Hello everyone, first time poster here.
I’ve been toying with the idea of a 4 input crossfader/mixer. The only way I can think to do this would be to take 2 crossfaders such as the Cadet VI and run each output into a 3rd crossfader.
To give a bit more context I’m trying to achieve something similar to how a 4 input summing amp/mixer would work in audio(but for video obviously).
Is there another or better way to go about achieving this?
Hi, and welcome!
There are a few options, depending on what you want to do.
BSO’s “Video Soup” and “Video Mix” will act as 4 input mixers, just like an audio mixer (with Video Soup being voltage controlled/quad VCA). BSO’s “Scanner” will sequentially fade through 4 inputs under voltage control.
See them here:
https://www.brownshoesonly.com/products
I’m sure others will have great ideas.
Oooh those all are very cool modules and seem to do what I am trying to achieve.
I wonder if perhaps a simple summing amp would work after all? If so, what should I be taking into consideration since its being used for a different application than its audio counterpart?
There’s the MHzxER, which is a quad summing video mixer: MHzxER DIY mixer
I believe it’s based on this old schematic LZX released:
Also, if you can do with only 3 channels instead of 4, you could use a matrix mixer like the LZX Color Chords or the reverselandfill matrix mixer and just use the knobs in a single column or row (depending on the layout of the matrix mixer).
I think that summing mixer might be exactly what I am looking for! However my plan is not to use this in a modular setup but rather a stand alone device for use with composite signals.
I imagine I may need to change the 100k resistors at the inputs to 75ohm. To be honest I am not all that familiar with the LM6172’s so I’ll have to take a look at the data sheet. Since its not going to be used with modular level signals I’d like to be able to configure them for single supply operation.
Does anyone have any other suggestions for amps? Would the AD8013 or similar be an appropriate substitute for the LM6172?
MAX4392 is a video rate op amp that can be used single supply.
Edit to add: I just realized that if you’re planning on using it with composite signals, I think you’re going to need to add some extra circuitry if using a single supply opamp. Composite signals are AC-coupled, I think, so there’s a positive and negative component to the signal. So you’ll need circuitry to offset it back into the only-positive range before putting it through a single-supply opamp mixer, I think. (I think this is called “DC restoration” in the video circuits world.)
You might want to go with a dual supply opamp that can run on +/-5V or even +/-3.3V and convert what I presume will be a DC power source into dual supply power with a 7805 & 7905 voltage regulator pair (or something like that). Then you don’t need to deal with offsetting the input signal.
re: using the mixer standalone with composite signals – Are the composite signals going to be genlocked? If not, you’re going to have sync issues since composite includes sync. You’ll also have color issues (if using color signals) if they’re not genlocked due to having multiple unsyncronized color bursts.
If not genlocked, you’ll get results closer to (perhaps indistinguishable from) using a simple passive mixer (sometimes called a “dirty mixer” in the video art world). The messy sync info will cause one/some/all inputs to roll, depending on the mix settings. Any color will almost certainly vanish and the output will just be b&w. Still usable if you’re going for a glitchy look (and your input can handle glitchy signals), but perhaps not what you have in mind.
@x1qt, Adrian built this exact module but only now have I read that you want this for stand alone use & composite signals. If your OK with SMT, the ADA4851-4 is a cheap and cheerful quad op-amp that has been recommended by Lars on here in other posts.
https://eu.mouser.com/ProductDetail/Analog-Devices/ADA4851-4YRUZ?qs=BpaRKvA4VqGU%2Fu%2FGddZJ%2Fg%3D%3D
For anyone else looking for a module, hit the Visible Signals site, it’s called the ONE Of Four.
In hindsight I probably should have mentioned that its for composite signals in the title of the thread… However I really appreciate the number of generous responses I have received.
@joem I imagine incorporating a gen-lock is going to add another level of complexity. Perhaps I was in a bit over my head, however I haven’t given up yet.
If you’re looking to mix between 4 composite signals, maybe look into a used Roland Edirol V-4 mixer. It even has effects that you can midi-sync.
Yeah, if you are going to use composite signals, then you will either need to take them from genlocked sources, or you will need to have a digital framebuffer for each of the sources and combine them digitally before converting them back to composite.
Thanks for the free promotion Robin 
but the One Of Four is a 4-input switcher (not a crossfader) for LZX/euro signals, so a bit different to what the OP is looking for I’m afraid.
Truth! I’ve been using your GrainBrain a lot recently and the ONE Of Four is a very new offering and fresh to the scene 
