Why is the TIA's color palette limited to 128 colors?

[Secamline]

Instead of say, 256 colors?

[Thomas Jentzsch]

I found the following, interesting answer here (not sure if it is correct):

The TIA manages a very large palette on NTSC and PAL systems because it takes advantage of the way that composite colour is encoded: three bits produce luminance, and the colour subcarrier is always exactly the same sinusoid, but four of the other colour bits set its phase.

So for both NTSC and PAL:

• three bits set the amplitude of one signal;
• four bits set the delay of another signal; and
• the two things are summed for output.

To verify this, look at an NTSC colour wheel, and look at which angle each of the Atari's NTSC colours appears. They're just steps around the outside of the colour wheel.

Even PAL suffers a little from the NTSC-first logic; it has 12 hues instead of 15 because the NTSC subcarrier is approximately 12/15ths the frequency of PAL's, and the delay steps are NTSC oriented.

SECAM doesn't work in the same way. Performing a phase shift doesn't actually make any difference — the colour subcarrier is a single channel in frequency modulation, not two channels in quadrature amplitude modulation. Nothing about a SECAM output is determined by phase. The colour-shifts that result from phase errors is exactly what SECAM sought to fix.

So more complicated electronics are required, and the elegant hack of just doing a phase shift isn't available. More logic is required, and Atari did what it was cost effective to do, which is to implement a palette much like the other RGB-oriented machines of the era.

In short: the Atari's disproportionately-good palette on NTSC and PAL systems is because Atari exploited the way QAM composite encodes colour. SECAM does not use QAM. Therefore a much more basic palette, closer to other machines of the era, was implemented.

[Secamline]

Oooh, I didn't expect this.

[Video]

I was going to say it used a seven (for some reason) bit code. That sounds more technical though, interesting read.

[scrummy]

That is really wild stuff from our favorite console. Thanks Thomas for this insight.

[freshbrood]

How does one make a pure yellow in Pal? It seems like certain primary colors are missing completely.
 

[+SvOlli]

I remember it was noticed in one of the videos I watched to prepare for my Ultimate Atari 2600 Talk. I think it was Joe Decuir who stated that they originally had 256 colors, 16 colors, and 16 hues. During development they noticed that 4 bits of hue led to being those colors "to close to each other" and that they could spare a couple of transistors on the chip just by dropping the least significant bit.

[Level42]

On 12/26/2020 at 1:13 PM, SvOlli said:

I remember it was noticed in one of the videos I watched to prepare for my Ultimate Atari 2600 Talk. I think it was Joe Decuir who stated that they originally had 256 colors, 16 colors, and 16 hues. During development they noticed that 4 bits of hue led to being those colors "to close to each other" and that they could spare a couple of transistors on the chip just by dropping the least significant bit.

Hmmmm....why would they need to drop a few transistors from TIA's design ? The only reason I can think of is that it is so crammed that else they couldn't make everything fit ? I mean, there is no cost advantage in leaving out a couple of transistors on a chip, right ?

By the way, the original question should have been: how on earth did Jay Miner and his team manage to get 128 whopping colors out of a 1977 machine while absolutely nothing else could manage that at the time......?  

The second post nicely answers that question and I actually never realized this neat trick.

 

Naturally the GTIA in the Atari 8 bit computers nicely topped this trick to 256 colors. I wonder how they did it in that chip because AFAIK both NTSC and PAL machines can both produce 256 colors.

 

To me, this has always been an incredible advantage of the VCS and A8 machines over anything else at the time: games already looked much more different by the freedom of colors that the programmers could choose. Compare to f.i. the Odyssey2/Videopac with it's basic RGB colors and, of course, the C64 where all games use the same purple and brown all the time.... for me this was one of the reasons to stick to Atari when I went from game console to computer.

Edited September 3, 2022 by Level42

[Keatah]

37 minutes ago, Level42 said:

Hmmmm....why would they need to drop a few transistors from TIA's design ? The only reason I can think of is that it is so crammed that else they couldn't make everything fit ? I mean, there is no cost advantage in leaving out a couple of transistors on a chip, right ?

I suppose there could be several reasons. One reason, like you say, is it gave them extra real estate for other functionality. Another possibility is a die-shrink. Less parts = smaller die = more die per wafer. It's more of a thing today than it was back then. And then less parts means less chance for failure. Less power consumption. Different spacing of parts in the area..

 

Could also be an inaccurate recount of the situation. Did they really need to cut the transistor budget at all?

 

I also thought the VCS and 400/800 had the best color palettes of the time. Thought that then and think that now. The vividness. The separation. The saturation. Compared to today's drab games, modern, remake, whatever, the vivid palette is a huge win!

Edited September 3, 2022 by Keatah