Hi Mike,

Could you please explain how that circuit works? I haven't had much time to
study it, but simple as it is, it isn't obvious to me.

Thanks,

Sean

At 08:29 AM 12/8/00 +0000, you wrote:


>-----Original Message-----
>From:   Nikolai Golovchenko [SMTP:golovchenko@MAIL.RU]
>Sent:   Friday, December 08, 2000 7:30 AM
>To:     PICLIST@MITVMA.MIT.EDU
>Subject:        Re: [sx]: SCENIX VIDEO VIRTUAL PERIPHERAL Design Challenge
>and Contest
>
>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.
>
>Have you DC restored the composite video?  If not you will see this
>problem as the video content changes.  I have attached a very nasty ascii
>circuit that shows a cheap and easy method of acheiving this.  Credits for
>this go to a Mike Day who published the Dirt Cheap Frame Grabber that used
>this circuit.
>
>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.
>
>ISTR that TV's use a "Flywheel" circuit, which is actualy a phase locked
>loop.  This would give a good degree of noise immunity.  This could
>probably be implemented with the scenix, although I'll leave that as an
>exercise for the reader :o)
>
>Regards
>
>Mike
>
><<dcrestor.txt>>

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