5 ms·

The Character Generator ROM

36 points by alraj 3y ago

JdeBP 3y ago
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.
chrisco255 3y ago
(1979)
jandrese 3y ago
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.
- sedatk 3y ago
  A 44-year old typo, to be exact. :)
rbanffy 3y ago
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.
- raphlinus 3y ago
  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 https://www.youtube.com/watch?v=QikO0WOAGWI&t=371s [2]: https://archive.org/details/bitsavers_hpterminaleriesCharacterSetGenerationOct1975_26617925 https://archive.org/details/bitsavers_hpterminaleriesCharact... [3]: https://www.masswerk.at/nowgobang/2019/dec-crt-typography https://www.masswerk.at/nowgobang/2019/dec-crt-typography [4]: https://retrocomputing.stackexchange.com/questions/6271/what-determines-the-color-of-every-8th-pixel-on-the-apple-ii https://retrocomputing.stackexchange.com/questions/6271/what... [5]: https://archive.org/details/byte-magazine-1979-06/page/n61/mode/2up https://archive.org/details/byte-magazine-1979-06/page/n61/m...
  - rbanffy 3y ago
    > 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.
ShadowBanThis01 3y ago
The Atari Character Generator ROM Missing a word there.
- DerekL 3y ago
  No, the title is correct. Even though it’s on atariarchives.org, it’s not talking about Atari computers.
  - ShadowBanThis01 3y ago
    Fair enough!

36 points by alraj 3y ago

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.

chrisco255 3y ago

(1979)

jandrese 3y ago

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.

sedatk 3y ago
A 44-year old typo, to be exact. :)

rbanffy 3y ago

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.

raphlinus 3y ago
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 https://www.youtube.com/watch?v=QikO0WOAGWI&t=371s [2]: https://archive.org/details/bitsavers_hpterminaleriesCharacterSetGenerationOct1975_26617925 https://archive.org/details/bitsavers_hpterminaleriesCharact... [3]: https://www.masswerk.at/nowgobang/2019/dec-crt-typography https://www.masswerk.at/nowgobang/2019/dec-crt-typography [4]: https://retrocomputing.stackexchange.com/questions/6271/what-determines-the-color-of-every-8th-pixel-on-the-apple-ii https://retrocomputing.stackexchange.com/questions/6271/what... [5]: https://archive.org/details/byte-magazine-1979-06/page/n61/mode/2up https://archive.org/details/byte-magazine-1979-06/page/n61/m...
- rbanffy 3y ago
  > 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.

ShadowBanThis01 3y ago

The Atari Character Generator ROM Missing a word there.

DerekL 3y ago
No, the title is correct. Even though it’s on atariarchives.org, it’s not talking about Atari computers.
- ShadowBanThis01 3y ago
  Fair enough!