Re: Interlaced Apple II video

["Joel" <joelbuckley54.nospam.@hotmail.com>]

----- Original Message -----
From: "Michael J. Mahon" <mjmahon@aol.com>
Newsgroups: comp.sys.apple2
Sent: Friday, October 25, 2002 3:17 PM
Subject: Interlaced Apple II video (was Re: Floppy Data Capacity)


> Joel Buckley wrote:
>

> >Sort of like my 384 vertical-line, interlace scan project for the ][+ and
//e.
> >I'm pretty sure I still have the working prototype for that somewhere in
the
> >basement. It was a simple design, and worked well but never got around
> >to putting it into production.
>
> Interesting.
>
> I assume that you page-flipped between hi-res page 1 and 2 during
> vertical blanking and used a hardware technique to delay the start
> of the even field by 1/2 line?
>
> More information would be very interesting.
>

That's exactly right. It seems so obvious, why didn't anybody else do it?
With my Apple ][+, all the ic's we're socketed, so you could lift up a pin
to jumper a signal out to a breadboard circuit and back.
If memory serves (I haven't dug out my old notes) I tried several
techniques.
One was a hardware page flipping toggled by Vertical sync or blank and
activated by an Annunciator output.
While that was the most straightforward, it involved intercepting on board
circuitry.
A second technique was to run the VSYNC into the NMI and switch the pages
via software.
I used NMI rather than IRQ because of the monitor IRQ zero page bug.
I had used 60 Hz into NMI before to have a real-time clock (good only while
power was on).
It was gated with an Annunciator output, because you had to turn it off on
any disk access.
(I used another output to control a relay for a telephone dialer. My old
JCat modem didn't have any programmed dialing support.)
It was easy to toggle the delayed VSYNC inside the Apple circuitry for my
prototype.
But if I had wanted this to be a practical project, you couldn't be running
wires all over the motherboard.
It would be more practical to extract the standard vsync from composite
video out, create a delayed version, and then recreate the composite
interlaced video.
It would be a bit more complicated and I never got around to doing it that
way.
I made some demo graphic images, diagonal line with half the staircasing,
etc.
It looked good.
It didn't work well with every monitor. With interlacing, you'd like 1/30
sec persistence. The regular Apple only needed 1/60th.
I considered designing a card for the //e slot 7 to do it. This should be
realtively easy because they brought out the composite video sync signal on
pin 19 (strangely slot7 only). So you're saved the trouble of separating the
sync from the composite video. I was disappointed that they doubled the
horizontal resolution, and didn't take the little extra effort to double
vertical as well.
The video-overlay card supports interlace scan. Most of the time it was just
used to line double the standard Apple video during video overlay. Though I
remember seeing a demo, that used it to depict a 400 vertical line desktop.

The ][+ was really a fun machine to tinker with. One person could understand
every line of code in the operating system, and every piece of circuitry on
the mother board. Those were the days!