Re: How does the apple2 RGB card work?

["Bryan Parkoff" <none@nospam.net>]

"mdj" <mdj.mdj@gmail.com> wrote in message 
1148878864.669334.237510@38g2000cwa.googlegroups.com">news:1148878864.669334.237510@38g2000cwa.googlegroups.com...
> Michael J. Mahon wrote:
>
>> The algorithms used by the common Apple II RGB cards and by the
>> IIgs RGB conversion are not fully understood yet, but Bryan is
>> working on it.
>
> While it's true the IIgs RGB conversion is not fully understood, the
> behavior of the Apple II RGB cards, in terms of their outputs is fairly
> well documented.
>
> http://www.freepatentsonline.com/4631692.html
>
> The URL above describes the technique used by VIDEO 7 for double hires
> modes. The card sold by Apple (AppleColour Extended 80 Column Card) was
> made under license from VIDEO 7
>
> There's a big difference between IIe RGB and IIgs RGB: The IIgs
> attempts to emulate composite DHR. IIe RGB on the other hand, provides
> nice, clear, purified picture. You could say that for HIRES, the
> approaches used by IIe RGB and IIgs RGB are equivalent in terms of
> functionality, but DHR is a very different story. The IIgs will by
> default provide a (IMHO terrible) approximation of colour DHR. This can
> be overridden with monochrome mode, provided by the control panel.
>
> The IIe RGB on the other hand, implements DHR as 3 seperate modes:
> Colour (Mode 1), Monochrome (Mode 2), and MIX (mode 3). These modes are
> implemented by toggling 80-Col and AN3 in certain sequences, which
> tells the card which mode to access. If a piece of software that used
> DHR is not RGB aware, it could possibly enable the wrong mode, and will
> require a preboot program to override the mode to whatever is
> appropriate.
>
> The modes are as follows:

    Thank you for a full explanation which it has the difference between 
Apple //e and Apple IIgs.

>
> Mode 1 - 140x192 16 Colour. Basically you get DHR colour, without
> artifacts, using whichever digital RGB colour most closely resembles
> the Apple Colour.

    It is Mode 3.
>
> Mode 2 - 560x192 Mono. Colour Information is ignored, giving full
> 560x192 monochrome graphics.
>
> Mode 3 - MIX. A byte is considered to be either colour or mono
> depending on the state of the high order bit in each display byte. In
> DHR, this bit is irrelevant in terms of colour information or raster
> pattern, so is reused in this case to provide an interesting mode.
> Dazzle Draw at least, makes it possible to utilise this mode for
> drawing DHR, and outputs its picture files in a manner that maintains
> compatibility with Mode 3.

    It is Mode 1.
>
> These are the three modes documented by Apple in the AppleColour RGB
> manual. The Video 7 hardware is actually capable of two additional
> modes that aren't documented by Apple, and are thus less likely to
> appear in third party implementations:
>
> Mode 4 - 160x192 16 colour. In this mode, all bits are considered,
> providing a nice unrestricted colour mode, that is also significantly
> easier to program, since pixels fall on byte boundaries.

    It is Mode 0.  I was able to convert EGA daa from 320x200 to 160x200 as 
640x200, but it was shrinked into 140x192.  I did demo The Black Cauldron 
(PC Version) into Apple //e's RGB card.

>
> There is one other mode, this one is really wierd:
>
> Mode 5(?) 280x192 16 Colour. This mode is essentially HIRES, treated as
> Mono, and the aux bank is considered as a giant palette register. I'm
> unaware of the specifics of this mode, but an example of how to use it
> was provided on the AppleColour RGB demo disk, even though it wasn't
> formally documented by Apple. One common use is to 'emulate' HIRES
> mono, which isn't supported by RGB cards, by filling the AUX bank with
> the appropriate colour information.

    It is undocumented.  The problem is that monochrome needs more data 
because 7th bit is not available to reset/set monochrome like DHGR.  Seventh 
bit is used to switch violet/green color and blue/orange.  The only way is 
to put monochrome data in the screen hole.  It needs 960 bytes, but screen 
hole has only 512 bytes available.  The only way is to store 480 bytes 
leaving free 32 bytes in the screen hole.  It has 2 1/2 bytes or 20 bits 
each horizontial lines.  It gives monochrome in each a group of two bytes. 
It is 20 group of two bytes.  It is great memory saving.  I may be able to 
demo at later time.

Bryan Parkoff

>
> So there you are: Apple IIe RGB in a nutshell. As has been mentioned in
> previous threads, I'm unaware of whether the IIc adapters made by VIDEO
> 7 supported all these modes, and await the sourcing of some
> documentation to find out. The absence of AN3 on the IIc makes me
> wonder if they do, but I suppose if it's possible to detect 80Col from
> the Video Expansion Connector, it's feasible.
>
> I obviously haven't tested every variant of Apple II RGB adapter, but
> I'd venture that since VIDEO 7 had a patent on the technique, that
> every other manufacturer produced theirs under license, and therefore
> most probably conformed to this "standard".
>
> Matt
>