Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!elroy.jpl.nasa.gov!lll-winken!uunet!munnari.oz.au!bruce!labtam!graeme
From: graeme@labtam.labtam.oz (Graeme Gill)
Newsgroups: comp.graphics
Subject: Re: 64K colors?
Message-ID: <10524@labtam.labtam.oz>
Date: 9 May 91 04:19:52 GMT
References: <1991May2.001518.30298@kuhub.cc.ukans.edu> <10497@labtam.labtam.oz> <1991May6.114625.6444@Informatik.TU-Muenchen.DE>
Organization: Labtam Australia Pty. Ltd., Melbourne, Australia
Lines: 32

In article <1991May6.114625.6444@Informatik.TU-Muenchen.DE>, roell@informatik.tu-muenchen.de (Thomas Roell) writes:
> >> While checking out the specs on the 4000-chip, my eyes bugged as I saw
> >> that it is (in theory anyway)  capable of producing 65535 colors on
> >> screen using 2 bytes to produce a  pixel.
> >
> >	As I understand it, this is some sort of HAM (Hold And Modify) scheme
> >where the chip is put in a special mode, and certain colour values are reserved
> >as "instructions" to interpolation hardware to generate colours between two
> >of the available palette colours.
> 
> No, you are dead wrong. The Et4000 is capable of sequencing 16bit pixels.
> There is just the normal timing, except per dot-clock cycle two bytes are send
> to the CLUT. Some CLUTs (or call them RAMDAC) can use this to drive their
> DACs directly. For example the Sierra RAMDAC uses 5 bits (of these 16) for
> each primary color and can thus display 32768 colors simultaneousely.

    I think I was confusing two of the newly released RAMDAC chips here.
The Et4000 controller chip, which (as you say) seems capable of providing 16
bits per clock into a RAMDAC, gives 32768 colours simultaneously when
combined with a suitable RAMDAC (ie.  the Sierra SC11482).
    I was confused because the DEFINICON RACE 1024 VGA card I have seen
combines the Et4000 with a Edsun Labs EL171CEG RAMDAC, meaning that
the 16 bit mode of the Et4000 cannot be used, but the Edsun RAMDAC provides
the HAM type anti-aliasing mode I described above instead. 
    The only trouble with the 16 bit/pixel mode is the data rate needed
at TTL voltage levels. At 100Mhz the multiplexing up to this rate becomes
a difficult problem, and the best solution is to do it inside the RAMDAC
(ie BT549, IMS G180 etc.), rather than having these high speed signals
go accross the PCB.

	Graeme Gill
	Labtam Australia