Muff's Modules & More

barto

Im working on integrating a multiplexer and would like to use T connectors for the different inputs so i could run my source into the multiplexer for monitoring and still be able to get my sources to a video mixer.

would my video signal be worse using T connectors rather than a distribution amp to feed the multiplexer and the mixer?

i have 50hm and 75ohm T connectors and BNC cables, does that make much of a difference?

bitSmasher · Posted: Mon Feb 06, 2012 11:40 pm Post subject:

Possibly irrelevant question - what mixer?

barto · Posted: Mon Feb 06, 2012 11:52 pm Post subject:

panasonic mx30

lizlarsen · Posted: Tue Feb 07, 2012 2:03 am Post subject:

If you're multing a signal in that way, then you only want the 75 ohm termination at one end (either the multiplexer or the mixer). The impedance of the T-connector doesn't matter for what you're asking, I don't think. Does either device have a way to turn the termination off?

If you plug a video signal into two 75 ohm inputs simultaneously, your signal level will drop to approximately two thirds of the original amplitude, and this will effect both chroma and luma amplitudes, as well as the sync levels (although some devices may still be tolerant there.) In other words, not good (and why you need a distribution amplifier.)

daverj · Posted: Tue Feb 07, 2012 2:54 am Post subject:

The difference between 50 and 75 ohm T-adapters won't make enough difference to notice. At GHz speeds it will.

For short cables you won't see much difference between 50 and 75 ohm cables. There's a slight difference in capacitance. At long distances you might see a difference (a bit more blur with the 50 ohm cable at 100 feet or more)

Video sources expect a 75 ohm termination at the end of the cable. If the signal "loops thru" more than one input, only the last one in the chain should be terminated with 75 ohms. The other inputs should not be terminated. They should be high impedance or "Hi-Z".

If a device has a single input connector and no termination switch, then you have to assume that it is internally terminated. If it has a switch, normally marked with "75 ohm" and "Hi-Z" then you would set it to 75 for the last (or only) input in a chain. All other inputs should be set to Hi-Z.

If a device has two input connectors with a line between them, then they are considered loop thru inputs. Essentially they have a T-adapter built in. Sometimes there is still a termination switch. Sometimes not. If there is no switch, then it is not terminated internally. Then you either have to connect two cables, one from the source and one going to the next device in the chain, or you have to plug a 75 ohm terminator onto the second connector. If it does have a switch, then set it to 75 if using one of the input connectors and set it to Hi-Z if cables are connected to both connectors.

The only time a T-adapter can actually be used is if the device you are putting it on has only a single input connector, but does have a termination switch. Turn it to Hi-Z when using a T-adapter. Of course in a pinch you can also use left over T-adapters as "barrel" connectors to connect two cables together to make a long one. Just be careful nothing touches the exposed male connector on the T.

If you "T" a video signal to more than one terminated input you end up with double or triple termination. That means that instead of a 75 ohm termination you end up with a 37.5 ohm (double termination) or 25 ohms (triple termination) or lower for more connections. This ends up dropping the signal lower than it should be. Some devices with auto level controls might handle it, but it adds noise and often sync instability.

barto · Posted: Tue Feb 07, 2012 12:39 pm Post subject:

thanks. the multiplexer only has inputs and no loop through or termination switches, same with my mixer so it sounds like i should use a DA which is a pain since id need 4 of them and only need 1 in 2 out. im not too concerned about having the best picture quality from the multiplexer as its just going to be used for monitoring, but i want to make sure my mixer gets a good signal as that will be used for final output

i also want to build some sort of matrix switcher where i take 4 sources, mult the signals and run them into two cheap source switches and then run those to my video mixer. that way i have 4 sources available to mix between my A and B bus' on my mixer and im able to save my ext camera input for genlock or feedback

lizlarsen · Posted: Tue Feb 07, 2012 1:24 pm Post subject:

daverj · Posted: Tue Feb 07, 2012 2:37 pm Post subject:

It doesn't say if it works, or if all the modules in it are composite video, but it looks like it's got ten 1-in 6-out distribution amps in it for a decent price:

http://www.ebay.com/itm/Grass-Valley-Group-Video-Distribution-unit-Cle an-Unit-/290665168320?pt=LH_DefaultDomain_0&hash=item43acfe7dc0

They did make modules for that system for sync pulses and subcarrier distribution, so it's possible that all the modules aren't composite video. You'd have to ask the seller.

Or you can pick up some cheap consumer 1-in 4-out distribution amps for about $20-$25 each.

barto · Posted: Tue Feb 07, 2012 3:36 pm Post subject:

lars - yeah thats pretty much the idea! having a module i could use in my modular would be way better than dealing with 4 bulky DA's all needing their own power connections. i was also thinking about modifying the DA's so i could possibly use one of those power supplies that guitar players use that have a bunch of effects pedals that need power, so that way i only have to plug in one and then i can feed the DA's from there.

dave - looks interesting though it might be too big for me. im just going to keep an eye out at my electronics recycler, ive found a couple there for about $5

lizlarsen · Posted: Tue Feb 07, 2012 3:39 pm Post subject:

One option for your termination problem you could try is to just open up the multiplexer and remove the 75 ohm termination resistors on the inputs. They should not be difficult to find (if it's a standard device.)

