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Thread: SCENIX VIDEO VIRTUAL PERIPHERAL Design Challenge and Contest
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face BY : Nikolai Golovchenko email (remove spam text)



James Newton wrote:
> I've been thinking about sync detect without using an external sync
> separator. Doesn't seem to be much on the web about doing that with a
> microcontroller...

> ...but it also seems to me like it ought to be very possible.

Sync detection is sure possible with a SX. I made a quick
test on the evaluation board (50 MHz SX28), just for
horizontal sync, using the comparator to catch the sync
pulses. It works in most cases, but sometimes, when video is
bad quality (noisy, but screen is stable) lines are missed
with that setup.

There are a few problems:

1) Comparator lowest input voltage level is 0.4V. With a 0
to 1V video, sync threshold is at about 0.15V. So it is out of
specs, though seems to be working.

2) Noise. I wonder how screen remains stable even when
signal is noisy. There must be some filtering in a TV. I'll
try to dig the sync separator circuit from an old TV.

3) SX counts the pulse width in a loop like:
       .
       .
       .
measure_ov2
       mov w, #5
       mov dx, w
measure3
       mov w, #128-100
       mov RTCC, w
       mov w, #compMode
measure3a
       mov !RB, w
       mov temp, w
       snb temp.0
        jmp measure_ok
       sb RTCC.7            ;or any other reg my be used for
                            ;counting
        jmp measure3a
       decsz dx
        jmp measure3
;too long - repeat pulse catch
       jmp wait_1
       .
       .
       .

The inner loop takes 8 cycles (without comparator that would
be 6 cycles). So the comparator is sampled every 8 cycles,
and maximum jitter in sync detection is 8 cycles. That can
be visible if SX frequency is too low. Probably, it will be
better to build an external circuit on a transistor and not
use the comparator altogether.

Well, the text over video is a cool idea! I'll play with it
a bit more. :)

Nikolai

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