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The Character Generator ROM

atariarchives.org

36 pointsalraj10 comments

Oh... That's just the start of the trickery. DEC and HP (and the Apple II, in high-res mode) had tricks to shift (delay, really) groups of pixels by half a pixel, giving us better A's and V's, and more rounded O's.

So the HP 2645 did use a half-bit shift and it's quite clever. It's shown in this video[1] by CuriousMarc (I've set the timestamp to the relevant part). The document shown in the video is available on the Internet Archive[2]. It also reminds me of techniques used in Epson dot matrix printer fonts, where the rate of pin firing couldn't be increased, but the half-pixel timing could be.

I'm pretty sure DEC VT100 and VT220 did not do this. There is some dot-stretching logic in the VT100 and VT220[3], which improves appearance but doesn't increase effective resolution.

Apple ][ is a slightly different story. The high bit (very similar to the HP terminal) does cause a half-pixel shift, but I believe this was almost entirely used to create more colors through NTSC artifacts, not to increase horizontal resolution. I'm unaware of a character ROM that uses the technique, but I could be missing something. The best writeup I've found is [4], and I also found an original description in BYTE magazine[5] from Steve Wozniak himself.

[1]: https://www.youtube.com/watch?v=QikO0WOAGWI&t=371s

[2]: https://archive.org/details/bitsavers_hpterminaleriesCharact...

[3]: https://www.masswerk.at/nowgobang/2019/dec-crt-typography

[4]: https://retrocomputing.stackexchange.com/questions/6271/what...

[5]: https://archive.org/details/byte-magazine-1979-06/page/n61/m...

> I'm unaware of a character ROM that uses the technique

I don’t think it worked in text mode, but in graphics modes there were tools and fonts using the trick.

Some of those block diagrams were a bit oversimplified. One cannot address 9 rows with only 3 address lines, for example. Or do block graphics with just 4 NOR gates.

I think there is a typo in the first graphic on that page, on the top row the second column is labeled 0011 when it should be 0001.

A 44-year old typo, to be exact. :)

(1979)

The Atari Character Generator ROM

Missing a word there.

No, the title is correct. Even though it’s on atariarchives.org, it’s not talking about Atari computers.

Fair enough!

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