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You have any links for this? It sounds both terrifying and clever as hell. Similar to the Amiga's HAM mode[1], where you could get 4,096 simultaneous colors, b

You have any links for this? It sounds both terrifying and clever as hell. Similar to the Amiga's HAM mode[1], where you could get 4,096 simultaneous colors, but the restriction was that the succeeding pixel could only differ from the preceding in just one of the R, G, or B values. [1] https://en.wikipedia.org/wiki/Hold-And-Modify https://en.wikipedia.org/wiki/Hold-And-Modify

jdblair 8y ago
The bit patterns are described in Wikipedia: https://en.wikipedia.org/wiki/Apple_II_graphics#High-Resolution_(Hi-Res)_graphics https://en.wikipedia.org/wiki/Apple_II_graphics#High-Resolut...
- nategri 8y ago
  Yes and the 64:1 interleave pattern on the display is also another "fun" feature to code around.
jdblair 8y ago
and this goes into detail about how the bit patterns worked: https://www.xtof.info/blog/?p=768 https://www.xtof.info/blog/?p=768
- function_seven 8y ago
  Thank you. I just had an epiphany. In elementary school computer lab, we'd all fight for the color machines. There were two of those and like 25 of the green screens. I remember wondering how the game "knew" what kind of screen it was on. How did it change from a solid color (for the lucky kids) to a pattern (for the rest of us)?? I just shrugged it off as the computer knowing the display type from the connection. Those two images in your link made it all just click right now, 30 years later.
dbcurtis 8y ago
"terrifying and clever" describes a lot of the Woz's work... So I am a little fuzzy on the NTSC standard, because it has been a while since I looked at it... but the hack goes something like this: Monochrome TV puts video on an upper vestigial sideband with the carrier at (IIRC) 1.25 MHz up from the lower edge of the 6 MHz channel. (The high frequency parts of the lower sideband of the video signal are whacked off with a filter.) The FM sound subcarrier is up closer to the top edge, I forget what the offset is exactly. Then color came along... so they added a color subcarrier just below the sound subcarrier. This reduced the bandwidth available for luminance, but got you color in return. The color subcarrier IIRC is kind of a flavor of independent sideband, but is really two double sideband signals on the same carrier, 90 degrees out of phase with each other, one skinny, and the other fat but with a notch filter applied to allow the skinny signal to ride in the middle. Or something like that. Anyway... if you send a monochrome signal with high-frequency components (high frequency == high dot rate) that fall in the range of the color sub-carrier, you will get color artifacts. This is because the color decoder simply responds to the rf it is getting, whether on a proper subcarrier or not. Another thing I used to know but am fuzzy about is I am pretty sure you need to put out the standard color burst on the back porch of the horizontal sync. Otherwise, the TV receiver will not sync to the "color subcarrier" that you are faking with artifacts.
- ddingus 8y ago
  You are correct. Something like a colorburst must be there. Turns out you can abuse that very considerably. And still get stable, useful displays.

jdblair 8y ago

The bit patterns are described in Wikipedia: https://en.wikipedia.org/wiki/Apple_II_graphics#High-Resolution_(Hi-Res)_graphics https://en.wikipedia.org/wiki/Apple_II_graphics#High-Resolut...

nategri 8y ago
Yes and the 64:1 interleave pattern on the display is also another "fun" feature to code around.

jdblair 8y ago

and this goes into detail about how the bit patterns worked: https://www.xtof.info/blog/?p=768 https://www.xtof.info/blog/?p=768

function_seven 8y ago
Thank you. I just had an epiphany. In elementary school computer lab, we'd all fight for the color machines. There were two of those and like 25 of the green screens. I remember wondering how the game "knew" what kind of screen it was on. How did it change from a solid color (for the lucky kids) to a pattern (for the rest of us)?? I just shrugged it off as the computer knowing the display type from the connection. Those two images in your link made it all just click right now, 30 years later.

dbcurtis 8y ago

"terrifying and clever" describes a lot of the Woz's work... So I am a little fuzzy on the NTSC standard, because it has been a while since I looked at it... but the hack goes something like this: Monochrome TV puts video on an upper vestigial sideband with the carrier at (IIRC) 1.25 MHz up from the lower edge of the 6 MHz channel. (The high frequency parts of the lower sideband of the video signal are whacked off with a filter.) The FM sound subcarrier is up closer to the top edge, I forget what the offset is exactly. Then color came along... so they added a color subcarrier just below the sound subcarrier. This reduced the bandwidth available for luminance, but got you color in return. The color subcarrier IIRC is kind of a flavor of independent sideband, but is really two double sideband signals on the same carrier, 90 degrees out of phase with each other, one skinny, and the other fat but with a notch filter applied to allow the skinny signal to ride in the middle. Or something like that. Anyway... if you send a monochrome signal with high-frequency components (high frequency == high dot rate) that fall in the range of the color sub-carrier, you will get color artifacts. This is because the color decoder simply responds to the rf it is getting, whether on a proper subcarrier or not. Another thing I used to know but am fuzzy about is I am pretty sure you need to put out the standard color burst on the back porch of the horizontal sync. Otherwise, the TV receiver will not sync to the "color subcarrier" that you are faking with artifacts.

ddingus 8y ago
You are correct. Something like a colorburst must be there. Turns out you can abuse that very considerably. And still get stable, useful displays.