Re: Video Question

[dempson@actrix.gen.nz (David Empson)]

Bryan Parkoff <BParkoff@satx.rr.com> wrote:

> Video Question
> 
>     How do Apple II make color in the screen?  How do it work?

Assuming you are talking about the (original) composite video output: it
is all in the timing.  The Apple II generates dot patterns which trigger
the appropriate colour response from an NTSC television set.

The hi-res graphics colours (green, violet, orange and blue) work by
generating a square wave at the correct frequency (every second pixel).
The timing of the square wave relative to the start of the line
determines which colour is generated.  The general principle is as
follows.  If you use green as a the base point, then orange is 90
degrees after green, violet is 180 degrees after green, and blue is 270
degrees after green.

Here is a rough picture:

Green        |-----|     |-----|     |-----|     |-----
        -----|     |-----|     |-----|     |-----|     

Orange  --|     |-----|     |-----|     |-----|     |-----
          |-----|     |-----|     |-----|     |-----|     

Violet  -----|     |-----|     |-----|     |-----|     |-----
             |-----|     |-----|     |-----|     |-----|     

Blue      |-----|     |-----|     |-----|     |-----|     |-----
        --|     |-----|     |-----|     |-----|     |-----|     

The high and low periods correspond to 1 and 0 bits in the hi-res
graphics video buffer.  The "half cycle" colours (orange and blue) are
generated when bit 7 is set, which delays the output signal by half a
bit time.

If two adjacent pixels are the same value (both 1 or both 0) then white
or black is generated.

There are some odd effects which can occur at byte boundaries, which
result in other colours appearing if the colour set (bit 7) changes
state.  This can result in other colours like yellow and brown, but they
only appear every 14th pixel.


Lo-res graphics mode works by generating a predetermined pattern which
triggers the appropriate colour to be generated by an NTSC monitor.


With an RGB monitor, the video controller interprets the dot pattern to
generate the appropriate colour.


If you want to know more, I suggest getting hold of a good book on the
subject, such as "Understanding the Apple IIe" by Jim Sather.


-- 
David Empson
dempson@actrix.gen.nz