Path: utzoo!attcan!uunet!mailrus!tut.cis.ohio-state.edu!ucbvax!pasteur!cory.Berkeley.EDU!soohoo
From: soohoo@cory.Berkeley.EDU (Ken "nmi" Soohoo)
Newsgroups: comp.sys.atari.st
Subject: Re: STE vs ST Shifter
Summary: Bit Planes vs. Bits per Gun
Keywords: DAC, STE, Shifter, Bit Planes, Bits Per Gun
Message-ID: <20207@pasteur.Berkeley.EDU>
Date: 2 Dec 89 16:40:31 GMT
References: <20157@pasteur.Berkeley.EDU> <1989Dec1.183920.12339@uokmax.ecn.uoknor.edu>
Sender: news@pasteur.Berkeley.EDU
Reply-To: soohoo@cory.Berkeley.EDU.UUCP (Ken "nmi" Soohoo)
Organization: University of California, Berkeley
Lines: 49

In article <1989Dec1.183920.12339@uokmax.ecn.uoknor.edu> cbdougla@uokmax.UUCP (Collin Broadrick Douglas) writes:
>In article <20157@pasteur.Berkeley.EDU> soohoo@cory.Berkeley.EDU (Ken "nmi" Soohoo) writes:
>>The STE shifter outputs 4 bits of color per gun, which means that
>>the DACs required to go to your video are gonna be different, no?
>>It would mean Atari would have to lay out another chip that's
>>an ST replacement shifter w/o the extra color bit... Seems kinda
>>pointless for them, lots of R&D $$, little return, plus they'd
>>need lots of packages to hit all the shifter versions out there...
>
>
>  Except for one thing.  In Medium res the Atari get 16 colors per screen.  I
>  may be stupid but doesn't this already mean that the shifter has to output
>  4 bits (4 bits = 16 colors)?  All they would have to change is the total
>  number of colors available (from 512 to 4096).  And that is just the 
>  difference of having more shades of the same colors.  The primary colors
>  (Red, Green and Blue) each with 16 shades gives a total of 4096 colors 
>  (16^3).

>cbdougla@uokmax.ecn.uoknor.edu
>
Collin,

You're confusing "bit planes" and "bits per gun"...
Lemme explain:
The number of bit planes in a specific resolution determines the number
of color _table_ entries you can access for a given pixel in that
resolution -- in Atari's low res case, 2^4 = 16, medium, 2^2 = 4,
high 2^1 = 1... Ok, no each color table entry can be filled with a
number that's going to determine which color is sent to the screen,
and in the ST's case that number has 3 bits assigned to each RG^B,
for a grand total of 2^9, or 512, colors possible to be assigned per
color table entry... So you can have 16/512 colors on the screen at the
same time without resetting the color table mid-screen...
Oh, anyway, so the STE's shifter has the same number of bit _planes_
per resolution, but an additional bit _per RG&B gun_, so that each
color table entry can be one of 2^12, or 4096 colors... SO now you can
get 16/4096 colors on screen at the same time.

A DAC takes the value in the color table entry and converts it from the
0's and 1's representation into a voltage value for the monitor.  With
an extra bit being sent to a DAC that only understands 3 bits, we have
a fundamental problem, and unfortunately I don't think it can be
ignored.


--Kenneth "kens" Soohoo				(soohoo@cory.Berkeley.Edu)
  Atari Hacker (Atari's Hacker...)
  "It could be worse, you could get hit by a bus..."
  My opinions are my OWN, _not_ necessarily Atari's. But "hey", who knows?