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Sync Separator/Recombinator (FIRST POST)
MUFF WIGGLER Forum Index -> Video Synthesis  
Author Sync Separator/Recombinator (FIRST POST)
enque
Hi all.

I am interested in making a video effect that modifies a sync signal to create glitch effects and, if possible, bend image.

My first thought was to input the sync signal to a microcontroller, edit it, and send it out. I have done this with a VGA source with some success.

Eventually I would like to do the same with a composite video source, but am stumped as to how to separate the sync from the signal (because composite has only one pin, whereas VGA had dedicated wires for the Horizontal and Vertical sync).

Of course once I separate the sync and modify it I will want to recombine it back into the signal.

I would really appreciate your ideas/help.

Thanks thumbs up
Mongo1
<i>Eventually I would like to do the same with a composite video source, but am stumped as to how to separate the sync from the signal (because composite has only one pin, whereas VGA had dedicated wires for the Horizontal and Vertical sync). </i>

Well, it's been a loooooong time since I've done any video stuff, but there's an interesting technique you might use. It's interesting because it was how we used to steal HBO and Cinemax back in the old days.

Back in the days of yore, the way cable companies would scramble premium channels was to suppress the horizontal and vertical sync signals from the video stream. Other that that the signals were untouched.

The way unscrupulous characters got around that was to build a board that had two phase-locked loops on it. One for the H-sync, and one for the V-sync. The idea was that even though the signals were suppressed, they were still present. Since the frequencies of those signals were well known, the PLLs could easily dig them out of the mush, and voila, free Boobs on your TV!

I think it would be fairly simple to build a box to dig the H and V sync out. In fact it wouldn't surprise me if there's already some box out there that does it.

Gary
daverj
Composite video uses a composite sync instead of the separate horizontal and vertical that VGA has.

You can pull the sync off using chips like the LM1881 or something more complex like this:

http://www.mouser.com/ProductDetail/NJR/NJM2257D/?qs=sGAEpiMZZMutXGli8 Ay4kFzjxRHOPt1H%2f5GBJCtu%252b%2f4%3d

Once you mess with the sync you will need to combine the H and V back into a composite sync signal. It's a more complex signal than simply mixing H an V since it has all sorts of small timing pulses around the V area.

Once you create the composite sync you need to then mix it back onto the video. To do that you'll need to make a switch that switches between the video and the sync using another timing signal called blanking, which you create from the H and V signals. You'll also want to clip off the old sync signals using a clipping or limiter circuit.
lizlarsen
Well extracting the sync signals as 5V logic out of a composite video signal is the easy part -- you can use LM1881 or LM1883 sync separator chips, which are fairly easy to obtain. Using a microcontroller to "modify" them is more difficult, because you'd want to make sure your microcontroller was running in sync with the video in the first place (by using a PLL or something similar.)

Actually removing the sync from the video signal, and then adding back in a modified sync is more complicated and you'll need some sort of DC restoration circuit and sync stripper. There was a recent thread on DC restoration circuits that may be useful:
https://www.muffwiggler.com/forum/viewtopic.php?t=40948

I've had some ideas about doing a module along these lines -- basically a send/receive for the sync signals, composite in/out, and then you'd patch the sync signals into various counters and boolean logic to mess it up.

I think there's probably a whole lot you can do glitch wise with something more primitive, like just interjecting weird logic signals or square wave VCOs into the composite signal intermittently, and using LM1881 to extract basic sync pulses and use those to sync the VCOs.

A lot of what any sort of glitch is going to look like, also, is entirely dependent on the display or capture device that is receiving the invalid/glitched video.

edit: the above two posts touch on what I ended up writing about. smile not trying to be redundant.
lizlarsen
Oh and welcome to the forum!! w00t w00t w00t
enque
Thanks for the input. I am going to order some parts. I'll will update when I get something working.
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