5

The IBM 5153 Personal Computer Color Display was a monitor designed to accompany the original IBM PC (albeit released a couple of years later) and provide a color display at sharp enough resolution to be usable with 80 columns of text.

Of course in principle you could use a color TV, but the fine detail was blurry and marred by NTSC color artifacts, so that it was problematic trying to work with much more than 40 columns on such a setup.

The reasons for the difference at the electronic level are clear: Some picture clarity is lost in the RF encoding phase, more in the encoding of color in the chroma signal that was necessary for backward compatibility with black and white TV sets. But what were the differences in the actual picture tube?

If you took all the electronics from a monitor like the IBM 5153 and substituted just the picture tube from a color TV, what would still be lacking in terms of display quality?

  • 1
    At the electronic circuit level, this doesn't really make sense as a question IMO. It's a bit like asking "if you replaced the dashboard of a Ford car with one made by Chrysler, would the instruments work?" Of course you could make them work by changing enough other things as well, but that doesn't seem to be the point of asking the question. – alephzero Apr 5 at 16:58
  • 2
    @alephzero Similarly here, my point is that I know about things like the chroma signal, that have been extensively discussed before, but what about the actual picture tube? And I'm hoping for an answer that talks about things like the phosphors and grid mask. – rwallace Apr 5 at 17:07
  • 1
    @rwallace I expect that the 5153 would have a triad shadow mask where a TV has an in-line shadow mask. It will be interesting to hear if the phosphor or anything else about the CRT is any different. – snips-n-snails Apr 5 at 17:18
  • 1
    Here's the reference guide for the 5153: minuszerodegrees.net/oa/… – snips-n-snails Apr 5 at 17:20
  • 2
    Well, we have a part number for the original tube 370RKB22-TC14 from @traal's reference, but nothing much more than that on the Web, it seems. I was taking the OP's question a bit more literally: if you just connect the 5153 output that drives the tube to a "colour TV tube" with different deflection sensitivity, different HT and intensity modulation and focus voltages, and different heater supplies, you will most likely get either nothing or a complete mess (and in the worst case, a loud bang and a lot of magic smoke). – alephzero Apr 5 at 19:17
5

If you took all the electronics from a monitor like the IBM 5153 and substituted just the picture tube from a color TV, what would still be lacking in terms of display quality?

The answer to your question depends almost entirely on which color TV CRT you wanted to use as a monitor. While any color computer monitor from the 1980s could serve as a color TV, just by adding the needed TV tuner circuitry, the converse of this statement is not true.

The suitability of monitor tube as TV vs. TV tube as monitor comes down to the physics of light, the discernment limitations of the human eye, and the tradeoffs between brightness and sharpness that are inherit in CRT shadow mask specifications. Fundamentally, a TV is (usually) meant to be viewed from across a room while a computer monitor is meant to be viewed from a couple feet away. For the former TV, brightness is critical as the light travels farther before reaching the viewer. For the latter computer monitor sharpness is critical because small text and fine line graphics are best viewed that way, and because broken areas caused by inter-pixel spacing are visibly discontinuous at this close viewing range.

With these considerations, it is obvious that certain specifications, like a higher dot pitch, may be perfectly suitable for a TV while being severely degrading to the quality of a computer monitor display. Specifically, a dot pitch of 0.28 or less is often cited as appropriate for computer monitor CRTs, while TVs can go significantly higher. So, that means you ought to choose a TV CRT that has the right specs to use as a monitor. In the case of you care greatly about monitor quality, you might spend the extra money required to build the CRT to your desired specification when there were no mass-produced CRT's already meeting those specifications.

A case in point would be Apple's early RGB displays. These were very high spec'd and expensive CRT's optimized for sharpness and a small dot pitch. The screens were often only 9 to 12 inches diagonal, which is usually too small for a consumer grade color TV. So Apple opted for the CRT that emphasized the specs that matter for close-up viewing, and their customers paid the extra cost to reap the benefit.

NOTE: I've seen the IBM 5153 dot pitch claimed to be 0.31. This would make it so-so quality, and would explain why many PC users preferred working in MDA/Hercules modes, or springing for a Princeton color monitor.

