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DIY CV video mixer?
MUFF WIGGLER Forum Index -> Video Synthesis  
Author DIY CV video mixer?

colossus

Anyone know if adding CV control to a simple Klomp-style video mixer is doable?

This is his really simple schem: http://www.karlklomp.nl/pro/dirtmx/dvmschema.jpg

My idea is to just replace the pot with a vactrol or LED/LDR but my fear is that 1K is relatively small compared to the resistance of a typical LDR. Thoughts? Does this seem doable? I'm waiting on some RCA jacks to come in so this is kind of a pre-experimentation question.


FetidEye

if you are into glitch it's fine.
check these schematics from the LZX diy page
http://www.lzxindustries.net/system/diy/


colossus

Thanks for the heads up. I just hit LZX up about if their new Cadet VC fader could be modded to use RCA jacks instead. Hopefully it works out!


gzifcak

You could certainly put RCA jacks on it but it won't really work outside of an LZX system. LZX modules have a 500 ohm output impedance, and commercial video gear expects 75 ohm. What will happen is the signal coming out of the Fader will be attenuated down to a very dark image by the input of the next thing in the chain (monitor, vcr, etc).

I would either just get a cheap used video mixer and add CV to that, or get the Fader, Sync Generator and Encoder, and a couple input modules and have a solid basis for expansion. You will officially only get monochrome mixing unless you buy three Faders, but it may pass color if you bypass the input modules, I haven't tried.

EDIT: but the LZX output module may not pass the color subcarrier, no idea


colossus

Ah, bummer. I'll probably seek out a cheap video mixer then. I'd love to build up an LZX system but it's a bit out of my price range at the moment. Thanks for the help!


snufkin

the problem here isn't just impedance mixing signals within the LZX before the encoder isn't the same as mixing video, entirely different problems.

I would certainly try adding CV to the dirty mixer either with a vactrol or with other better methods (DIY VCA) however you will still be crunching all your sync information and a continuously variable resistance may result in even less stable images than even the dirty video mixer offers with a static pot.

Two approaches I would use:

some people have added an audio glitching input to the dirty video mixer, this might give you the control you need, although I wouldn't push CV through the DVM as you will be sending it straight to your output device.

the second is a sync striping design like http://gieskes.nl/visual-equipment/?file=schele-mixer maybe contact Gijs and see if he can advise on cv control.

The easiest is to buy a mixer with TBC and sliders that are simply attenuating 5v like most of the panasonics and hack it.

Even with an LZX you can only really mix video if you have 2 TBC modules as you need to solve the problem of reconciling two asynchronous feeds in to a single output, not to mention dealing with RGB colour space.

That is unless your sources have a genlock input and you slave them to your master sync source.

Hope that helps


gijs

snufkin wrote:

the second is a sync striping design like http://gieskes.nl/visual-equipment/?file=schele-mixer maybe contact Gijs and see if he can advise on cv control.

could work with 2 vactrols.


gzifcak

snufkin wrote:
the problem here isn't just impedance mixing signals within the LZX before the encoder isn't the same as mixing video, entirely different problems...
...Even with an LZX you can only really mix video if you have 2 TBC modules as you need to solve the problem of reconciling two asynchronous feeds in to a single output, not to mention dealing with RGB colour space.

That is unless your sources have a genlock input and you slave them to your master sync source.


mixing signals within the LZX before the encoder IS mixing video, you just have to know what hurdles you are trying to jump over if you're adapting them for use outside the system's in and out modules:

1. sync - if you're using the dirty video mixer then you don't care about sync. if you do want sync and don't want to use the LZX input or output modules, genlock your sources before mixing, and don't include any signals that go below 0v or they'll interfere with the sync signals.

2. RGB - if you are using monochrome this doesn't matter. if you want color and you're using the input on the sync generator, you can change a jumper so it doesn't filter out the color subcarrier. the LZX modules shouldn't either (but my guess is that there are plenty of things you could do to inadvertently distort it). if you're not going through the CVE/Cortex, color should be retained if you're just using a fader module to mix two sources.

3. impedance - if you're not using the CVE/Cortex, the fader would have to be followed by a buffer with 75 ohm termination like i mentioned above.

there are a bunch of other issues like dc restoration on the input, clipping, and blanking that the LZX input and output modules take care of (and which are very important to have if you're doing stable synthesis that conforms to NTSC/PAL), but if you're going for something as quick and dirty as the original mixer circuit, you can definitely find ways to make LZX modules work.

i stand by my original point that the fader module can definitely work as well as the dirty mixer with a nice CV input, as long as you get the outputs to the right impedance to drive the inputs of other gear.

