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modifying a video mixer
MUFF WIGGLER Forum Index -> Video Synthesis  
Author modifying a video mixer
barto
i remember seeing an auction for a panasonic wj-mx30 mixer on ebay a while ago that was modified to add brightness/contrast/chroma adjustments for each channel. does anyone here have any knowledge of these modifications?

heres the auction page with some discription:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=310277548236&ssPage Name=ADME:X:AAQ:US:1123#ht_1970wt_1104
daverj
This is just a guess, but... This mixer no doubt uses a couple of standard video decoder chips to get the video in. Those chips pretty much all have the ability to set the brightness, contrast, hue and saturation by sending signals via an I2C serial control signal.

So I'm guessing that if the mixer board didn't already have trimpots which they simply brought out to regular pots, then perhaps they added a small microcontroller with A-D inputs which reads the pots and sends the I2C signals to the decoder chips.
lizlarsen
Yeah, trimpots brought out seems most likely. Adding the circuitry to do these kind of manipulations would be pretty involved.

It does say some company buys and modifies these, and the labels on the frontpanel look like some work was put into them.

It always seems difficult to find service manuals for video mixers, or else it might be possible to confirm the theories here.
bitSmasher
I had a quick search on VJForums and it appears there are (well, were) lots of people wanting details of this mod but all the links there are 404s now.
Might be worth going there to ask the question again, see if anyone can help.
barto
thanks guys. i just got a wj-mx12 at my local electronics recycler for $40! are these types of mixers capable of being circuit bent at all? if so, are there certain areas i should stay away from so i dont break things? maybe i should try to contact karl klomp about circuit bending video devices
lizlarsen
This is just my opinion, but circuit-bending video devices results in only mildly useful results. Mostly you just end up destroying the integrity of the video sync/signal. And when that happens, you can't properly view the results. This can be cool for glitch art and effects, but the most interesting results I've seen are more in the classification of "modifications" than bending, that is -- things like adding in external modulation over a keyer (look for comparator chips or see if voltage is coming off of any of the pots so you can add voltage control or external triggers to buttons, etc.) The coolest bending (to me) has been with things like video title generators or graphics displays that have been made to spit out random characters or data.
barto
thats pretty good logic. i circuit bent an archer video processor in hopes i could use the glitch effects as a luma key source but when put it through my wj-mx30, it did not seem to like the signal that came out. when i plugged the circuit bent processor directly into a tv however it looked fine.
lizlarsen
Yep, the best way to view the output of a glitchy device would be on a CRT monitor/television, since with an LCD television, projector or any display that digitizes the incoming video before displaying it, any sync errors will possibly result in an ADC error which prevents display of the signal. So glitch effects are highly dependent on the display device that is being used to display the signal. You could use a time-base corrector, but these are meant more to correct timing errors than make a completely destroyed signal viewable. I suppose the best way to record a glitchy signal output is to rescan the monitor that gives you the results you want with a camera, but that also has complications. So if you're aiming for glitch effects, just be sure you're testing your bends on the display you want to use, and have some sort of workflow worked out for how you're going to record or display the output.

And, if you don't want to glitch out your output, the bends you want to find are ones which don't alter the timing or integrity of the device's video sync on the output -- that's gotta stay consistent. So you're limited to stuff that deals with the image content itself, which is why title and character generators have some interesting potential.
daverj
When my Output Amp/Genlock modules become available later this summer they are good for making some glitch/bent machines put out a viewable and recordable signal.

If it's something that processes video then you can genlock to the signal you're feeding into the bent box and feed the output of the bent box through the Output Amp. It won't mess with the distortion of the bent video, but it'll put on fresh sync which makes it viewable and recordable.

And even if you don't genlock to the signal going into the bent box, or are looking at a box that generates instead of processes, you can still use the Output Amp to put clean sync on it. It'll just be "free running" sync, meaning that the bent images will be slowly drifting side to side all the time.
bitSmasher
That sounds great - making a bent signal useful is going to be very, err, useful.
lizlarsen
Yep, the Jones Output Amp would be pretty good for this case! The LZX input/output modules can also strip and restore sync to a faulty signal, however with the RGB signal path and workflow with the current output module (Color Video Encoder), it is difficult to get a glitchy color signal in and out of the system, assuming you've got a composite video source. A Component and RGB source would work fine with the right input module, but a Composite source requires decoding (Color Video Decoder) and a glitchy input source may cause instability of the decoder. If your glitchy source is a monochrome source like a black and white camera, or you plan to colorize the signal in the system anyway, though, it wouldn't be an issue.
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