Is a normal B&W TV good enough for 80-column text?

Question

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?

Answer 1

"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.

Answer 2

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.

Answer 3

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.

Answer 4

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.