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S-100 bus machines normally had displays that had 64-chars-per-line. The VDM-1 used a 7x9 format, and I think most cards used that, so that would be 448 pixels. I understand that an NTSC television normally can't go much beyond 320 pixels. Yet it seems one could easily modify a B&W TV to use with these machines.

So I'm wondering what's going on here.

One possibility is that the extra resolution is there if you bypass the RF section, which I believe all the S-100 cards did.

Another is that it's the color information that limits the resolution, and if one ignores color, you can get higher resolutions.

I know there were modifications for the Atari that added 80-column support, including color. So in this case I assume the monitors would have to be very different. But I also know they worked great with Commodore monitors. So how did these monitors extract the right scanning rates and such?

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    Early versions of the WebTV box had a telnet client built in. 80-column text on a small television was perfectly readable. IIRC the device had some custom hardware intended to improve the readability of text on composite NTSC, but I don't recall any details. Whether or not you'd want to sit in front of it for an extended period depended on how much dot drawl annoyed you. – fadden May 29 '18 at 17:28
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    The Amiga 1000 was known for its maximum adherence to the NTSC standard and for having RF. Composite, Digital RGB, and Analog RGB video out. The original OS specifically supported 60 or 80 column default text, with 60 as the recommendation for RF and Composite Color monitors, and 80 as the recommendation for Digital or Analog RGB monitors. You could select 80-column for any, but it was too blurry for normal usage on non-RGB displays. – Brian H May 29 '18 at 17:59
  • @BrianH - very interesting! Oh, and was the 60-column mode color? – Maury Markowitz May 29 '18 at 18:06
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    "good", and even "good enough" are relatively relative - On some TVs, even the 32-character screen on my humble ZX-81 was barely readable. Today, I would probably return it... – tofro May 29 '18 at 21:31
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    @MauryMarkowitz all the Amiga video outputs are in color. There was no luma only output. You are correct that a common way to get crisp 80 columns was to eliminate chroma from the signal, and connect luma only to a cheap mono display monitor with nice green or amber phosphors. This was common with Apple //e and also possible with the Commodore 128. – Brian H May 30 '18 at 0:29
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"I understand that an NTSC television normally can't go much beyond 320 pixels" ... this is an exaggeration of the issue. Try playing back an old 1980s era VHS video onto a TV versus a higher quality source (e.g. broadcast TV or DVD) and you should see a noticeable difference in quality. VHS (not the revised SuperVHS) has a horizontal resolution that is approximately equivalent to 320 pixels (352 is often quoted -- that's the resolution of SIF (or CIF, the PAL equivalent), a digital format intended to replicate VHS quality, and used for example in the Video CD format). Theoretically, an NTSC TV could manage up to 704 pixels horizontally, but this seems rare.

SuperVHS has a horizontal resolution of about 400-420 pixels, and to my understanding this was usually fully displayable by most NTSC TVs, even when connected to the VCR through an RF connection, which I understand was by far the most common approach in the US market (I've only ever used it with PAL so can't comment directly, but PAL TVs could definitely exceed even the SuperVHS quality, and SCART was commonly used as an interconnect that provided better quality than RF). The 704 pixel figure is from 4SIF (or 4CIF for the PAL equivalent), a digital image format intended to be approximately equivalent to the best a standard definition TV could be. In reality, you're unlikely to ever see that, but it is at least a theoretical possibility (and can happen with a digital TV sampling an analogue input signal).

So it seems that an NTSC TV, at least if it is reasonable quality, ought to be able to produce at least 420 horizontal lines. The question is, is this enough for legible 80 column text?

The answer is probably, as long as your font is appropriately designed. 5x7 fonts are commonly used (e.g. in LCD displays on printers and similar equipment) and are perfectly legible. These fonts usually don't include horizontal space, though, so you'll need 6 pixels per character to match them, meaning you should be using a display that provides 480 horizontal pixels. This is potentially pushing the limits of what an NTSC TV can display, but as long as the quality of the TV and your connection to it are good enough, it ought to work.

It's worth noting that the comments to the question mention an Amiga 60-character mode that worked well -- 60 characters and an 8x8 font would suggest that this used a 480 pixel wide display mode and therefore suggests that swapping to a 6x8 font should get you 80 columns working well enough.

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A typical black and white television will be capable of displaying 80-column text that can be read, but such text will generally be sufficiently unpleasant to read that some other kind of display would be preferable.

