Re: Video Question

["Bryan Parkoff" <BParkoff@satx.rr.com>]

Where can I get "Understanding the Apple ][" and "Understanding the Apple
//e".

--
Yours Truly,

Bryan Parkoff
BParkoff@satx.rr.com
"David Empson" <dempson@actrix.gen.nz> wrote in message
1fnipif.beubkl1nirzggN%dempson@actrix.gen.nz">news:1fnipif.beubkl1nirzggN%dempson@actrix.gen.nz...
> 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