CVBS Video Switch (non-genlocked)

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I dont see exactly how these can be used, unless i convert the composite signal to digital and back again. Whats the best way to do that ?

Alex.

But that does not accept component video inputs, so i would first need to convert my signals into digital (saa711x? ic), then back to composite after :(. No easier way ? If not, what is the best way to go about it.

Surely there must be some examples or good documentation around for making a frame synchronizer ?

Alex.

This guy should do it:

Or its bigger brother:

And if you need something boxed and professional:

"Quack" schrieb im Newsbeitrag news: snipped-for-privacy@o13g2000cwo.googlegroups.com...

If you want to see all signals at the same time split screen, then you have to do that. Otherwise you could maybe modify the monitor to mute the picture until the sync has locked again, but this is not so easy as well. Ideally there would be 4 sync circuits multiplexed, then there would be only one frame timeout, well I don't know if the line transformer likes this. Maybe somebody knows more of video than I do.

Without genlock you are basically lost.

I think that you need a synchroniser/framestore, or a little digital video mixer such as

It will be a lot cheaper than building your own

martin

Well, they convert the video image for you from component to digital and store a frame. You can read the frame out at a different speed than you write it into the chip, allowing you to 'resync' the video source. And yes, you'd have to convert composite to component and back as well.

But as another poster stated, there are relatively cheap consumer videomixers out there that can do this for you.

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I dont see exactly how these can be used, unless i convert the composite signal to digital and back again. Whats the best way to do that ?

Alex.

Many thanks everyone!,

It seems far more complex than i ever imagined. Unfortunate, i know its possible but it would be too big and expensive for what i am trying to do. I can not use a consumer grade mixer, this device must be small and light (max: 6cm x 6cm x 200grams), which it is currently but it switches dirty.

I had an idea though, how does this sound;

The output of this switch goes to a digital encoding device anyway, a video capture system (digitizer), but this system seems to be extremely poor on 'resyncing' after a dirty switch (sometimes, random of course).

By that i mean sometimes its fine after a switch sometimes, and sometimes its screwed up for a few seconds or more due to the random location of the 'cut' when switching in the signal.

So my idea is to use a LM358 to monitor the position of each of the 4 incoming composite signals (detecting the vertical sync and counting).

I would then swith video like so; I would 'hold the current voltage level' of the signal (on the output) when i reach the next vertical sync in the currently selected video.

I would then internally switch to the desired video input (circuit output still in sync-level voltage)

I would wait for its next vertical sync (the desired input) and then let it go out.

So that the video signal would always be 'complete frames', just that when switched, a particular frame has a very loooong SYNC voltage level, perhaps this would _force_ the digitizing device to recalibrate itself quicker, in that it gets a known stuffed up frame, rather than just 'random garbledness'.

My question here is that perhaps a 'long sync' can be useful, or perhaps not, what other methods are commonly used (using the lm358 and a video switch IC [eg: max4545]) to force a receiving device to re lock on a new signal.

Perhaps i should just 'break before make' ... i think i will start experimenting this way rather than try to buffer the video and remove the bad frame completely, if i can force it to be bad 'in a good way', then thats okay too - i can pause the digitzing device (not capture data) for upto 200ms, so the goal is to have it resync within

More info: Digitizing device is based on phillips SAA7113 with NT1004 usb interface.

So relating these devices to the above description, how can i force the SAA7113 to resync nicely within 200ms after a switch, by;

-forcing all switched frames to be short

-forcing all switched frames to be long

-holding at sync level voltage for the time difference between signals

-holding at other? level voltage for the time difference between signals

or something else perhaps ?

Thanks again, and i hope this makes some kind of sense :) Alex.

Hello Alex,

You can't store a frame any other way. There used to be CCD buffers for one line of video (but not a frame). However, AFAIK even those are disappearing.

Just one word of caution: You might find chips that combine a lot of the AD/DA and sync separation functions you need but they can disappear quickly. Usually that happens the minute such a chip is no longer needed for the production of some consumer equipment. Often there is no 2nd source. So be careful if this is for a product with a long production life.

Regards, Joerg

Hi Trond,

Would it be best to cut the signal at the vertical sync or any other particular place, and re-establish a signal at its vertical or other sync, or just randomly cut it and return it at any location ?

Alex.

That jogged my memory. The trick is to fade to black, switch to the new video source which is held to black, pause for sync to occur, then "fade-up".

...Jim Thompson

But thats only for in-sync signals no ? What do i do with the broken short (or long) frames ?

Alex.

This is probably the easiest solution. By removing the signal completely for a few frames, the decoder will probably lock to the new signal a lot faster, when you apply it. You can experiment with the length of the "no-signal" duration to find the optimum time.

Trond-I.

I know it works for monitors, due to black-level clamping, but I don't know how your digitizing circuitry might respond.

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

I havent fully read the two website quoted, but it should give you an idea of the complexity and if it is within your expertise

martin

That sounds like an interesting idea.

I am using a sync seperator chip to read the sync, how should i generate it, just go from black (0.3v) to 0v ? or is generating a sync more complex than that ?

Alex.

My guess is that the whole problem is the decoder not realizing that the signal has changed for up to several seconds after switching. By removing the signal completely for a few frames, the decoder will be forced into some sort of "search for sync" mode, and locking will probably occur sooner when you apply the new signal. I don't think it will matter much at which point in the video frame you do the cutting, but you can always do some experiments.

Trond

If cutting out the sync works depends on the digitizer. If it does not work, you can try this:

Since at this point, you are extracting the sync signal anyways, you could remove the H and V sync from the input and generate new syncs using a uC. Then, figure out by how much your timing can be off between frames to not get a hickup in the digitizer and adjust your artificial sync until it matches the input sync.

The effect would be that the contents of the video frame would move around until the sources are synced again.

But this is more of a programming job than an electronics job.

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Hello Alex,

A good sync separator chip should be able to generate sync pulses that are within spec while it isn't receiving any. Kind of a "free running mode". It's been a long time but I believe the chip I did that with was the SAA1043 or something (could be wrong on that P/N), probably obsolete by now. AFAIR I had to design my own PLL for one of the syncs. Anyway, Philips used to always have a chip like that. Then you don't need a uC.

If V-sync was way off from one source to the other it will take quite a while to nudge over to the new sync without deviating too much from standard. Else the V-sync of the monitor or whatever is connected could briefly "hick up".

Regards, Joerg

Hi Matthias,

Can you point me in the right direction for 'removing the H and V sync and generating them in a uC' ? :).

I have tried all else mentioned here, ill give this a try.

Thanks