Re: How does the apple2 RGB card work?

["mdj" <mdj.mdj@gmail.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:

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

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.

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.

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.

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