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Thread: M4 pixels not all the same size. really?

  1. #11
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    Quote Originally Posted by steaminghacker View Post
    So the 10 scan lines per cell mean 24*10 = 240 which is why that's the same for the hi-res card. bandwidth?
    It's not "bandwidth", per se, it's just the fact that Radio Shack used an off-the-shelf NTSC monitor chassis when they built the Model III and they elected to make the Model 4 electrically backwards compatible with it. If it had a PAL monitor you'd have no problem, you could push 300 lines per frame. (Albeit at a lower refresh rate, 50hz instead of 60hz.)

    If the Model 4 had a "regular" 6845 CRTC handling the video timing you could *try* programming it to give you more lines per frame, IE, come up with a custom video mode that, since the pixel clock is fixed, would run at something like a 56hz refresh vertical refresh and enough lines to do 24 twelve-pixel-tall character lines, and see if the monitor would sync with it. But my vague impression was that the Model 4 used some kind of mask programmed variant of the CRTC that mostly limits it to the Model III and 4 modes? (Also, in theory at least an oddball video mode like that might put unusual stress on some components in the monitor even if it decided to sync with it, which it may well not.) At the very least you'd probably have issues like there not being enough travel in the sizing pots to fit it onscreen.
    My Retro-computing YouTube Channel (updates... eventually?): Paleozoic PCs

  2. #12
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    I wanted to give this small tidbit as it may in some ways be related and if not it is still an interesting bit of TRS80 CRTC history.

    A long long time ago perhaps mid 1980s I listened as a man explained to Roy Soltoff that original CRTC had defects. I do not remember what they all were except one...scroll.

    Seems this chip had more than one glitch when released for production but no customer had been large enough to demand a repair.

    One fault or defect was scroll would not work. When Roy heard this he commented that explains why scroll didnt work on model 4 and had to be done in software.

    Roy said something like....had hardware scroll worked things sure would have flew by.

    If I can find this conversation I will post here.

    My point is that there seemed to be several bugs with this controller and as I recall that was corrected on later releases. Later releases hardware scroll was fixed and if M4 has newer corrected chip you could redo driver and make use of fixed scroll feature.

    Is it possible pixel quirk could be related to overal quirks with this chip?

    It is just a thought and sorry if I am off base.

    Thank you,

    Daniel

  3. #13
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    Quote Originally Posted by danielbooneamerica View Post
    Is it possible pixel quirk could be related to overal quirks with this chip?
    No. It’s simply due to Radio Shack electing to generate the dots for the graphics characters with the same hardware in Model III and 4 modes even though the cells are different heights. The dots themselves aren’t actually generated by the CRTC.
    My Retro-computing YouTube Channel (updates... eventually?): Paleozoic PCs

  4. #14

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    Thanks for all your inputs on this.

    Despite the M4 being predominantly a business machine, the decision to make inconsistent sized pixels, in my view, was crazy. Obviously, business software will want to make diagrams and charts and this makes it largely impossible. Consider a bar graph for example, where the height of each pixel is not constant.

    Insane.

    So, for my purposes, i want to make game sprites. I tried redesigning these for M4, but they still look poor. So bad, that i may as well run with the same designs for all machines and save memory.

    I actually considered whether i could "hack" the CRT display chip. I know nothing on how to do this, but it also occurred to me that it was probably a bad idea as it may fail on many machines or, at best, work badly on some.

    I think the long term, universal solution would be to use high-res graphics card for the M4 version, with a fallback to graphics as they are now (lumpy) where not present. Or, of course, M4 users could run in M3 mode with less screen if they prefer.

    At some point will do some drawing tests to see how fast i can draw high-res sprites. If they're not too big (like now) + 4MHz, i might get away with it!

  5. #15

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    One last question;

    I understand the hi-res card can "overlay" the usual text. Is this "text", the usual content of the video memory? So what happens if there are graphic characters also in the video memory, do they appear as well?

    I'm currently using the 2K video mode at 0xf800, will that also work with the hi-res card?

    thanks.

  6. #16
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    Quote Originally Posted by steaminghacker View Post
    I understand the hi-res card can "overlay" the usual text. Is this "text", the usual content of the video memory? So what happens if there are graphic characters also in the video memory, do they appear as well?

    I'm currently using the 2K video mode at 0xf800, will that also work with the hi-res card?
    Yes, the overlay text is the normal text screen. The pixels from the text display and hi-res display are XORed together. So they appear as normal unless both are set in which case they result in black.

    The memory mapping of the text screen has no affect on its output to the display. The linear map at 2K will work fine.

    However, note that the Tandy hi-res board and the Grafyx (and its clones) use a different bit to control the text overlay. You'll need to detect them in order to overlay text properly.

    Also note that the text overlay exerts some control over the graphics mode. If text overlay is off, resolution if 640 x 240. If the text overlay is on then resolution depends on the text mode. If 80 column it remains 640 x 240 but if 64 column you get 512 x 192.

  7. #17
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    Quote Originally Posted by steaminghacker View Post
    I understand the hi-res card can "overlay" the usual text.
    I know you're talking about the Model 4, but FYI this behavior isn't available on the Model III Tandy hi-res card. It's one mode or the other.

  8. #18

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    Thanks for the text overlay info. @gp2000, i'm actually using your card detection code!

    Sounds like i'm good for the overlay idea. I don't think this is going to be for version#1. The M4 will just have to look silly with short pixels for now...

    I actually think drawing smallish hi-res sprites should be OK performancewise.

  9. #19
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    Quote Originally Posted by steaminghacker View Post
    Despite the M4 being predominantly a business machine, the decision to make inconsistent sized pixels, in my view, was crazy. Obviously, business software will want to make diagrams and charts and this makes it largely impossible. Consider a bar graph for example, where the height of each pixel is not constant.
    For something like crude business graphs probably the best way to deal with this on the Model 4 would be to proportion things like you're using a 5x2 dot matrix instead of 3x2 and just accept that the top two and middle two "virtual" pixels are stuck together. (IE, pretend like the screen is 160x120 instead of 160x72 and render accordingly.) An effective 1-in-120 error for the height of a bar graph isn't going to matter much.

    But, yeah, it obviously throws a major monkey wrench into any sort of game scenario.
    My Retro-computing YouTube Channel (updates... eventually?): Paleozoic PCs

  10. #20

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    One thing to ask yourself is are there any existing programs out there that use Model 4 standard graphics? My guess is very little due to this issue.

    Btw, if you program just to the hires screen you could potentially have decent portability across the II/III/4/12/16/16B/6000 when using Z80 assembly. They all use essentially the same port driven API for the hires board. Other than memory origination, only the keyboard, file and serial i/o routines would need to change.

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