Re: DHGR Color Question

[mjmahon@aol.com (Michael J. Mahon)]

Bryan Parkoff asked:

>    DHGR's 7 pixels are in 7 bits that contains 8 colors in the even and
>another 8 colors in the odd that are total 16 colors.  Who invented double
>high resolution on all Apple II Family?  Who overcame the limitation of 16
>colors in 7 pixels per byte that 6502 can only handle 16K space.  One 8K in
>even bank and another 8K in odd bank.  It is true that it does not look
>neat, but it should be 16 colors per pixel in 7 pixels.  Is it what they are
>trying to save memory space so they overcame the limitation?
>    Think about Apple IIgs and VGA -- they look much better than double high
>resolution because each pixel can have 16 colors per byte however double
>high resolution can only have 8 colors or less per pixel in 7 pixels?
>    Can you please explain how clever they did?

The cleverness was in finding a way to integrate an 80-column text
display within the Apple II video framework.  This is why the video
bandwidth was doubled to 560 monochrome pixels per line.

DHGR was a straightforward extension of the double-bandwidth
video system that was not fully implemented on the first //e revision.
(An interesting "echo" of the lack of implementation of two HGR
color sets in the first revs of the Apple ][+.  ;-)  To complete the
DHGR implementation, it was necessary to "steal" the AN3 output
to do the mode switch.

When in DHGR mode, there are 140 pixels per line, with 16 color
possibilities.  The colors are still "artifact" NTSC colors produced
by four bits per pixel (two from the main bank and two from the
auxilliary bank).

When DHGR mode is used in monochrome mode, there are
560 pixels per line, each under software control.  On the //e
and //c, there is no hardware monochrome mode (the color
burst is always generated for graphics modes), but the color
interpretation of the pixels can be cancelled by using a mono-
chrome monitor, or putting a color monitor in monochrome mode.

-michael

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