Re: DHGR Video Riddle Solved!!!

["Arkain" <rwking@no.usa.spam.net>]

Close, but not quite David...

There are 14 edges in the clock pulse before the Video Phase signal falls.
In 40 column mode, the rising edges of these pulses are used to both
generate a 7MHz clock and load data bits onto the video bus. In B&W mode,
the rising edges of the 7MHz signal are used to shift the bits out onto the
video display. In color mode a 3.5MHz burst causes the bits to be shifted
out on the falling dges instead.

In 80 column mode, the only difference is that no 7MHz signal is generated.
The rising edges of the 14MHz signal is used to shift the data onto the
display. Once again, a 3.5MHz color-burst signal causes the bits to shift on
the falling edges of the clock in color mode.

Even in 80 column mode, you get a 1/2 pixel shift. This is why 80 column
text in mixed mode under a hires display is prone to color-bleeding on
composite displays. The 1/2 pixel shift is better described as a phase
shift.

As it turns out, there is a color generator table in the character ROM that
the system uses to display colorized blocks on the screen. It seems that in
DHGR mode, if bit 7 of the byte read is clear, then the color table is
accessed to grab a color pattern. If the bit is high then the data is
displayed in the same way as the HGR display. It seems that only the even
byte Aux Memory bit 7 actually matters, but I'm still testing this... I came
to this conclusion from using tests at the Monitor, but since the Monitor
plays with soft-switches itself, it may have been corrupting my results. I'm
writing a pixel-level driver in assembly to do further testing.

Arkain

David Wilson <david@uow.edu.au> wrote in message
news:39bc35eb$1@news.uow.edu.au...
> Matt Jenkins <mdj@bilby.cs.rmit.edu.au> writes:
>
> Yes, I have the file://e Tech Reference Manual and Jim Sather's
"Understanding the
> Apple IIe". In HiRes mode bit 7 determines the color set by causing a 1/2
> pixel shift in the output bitstream. In Double HiRes you are putting
pixels
> out at 14MHz already so no shifting can take place. You only have 560
pixels
> to display and that is 7x80.
>
> With composite video you effectively always have a line of 560 pixels of
> black or white. The color burst tells the monitor to interpret this as
color.
>
> With RGB cards the otherwise unused bit 7 in DHR can be used to modify
> how the color is generated by the RGB card.
> --
> David Wilson  School of IT & CS, Uni of Wollongong, Australia