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CGA on the original IBM PC produced sixteen colors, one bit each for red, green, blue and overall intensity modifier.

The preferred output device was the later-arriving 5153 color monitor, which accepted the RGBI signal in digital form, four bits (each on a separate wire, using TTL voltage levels) and performed its own digital-analog conversion.

This is a contrast with later VGA, where DAC was on the video card and the monitor accepted analog signals. Of course, the number of bits per pixel in VGA was much greater, so it's easy to see why VGA did DAC on the card.

Why did CGA leave DAC to the monitor? One possible answer would be that because there are only four bits per pixel, there is no particular disadvantage, so basically: why not.

But the other CGA output option was NTSC, and that involved doing DAC on the card after all. Admittedly not to the same analog format as the monitor ends up using, but it still seems intuitively likely that some of the circuitry could've been shared. Which in turn suggests there should be some offsetting positive advantage to leaving DAC in the monitor.

So why did CGA leave DAC to the monitor instead of putting it in the video card?

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Conjecture: Cable material that could cleanly transfer three analog video signals, plus sync pulses and identification pins, over several feet was probably not a cheap, mass produced item at the time CGA was introduced. Mind that VGA uses a quite special cable stock (several simple wires and three 75 Ohm coaxial lines), and there was likely only a reason to produce this in large quantities, at consumer friendly prices, once VGA hardware became widespread. In 1982, it would likely have been a niche or bespoke product adding extra cost.

The only other widespread consumer application at that time needing a similar type of cable material would have been SCART - and this cable might simply have been too thick to fit into a DE-9 plug and/or have been harder to mass produce configured cables with due to there having been TOO MANY extra wires. Also, a cable type used in europe might not even have been well known to american engineers in the early 1980s....

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    a cable type used in europe might not even have been well known to american engineers in the early 1980s -- in 1981, I don't think the system was well known in most of Europe, either. The French were committed to it, but the rest of us didn't catch up until the mid 80s, as I remember it. – occipita Apr 9 at 1:54
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But the other CGA output option was NTSC, and that involved doing DAC on the card after all.

I think here the basic logic error of your question is hidden. Colour in NTSC is neither an analogue level, nor tied to intensity. Intensity is the base b&w part of a colour TV signal and formed independently as level (I wouldn't dare call it a DAC, it just emits two levels), while the colour signal is added and encoded as timing-based information.

Admittedly not to the same analog format as the monitor ends up using,

Both are not what a/the monitor is using:

  • With NTSC input, intensity and colour shift of the signal has to be translated into three colour signals with intensity varying on each of the three signals - which means that an even more complex mixing scheme has to be used than the simple DAC aboard the CGA.

  • When using RGBI input, the three colour signals are already decoded and ready to use after intensity gets added (which is a quite simple circuit). Not much to do here.

but it still seems intuitively likely that some of the circuitry could've been shared.

It's less a shared circuitry than leaving out any circuitry at all. the RGBI signal is directly delivered from the digital video logic and handed toward the NTSC encoder - the digital outputs are nothing but grabbing and offering these signals prior to the NTSC encoder.

So if anything, the question is rather why IBM went to all the length of adding an internal NTSC encoder - having it external would have been more appropriate (*1).

Which in turn suggests there should be some offsetting positive advantage to leaving DAC in the monitor.

Since there is none, at least none worth that name, there is nothing to be left to the CRT electronics :))


*1 - In fact, Apple went exactly that way with the Apple /// XRGB video. Its output is 4 TTL level signals - which can easy be used as RGBI. Instead of adding a colour encoder to the video card, they left it to external devices (like the monitor) to decode it into colour or gray levels (a method that Macs also continued to use for quite some time). Due to the low number of signals, any conversion into RGB is just a bunch of resistors. Converting it into NTSC is much more work.

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  • So basically the answer is, RGBI DAC is so trivial, there isn't really anything to be concerned about? Is the same true of EGA, which has an extra bit for each of RGB instead of just one extra bit overall? So then the point at which DAC starts becoming nontrivial is 9-bit color a la the Atari ST? – rwallace Apr 5 '19 at 23:52
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    @rwallace DAC are (almost) always trivial parts. It's only about the addition of levels. A job resistors fo quite fine without any complicated circuitry. For an Atari ST it's just 3 resistors per colour. More level only means adding more resistors. Don't mix this up with the other way around. ADC are pure beasts of hell. – Raffzahn Apr 6 '19 at 0:11
  • The addition of the NTSC encoder circuitry in the CGA is made more puzzling by the fact that a simple resistor DAC to generate 16 gray levels would have been simultaneously more useful and cheaper than the encoder circuitry. – supercat Jan 22 at 22:24

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