Re: Interlaced Apple II video

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

Joel wrote:

>----- 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.

Actually, there were several products that used milled-pin sockets or
small PC boards to tap/intercept/modify main board signals.  I suppose
there is a small upper bound on the number of such "daughter boards"
that could be tolerated in one system.  ;-)

>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 much easier just to knock out most of the first half-line of the
vertical sync pulse (which is not serrated on an Apple).  I would add back
to the composite video on alternating fields, a signal which starts where
the horizontal sync pulse would end and ends halfway through the line.
If the signal were added with the same amplitude as the sync, the result
would be a 1/2-line delayed vertical sync pulse every other field.

It might be necessary to lengthen the end of the shortened vertical sync
pulse to keep its total length nearly constant, but most modern monitors
would probably sync stably without problems.

A related issue is that each alternate field occurs 1/2-line later and then
1/2-line earlier, rather than always 1/2-line later in a standard video signal.
This would probably result in the lines of the two fields not being equally
spaced from each other, but could always be compensated by shifting
the start of the delayed vertical sync slightly.

The actual "knockout" pulse could be generated by an annunciator,
assuming that solid "vapor lock" to the horizontal sync was achieved.
It sould be possible to accomplish this within the vertical blanking
interrupt routine.

>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.

This would be a nice complement to the pseudo-double hi-res technique
in which different color sets are used to plot points with 560x192 resolution.
Of course, the a monochrome display is used, and the standard waivers
apply about plotting points with different "color sets" in the same byte.

This technique also cuts the stairstepping by a factor of two, particularly
visible on nearly vertical lines.

>It didn't work well with every monitor. With interlacing, you'd like 1/30
>sec persistence. The regular Apple only needed 1/60th.

This would be another plus for a monochrome display, since they
typically have a longer persistence phosphor.

>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 sync signal is dostributed to slot 7 because Apple intended it to be
used to adapt the Apple output to different video standards--like PAL.

>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.

Actually, the VOC supports all the IIgs video modes!  Interesting that no one
has written any software to use it that way--to my knowledge.

The simplest--read lowest chip-count--way to get the Apple II video generator
on a card was to use the IIgs video generator.

>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!

Amen to that!

In my opinion, computer hardware is personally interesting in proportion to
how understandable and manipulable it is.  Practically every personal computer
since the Apple ][+ (including even the //e, with its increased integration,
and
the IIgs, with its overlayed kludges) has been less transparent and, therefore,
less interesting to me.

This is a general problem in the electronics field, since higher levels of
integration have made most equipment unknowable and unmodifiable.
I learned electronics by taking apart old radios and TVs and making new
circuits out of the components.  The complexity barrier has virtually closed
that path to today's kids.  ;-(

And what is true in hardware is even more true in software.  The Mac used
to boast that the manuals required to understand it and its software weighed
less than the computer.  While this is certainly true for the 8-bit Apples,
both
the IIgs and the Mac fail the test pretty badly.

-michael

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