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Re: How does the apple2 RGB card work?

General_Failure wrote:

Bryan Parkoff Said:


  I want to make sure what I understand your view.  You want video modes
(TEXT40/80, LORES40/80, and HIRES40/80) from aux/main memory to be mapped
into video memory.  Then RGB card reads video memory and displays pixel by
pixel to the screen.



Yes. Well, partially. It's more of a mirroring than mapping. I was
thinking of a card which keeps a copy of the video data on its own
board. The video data is based on listening to write accesses performed
on the video area of memory. The card would never need to interact with
the system bus, except in its passive listening state.


And you would also have to track Main/Aux bank switches, too.


  I do not think that 80 Col / RGB card have video memory.


The 80 column/language card in my //e does. It has 64kb. However I
suspect that the RGB card does not. Having never seen an actual unit or
a schematic of one I cannot tell.


Virtually all AUX slot cards contain memory, since a primary function
of the slot is to support AUX memory for "80-column" modes.  2KB is
the minimum, 64KB is typical, and 1MB+ was widely supplied by third
parties.

However, no RGB card for any Apple II (with the exception of the VGA
SecondSight card) "mirrors" all of video memory--only the AUX memory
is on the card, and it is not accessed *by the card* for video display.

Instead, the Apple accesses both main memory and AUX memory to generate
composite video, which is supplied to the AUX slot.  An RGB card decodes
this composite signal to generate R, G, B, and "I" signals for a digital
RGB monitor (originally, the AppleColor 100).


It only runs
at 1/14M or 70 ns to read NTSC pulse from aux or main memory.  NTSC pulse
goes through the RGB circuit to translate into R, G, B, and I signal.


By NTSC pulse from memory, what exactly do you mean? Are you saying
that the colour data in memory is pre-encoded with NTSC modulation. Ie
with the phase shifts coded into the colour information?


Basically, yes.  All Apple II color is "artifact" color resulting from
video signal pulses which contain 3.58MHz "color" information, in
various amplitudes and phases, causing an NTSC monitor to display color.

The sync, blanking, and color burst are generated by logic, not read
from memory, but the entire structure of the video line is created from
information read from memory--without any translation in the case of
hi-res and double-hi-res graphics modes.

-michael

Parallel computing for 8-bit Apple II's!
Home page:  http://members.aol.com/MJMahon/

"The wastebasket is our most important design
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