Among other things, a monitor which is adjusted to have extremely sharp focus and modulate the beam very sharply will generally produce a television picture which is less aesthetically pleasing than one which has a softer focus and slower transitions. Displaying 80-column text doesn't require using any special scanning rates or doing anything that would be incompatible with a typical black and white television, but the amount of blur required to render 80-column text unpleasant to read (if not totally illegible) is only half that required to do likewise for 40 column text. The amount of blur produced by many sets falls somewhere between those two levels.

  • Got it. Ok, but what about the color 80-column systems? – Maury Markowitz May 29 '18 at 17:26
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    I think the main advantage of a b&w set is that it isn't going to try to separate luminance and chrominance, which generally involves some sort of low-pass filter. So a colour 80-column system that conforms to the standards might be workable because the colour subcarrier is deliberately a non-integer multiple of the line length, so that it'll cancel itself out psychologically even if displayed. A system like the Apple, Atari, MSX, etc, that subverts that with a deliberately-off line length might not be so pleasant. – Tommy May 29 '18 at 17:34
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    @MauryMarkowitz: Color television sets would need to interpret a uniform pattern of black and white stripes with a certain spacing (called the "chroma" frequency) as a solid color, which would require interpreting such areas as having a solid luminance value. That stripe spacing is very close to what would be produced in e.g. the middle of a row of eighty letter "U"'s. Some sets will pass through luminance content at frequencies which are either higher or lower than chroma, but it's simpler and easier to filter out anything which is greater than about 2/3 chroma so many sets do that. – supercat May 29 '18 at 17:42
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    @MauryMarkowitz: Some black and white sets may also try to filter out chroma, but in cheaper ones it simply shows up as a "moving checkerboard" pattern on solid areas. – supercat May 29 '18 at 17:43
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    @MauryMarkowitz: The lower resolution for character colors generally stems from efforts to minimize the memory requirements for the display. Storing one color value for each 8x8 box will only cost 1/64 as much memory space, and 1/8 as much memory bandwidth, as storing a color value for each pixel. – supercat May 29 '18 at 18:46
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I once owned a black and white monitor (not sure if it was a computer monitor or CCTV monitor) that I hooked up via composite to a CGA card, and 640x200 (enough for 80 columns) was very sharp. But while it was NTSC, technically it wasn't a television in the normal sense, and if you hooked up through RF, I think you would not be very pleased with the results.

  • So do you think the RF section is a non-no for 80-column generally? – Maury Markowitz May 29 '18 at 17:43
  • @MauryMarkowitz The image through RF will be both soft and noisy, so I would definitely try to avoid it. RF < Composite (CVBS) < S-Video (Y/C) < Component (YPrPb) / RGB < VGA (RGBHV). – snips-n-snails May 29 '18 at 18:12
  • @MauryMarkowitz When my family first got a C64 we hooked it up to an old B&W TV and the picture was pretty bad in even 40 columns. On the other hand my 80 column card worked well on B&W monitors, but not as good on colour monitors. – Ross Ridge May 29 '18 at 19:48
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    Hooking up through a broadcast-quality RF modulator might yield decent results. Most computers, especially those which include sound in the RF signal, shipped with rather junky RF-modulator circuits, however. From what I've read, the picture quality of many cheap RF modulators can be improved by cutting out the circuitry that generates and mixes the sound sub-carrier. – supercat May 29 '18 at 21:25
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I used several B&W television sets as 80 column monitors "back in the day" with acceptable results, but I always modified them to bypass all the RF stages. This required finding a point to inject the video signal into the video amplifier input. One problem was that virtually all B&W sets are "hot chassis", meaning that the AC power cord is connected directly to the chassis. Being young and adventurous, I just measured the voltage at the chassis and made sure the cord was plugged in "the right way" to not get fried. Today, more cautious and with more money, I would add an isolation transformer to the mix. I never tried to use a color set, but I do know your chances of success are much less. (Worked my way through college as a TV repairman. Remember those?) There is seldom perfect registry between the color guns, leading to ghosting. In addition, shadow mask in most picture tubes is coarse enough to be similar in pitch to the dot pitch of the characters, leading to moire patterns. High resolution color monitors of the era typically cost thousands of dollars and were FAR more complex than TV's. An 80's vintage CAD system monitor I worked with had 1024 X 1024 resolution and weighed over 100 pounds. It also had to be in a darkened room to be usable due the the very fine pitch shadow mask restricting most of the beam current.

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