EDIT: you may even be able to get the LT1251 in the fader to drive low impedance loads without buffering.

but yeah if you can pick up a video mixer for the same price and figure out a cv hack, you'll also get sync. fwiw the panasonic mx10 doesn't use a voltage divider for the mix circuit, so you'd be back to square one vactrolizing a pot.


snufkin

gzifcak wrote:
snufkin wrote:
the problem here isn't just impedance mixing signals within the LZX before the encoder isn't the same as mixing video, entirely different problems...
...Even with an LZX you can only really mix video if you have 2 TBC modules as you need to solve the problem of reconciling two asynchronous feeds in to a single output, not to mention dealing with RGB colour space.

That is unless your sources have a genlock input and you slave them to your master sync source.


mixing signals within the LZX before the encoder IS mixing video, you just have to know what hurdles you are trying to jump over if you're adapting them for use outside the system's in and out modules:



to clarify I mean that once inside the LZX your not dealing with a composite video signal, thats generated by the encoder at the end, you are pushing one of the components of composite video through one of the channels, semantics really but helps to differentiate for the less experienced the difference.


theflyingfridge

hmmm.....

Wondering if anything ever came of this? I'm looking at slapping a dirty mixer together, and would be interested in giving it come CV control. Just not sure if it's worth chasing down...

Also, why are video mixers so dang expensive? Even shitty old ones on eBay are pricey?

I can't find anything under $200 that will simply mix two video signals.

Are the components for syncing video that pricey?

Edit: Should clarify that I'm a total noob at all this video stuff...


Lento_Zoom

You have to keep an eye out on the Video Mixers. I managed to get a decent SIMA one with TBC for $60, but I had to do a lot hunting, and that was after I bought I Panasonic one on auction for $50 that wound up having a dead input. I think that you see them go for a lot because there is a more limited supply compared to other electronics and that they were/are pretty expensive brand new (Roland V-4, V-8). Look for Panasonic, SIMA, and Videonix. Just make sure with the SIMA and Videonix stuff that it is a true mixer. Those guys made a lot of bargain basement stuff without TBC, so you can't do a straight fade or wipe without dipping to black first.

Stay vigilant and don't forget to hunt thrift stores and university equipment auctions/sales in addition to e-bay.

I'll post stuff here when I get around to adding vactrols to my mixer for CV control.


maggan

Lento_Zoom wrote:
I'll post stuff here when I get around to adding vactrols to my mixer for CV control.


Looking forward to see a guide on how this is done. Been thinking about doing it to some of the circuit bent audio gear I have but don't know where to start really. Is it the same procedure?


Lento_Zoom

It should be! From what I understand, a vactrol allows you to convert a control voltage to resistance it shoud work in-line with most pots. I don't have any direct experience yet, but I'm thinking about trying the Synthrotek vac pak, since it is pretty much a drop-in complete package. http://store.synthrotek.com/Vac_Pak_Kit


colossus

I never made much headway here, but I have a vac-pak and plan to fiddle with it over my winter break in a couple weeks.


theflyingfridge

colossus wrote:
I never made much headway here, but I have a vac-pak and plan to fiddle with it over my winter break in a couple weeks.


Keep us posted. I'm going to pick up parts for the dirty mixer build in the next few weeks. If it's simple enough to just add a vactrol, I'd love to know how well it works...


theflyingfridge

colossus wrote:
I never made much headway here, but I have a vac-pak and plan to fiddle with it over my winter break in a couple weeks.


Keep us posted. I'm going to pick up parts for the dirty mixer build in the next few weeks. If it's simple enough to just add a vactrol, I'd love to know how well it works...


evolve

I have had a similar idea!
In fact, I've been brainstorming a euro version... the issue being that with LZX you would have to go to output, then back to input in order to use it.

You will need to supply power to your circuit if you are using a vactrol, as the CV itself would not be enough to power it. Typically an op-amp is used, like in the Synthrotek vac pack.

As for 200$ for a video mixer....certainly not. Scour ebay, goodwill, craigslist, and you will find that 10-50$ special video mixer. Panasonic, Videonics, Sima, etc. They can be modded for CV either with a vac pack or with a voltage straight to the xfader circuit (YMMV, I am not responsible for you ruining your video mixer, please recycle)

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