[Coco] [Color Computer] Re: Coco3 256 color mode
jdiffendaffer at yahoo.com
Sun Jun 6 17:58:21 EDT 2010
Quoting from the mailing list is a pain but...
>jdaggett at gate.net jdaggett at gate.net
>Sun Jun 6 14:52:53 EDT 2010
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>On 6 Jun 2010 at 6:57, jdiffendaffer wrote:
>> Even if the mode was left in the chip, the number of bits for DA
>>conversion of the video output was reduced so only 64 colors would
>>be possible even if it's still there. If the DA conversion hadn't
>>been messed with, a 256 color pallet would have been possible for
>>other modes instead of just 64. Whether or not Tandy would have
>>supported 256 colors in other modes is anybody's guess.
>For 256 color mode you do not need the color palletes as the raw
>data from memory would
>be fed directly to the D/A. The question still arises "does the GIME
>D/A convertor have 8 bits
>or is it 6 bits"? No one knows for sure.
Um... yeah.... too bad that's not what I was talking about.
Notice the words "other modes".
>> If I remember right, the color palette info was 1 bit for each
>>color and 5 for intensity. But that doesn't really make sense to
>>me, I would think it was 2 bits each color and 2 intensity... but
>>that's just my opinion.
>In RGB mode it is 2 bits for each color. In Composite mode it is two
>bits for intensity and 4
>bits for phase.
I was referring to the Tandy engineers comments on the 256 color mode. I think he said 1 bit for each color... which makes no sense and doesn't match the standard palette registers in any way.
*IF* there were a 256 color mode and it passes the values directly to the D/A part of the video circuit, then it stands to reason that in other modes the palette registers would use the same data values. It takes more hardware to convert palette data than to just pass it through. Hence, a 256 color palette to choose from for other modes. (16 of 256 for example)
However, you could save a few gates by making palette registers use fewer bits and hard wiring unused bits as high or low. For 16 registers the savings isn't exactly huge though. And if you drop those bits, how do you keep the palette from being too bright or too dim? A lookup table? That would be bigger than using 8 bit palette registers so I think it's obvious the D/A converter got chopped down. That reduces the number of colors but they are spread out better.
For RGB, I would think the 256 color mode would use an extension of the 2 bits for each color... so with only 2 bits left you can't add another color bit but you could add 4 intensity levels of a color combination.
For Composite... you either add more intensity bits or phase bits. Since I'm no video engineer I don't know what the impact would be of either. I think you would need more intensity bits but I really don't know.
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