  • 3
    "A case in point would be Apple's early RGB displays. These were very high spec'd and expensive CRT's optimized for sharpness and a small dot pitch" - the Apple 13" RGA remained the best color monitor I used until I had already moved off CRTs. I used a number of high-spec Multi-Syncs and so forth (albiet not the really high end ones) but the Apple trinitrons crushed them for clarity and saturation. – Maury Markowitz Oct 8 at 19:21
  • 2
    My experience is that the IBM 5153 was indeed so-so quality. Could see some fuzz to it when I used one after having used a higher quality CGA display earlier in the day – Brian Knoblauch Oct 10 at 14:36
  • The question is not about some high quality workstation screen, but the 5153, which was not any differnt for a TV at the same time. – Raffzahn Oct 10 at 21:40
4

The question doesn't make much sense.

CGA works within TV spec of a standard line rate close to 16 kHz. Thus any CRT fitting must be capable to operate at ~16 kHz. Which is what each and every TV tube does. The mask of any colour CRT at this time was way better than needed for 640 pixel (80 columns with 8 pixel each). As a result next to any colour CRT can be used.

There is no commercial sense in making a special CRT for a few thousand IBM displays when standard TV CRT where already produced by the million.

Te real difference is how the data (signals) are presented to, and turned into beam motion, by the display electronics. When it reaches the tube it's all the same.

  • 3
    I strongly suspect that "The mask of any colour CRT at this time was way better than needed for 640 pixel (80 columns with 8 pixel each). As a result next to any colour CRT can be used." is exactly the answer that was being sought. – Tommy Apr 5 at 20:43
  • 3
    I believe TVs used a slot mask to optimize brightness while monitors used a dot mask to optimize resolution/clarity. – snips-n-snails Apr 5 at 23:54
  • 2
    @traal You're right, different mask designs are optimizations toward different goals (as always in in real life), but at the level we're talking it isn't as big as it may seam. Then again, the dot (or shadow) mask is the default for TV. It wasn't until 1980 that Sony Triniton became a mainstream success with their slit masks. Slot (or EDP) mask is a development of Hitachi of 1987 (way after the 5153) combining both ideas - large elliptical slots grouped as a triangle. So when looking at the 5153 time frame, it's dot masks either way (unless they where Sony - which they were not). – Raffzahn Apr 6 at 0:24
  • 3
    "The mask of any colour CRT at this time was way better than needed for 640 pixel (80 columns with 8 pixel each)" - er, no. repairfaq.cis.upenn.edu/sam/crtfaq.htm#crtdpt vcfed.org/forum/showthread.php?14701-Pre-VGA-Monitor-dot-pitch – Bruce Abbott Apr 6 at 1:00
  • 3
    Sorry I can't find more 'reliable' ancient CRT specifications at short notice, but it is a fact that the dot pitch of standard TVs was way too low to resolve 640 pixels. It is the reason I bought the Amstrad CPC664 with mono monitor, because the color monitor used a standard TV tube that could not display 80 column text clearly. Even 40 column text was marginal commons.wikimedia.org/wiki/File:Amstrad_CPC_screen_closeup.jpg – Bruce Abbott Apr 6 at 2:14
4

CGA, EGA and VGA each have scan rates broadly comparable with broadcast TV signals. There should be no real difficulty in getting a reasonably good-quality TV picture tube to display a CGA signal, regardless of its physical construction (of which there were many, many variants).

The major difference you would probably run into is in beam focus. TVs were designed to display mostly photographic material, and broadcast stations learned to adapt overlaid text and graphics to the typical TV's capabilities. This included Teletext, which adopted a 40-column text format and character generators optimised for TV characteristics.

A computer display, by contrast, is much more likely to have a viewer interested in pixel-level detail, and the pixels of an 80-column terminal are 2-3 times narrower than the typical beam width of a TV. I remember using a (non-CGA) RGB monitor of the time with a BBC Micro, and noting that individual horizontal pixels could not be resolved in the 80-column modes.

On some TV tubes, you may be able to adjust the beam focus to a narrower pattern and thus improve resolution.

Another difference is likely to be in the RGB phosphor colours. These have different specs between NTSC and PAL, never mind computer monitors. However, CGA is not exactly reputed for high colour definition, so it probably won't matter.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.