and refer to it as "artifacts" that somehow "trick" the television or monitor, but in actuality it was just a clever way of having the hardware produce various signal amplitudes and phases, just as "real" color does. It was clever enough to be patented, but it's not in any way "wrong" or "fake". When other devices have an issue with Apple II video as a source, and don't reproduce the colors properly, in my experience it is usually the 228 vs. 227.5 color carrier cycles per scan line that is at fault. In the early 1980s, Video Associate Labs sold a product for the Apple II called the VB3 Microkeyer. This consisted of a long slot 7 card, and a larger card that installed atop the power supply, connected by a ribbon cable. You had to pull about a dozen chips out of the Apple II motherboard (thus it would only work with the II and II+, but not the IIe or IIgs), and install in their place ribbon cables to the VB3 boards. The results were: 1) The Apple II would generate fully-compliant broadcast-quality NTSC output. (IIRC, there was a way to selectably disable interlace, which would break NTSC compliance.) 2) proc amp and gen lock: The Apple II video could be overlayed on an external video input. This could be done selectively (keying), and it was also possible to do software-controlled "wipes" between two video inputs, using the Apple video bitmap as the input selector. 3) A special "linear" high-res graphics mode was added, in which the addressing of the frame buffer was linear rather than having the normal Apple II interlaced addressing. This made it easier to program keying and wipes, but could be used for other purposes. I used and programmed one of these in the instructional television studio of a community college. I wrote some "wipe" programs for it, and licensed them to Video Associate Labs in exchange for a VB3 of my own. Alas, I no longer have it. :-( Years later Apple introduced a Video Overlay card, which did produce broadcast-quality NTSC output, and would work in the II, II+, IIe, and IIgs. It supported all of the IIgs graphics modes, even when used in an earlier model computer.