Non-glitchy analog video mixer?

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SynthularModulus
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Non-glitchy analog video mixer?

Post by SynthularModulus » Tue Mar 10, 2020 2:45 pm

Does such a thing exist?

I already have several "glitch video mixers" that combine two analog signals, with no care given to sync, and a resulting glitchy output.

Is it possible to build a non-glitchy fully analog video mixing circuit that will mix (crossfade) between two analog signals while maintaining video sync?

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Re: Non-glitchy analog video mixer?

Post by Jefro » Tue Mar 10, 2020 3:03 pm

Does it have to be analog? There are plenty of digital mixers that could do this.

SynthularModulus
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Re: Non-glitchy analog video mixer?

Post by SynthularModulus » Tue Mar 10, 2020 3:09 pm

Doesn't have to be analog, but if there is such a thing, I would like to attempt a DIY build. If I'm going digital, I'll just buy an edirol or something, but wondering if there is something buildable.

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Re: Non-glitchy analog video mixer?

Post by Jefro » Tue Mar 10, 2020 3:38 pm

I’m not aware of any that are diy. You would need some sort of timebase correction in there, so that complicates things. An Edirol is probably your best bet, but depending on your needs, maybe a Videonics or Panasonic video mixer could cut it as well.

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Re: Non-glitchy analog video mixer?

Post by FetidEye » Tue Mar 10, 2020 4:11 pm

Look at the (open source) LZX Cadet series.
B/W analog mixing is easy. Color is possible, but a bit more complex. There was a thread on the LZX community forum about this.

if you are up to it, you could build a big standalone machine from the schematics.

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Re: Non-glitchy analog video mixer?

Post by joem » Tue Mar 10, 2020 4:55 pm

FetidEye wrote:
Tue Mar 10, 2020 4:11 pm
Look at the (open source) LZX Cadet series.
B/W analog mixing is easy. Color is possible, but a bit more complex. There was a thread on the LZX community forum about this.

if you are up to it, you could build a big standalone machine from the schematics.
Cadets will only work for mixing if the video sources are genlocked. If the sources to mix are genlocked, then definitely go the Cadet route (they're so much fun!), but if not genlocked, then this isn't going to work.

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joem
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Re: Non-glitchy analog video mixer?

Post by joem » Tue Mar 10, 2020 5:24 pm

To explain the problem a bit...

The problem, as others have noted, is time base correction (TBC). If you take two analog video sources, the chances of them already being in both line- and frame- sync is very very very slim, but in order to mix them, you need them to be sync'd (and if you want to mix color composite signals, you need them to be very closely sync'd). Even if you get lucky when you turn your sources on and they randomly end up being synced, they almost certainly won't stay synced for long. To synchronize unsynced video, you need to delay one of the signals by a certain precise amount, and that amount will likely change over time. Additionally, you may even need to shrink or stretch the time it takes one signal to display a line of video. These are fairly complicated tasks for analog circuitry.

Usually, to get around the difficulties of time base correction in the analog realm, you'd just use video sources that could be genlock'd (aka synchronized). A lot of old analog pro video gear has genlock/sync -- it's a lot easier to get things in sync at the source, where the video signal generation happens, than it is to time base correct. When the sources are genlocked, mixing video isn't too much more complicated than mixing audio is, but if it's not genlocked, you need to time base correct it. This is why you see so many glitchy DIY video mixers out there and no clean DIY video mixers. (If you do have genlocked sources, look up opamp mixer circuits, and recreate one using very fast opamps, like the LM6172. That's pretty much all you have to do, I think.)

Edirol mixers and other similar ones have built in time base correction, btw.

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Re: Non-glitchy analog video mixer?

Post by FetidEye » Wed Mar 11, 2020 4:07 am

hah. for some reason I forgot to mention this. sorry about that.
I was already thinking about the next stage, mixing with the Fader module and how that would work with 3 video signals.

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Re: Non-glitchy analog video mixer?

Post by SynthularModulus » Wed Mar 11, 2020 9:18 am

That makes a lot of sense about TBC/sync and using genlock. That must be at least one reason why genlock was such a big thing in analog broadcast video.

It's interesting that black and white doesn't seem have these same sync problems, or it's not as bad, because you can actually get some half decent results with black and white video and audio mixers. Is the sync signal much simpler on B&W video than it is for color? I guess it would be...

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Re: Non-glitchy analog video mixer?

Post by FetidEye » Wed Mar 11, 2020 12:18 pm

disregard my previous post. I was linking the easy part to the actual mixing of B&W video,
which is less complicated because you would need only 1 Fader module

the sync signal is the same for B&W as for color (as far as I know)

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Re: Non-glitchy analog video mixer?

Post by joem » Wed Mar 11, 2020 1:47 pm

As FetidEye says, the sync signal is the same for color and for B&W. One difference though is that for color to work, you need a colorburst signal which is used to synchronize the color decoding (b&w signals don't need the colorburst, so most decoders assume that signals with missing or unreadable colorbursts are b&w, I think). Color is encoded in composite video with some phase shifting in a precise way, which is kind of complicated (IMO) but it's done the way that it's done so that a color composite signal degrades well to B&W. If the colorburst is off (or if the color decoder is off), you get odd color shifting. This colorburst and phase encoding stuff is why when you mix two color composite signals through an audio mixer, you often lose the color and only get b&w output. Sometimes it's due to color decoder confusion since the two signals are encoded with slightly differently timed colorbursts, and sometimes it's due to the audio circuitry stripping out the higher frequency colorburst signals altogether so the decoder only sees them as b&w.

Is this what you mean by b&w mixing seeming not as bad as color mixing, using just audio mixers? Because the overall sync issues should be the same... One of your video sources will most likely not be correctly aligned horizontally and/or vertically, and might be scrolling when the other source is still. And with the mixer in the right position (usually fairly equally in the middle), you probably won't get ether signal to sync stably since the decoder will probably be switching between the two sync signals.

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Re: Non-glitchy analog video mixer?

Post by SynthularModulus » Wed Mar 11, 2020 2:03 pm

joem wrote:
Wed Mar 11, 2020 1:47 pm

Is this what you mean by b&w mixing seeming not as bad as color mixing, using just audio mixers? Because the overall sync issues should be the same... One of your video sources will most likely not be correctly aligned horizontally and/or vertically, and might be scrolling when the other source is still. And with the mixer in the right position (usually fairly equally in the middle), you probably won't get ether signal to sync stably since the decoder will probably be switching between the two sync signals.
Yeah, I guess you are right, the sync thing is still a problem with B&W signal. B&W maybe just seems somehow slightly less glitchy when using a glitch or audio mixer to me, but maybe I'm wrong.

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