If you do that, you could use a T-connector -- you just wouldn't be able to use the multiplexer independently (it would need termination on the other end.

barto · Posted: Tue Feb 07, 2012 3:48 pm Post subject:

better yet, could i solder a switch in there so i could have my own termination option?

lizlarsen · Posted: Tue Feb 07, 2012 3:49 pm Post subject:

Yep, electronically speaking that should be pretty easy (finding a way to mount a switch in the case is another matter.)

daverj · Posted: Tue Feb 07, 2012 4:16 pm Post subject:

Before you start drilling, check to see if there is a 75 ohm termination resistor first. Some circuits have distributed termination because part of the circuit causes a partial termination, and a non 75 ohm resistor is added to bring the total to 75 ohms.

So, if you look inside and find a 75 ohm resistor (ideally near the connector), then putting it on a switch is going to be fairly simple.

Normally, to reduce adding extra wires (and noise) to the input itself, I prefer to disconnect the grounded end of the resistor and put the switch between the resistor and ground.

bitSmasher · Posted: Wed Feb 08, 2012 9:28 am Post subject:

daverj · Posted: Wed Feb 08, 2012 4:32 pm Post subject:

I saw an 8x8 video matrix on ebay yesterday. Probably find it with a search on Video Routing.

I used to make 8x8, 8x16, 8x24, and 8x32 video switchers in the 90s, and a 32x32 about 5 years ago. But they were all way too expensive for home use. They were for Hollywood studios and commercial rental companies. A couple of them were used in the movie "Sliver" to route 24 frame video to a wall of monitors. Others ended up on stage for a few mega sized rock concerts switching banks of projectors. Still others are buried inside some major video art installations.

barto · Posted: Fri Feb 17, 2012 2:52 pm Post subject:

hmmmm
http://seattle.craigslist.org/sno/ele/2850715109.html

looks tempting. it would be cool if there was a loop through so i could monitor my inputs. the BNC inputs are RGBHV though it says its compatible with component video, S-video, composite video

lizlarsen · Posted: Fri Feb 17, 2012 2:53 pm Post subject:

That's a good price for something like that! It may be overkill for what you need it for, though.

barto · Posted: Fri Feb 17, 2012 2:59 pm Post subject:

true. and not very portable. i think i just need to find one more cheap DA then ill be able to construct my DIY matrix switcher

lizlarsen · Posted: Fri Feb 17, 2012 3:00 pm Post subject:

Distribution amps should be pretty easy to DIY, too, if you're willing to protoboard them. I think I posted a schematic in a thread here a few months ago.

barto · Posted: Fri Feb 17, 2012 3:05 pm Post subject:

http://www.ebay.com/itm/Extron-MAV-62-6x2-Audio-Video-Matrix-Switcher- /200621051546?pt=LH_DefaultDomain_0&hash=item2eb5f24e9a this looks pretty gooood too. though if i do my DA, ill have enough outputs for monitoring. ill have to look up those schematics lars

lizlarsen · Posted: Fri Feb 17, 2012 3:06 pm Post subject:

Here's that link:
http://www.muffwiggler.com/forum/viewtopic.php?t=47626&highlight=distr ibution+amplifier

daverj · Posted: Fri Feb 17, 2012 4:06 pm Post subject:

4 outputs and 2 outputs are not a lot for switchers. If you go for a matrix switcher, I'd look for one with at least 8 outputs.

barto · Posted: Fri Feb 17, 2012 4:47 pm Post subject:

holy crap! the thrift gods have blessed me this week.

new in box, 4 inputs on the back and two outputs, each with source selectors. "TV Viewing" and "VCR Recording" outputs plugged into A and B busses of my mixer. great success! also found a pico projector for $80 at a goodwill in portland last weekend

lizlarsen · Posted: Fri Feb 17, 2012 4:49 pm Post subject:

That's exactly what you wanted!
I used to go thrifting all the time, I should go more often.

How do you like the pico projector? I've thought about getting one of those for some "in-patch" rescanning experiments since I've got multiple encoders in my system now.

barto · Posted: Fri Feb 17, 2012 4:56 pm Post subject:

i mounted it on my gorillapod, then wrapped the gorilla pod around my wrist and plugged my iphone as a video source and i felt like a visual superhero seriously, i just don't get it

portability is killer, i like being able to have a projector that fits in my pocket. its basically the same size as my iphone except thicker. only limitation obviously is its ouput is only like 12 lumens but i can still see it in a dimly lit room during the day. it runs off rechargeable batteries and comes with av cables to connect composite or vga and a mini tripod. i dont know if i would have purchased it at retail price though which i think is about $350

daverj · Posted: Fri Feb 17, 2012 9:03 pm Post subject:

That's a great price for the projector. I'd love to have a couple of those, but at the retail prices they just aren't worth it.

bitSmasher · Posted: Sat Feb 18, 2012 6:42 am Post subject:

Yeah same, a few micro projectors would be fun for installation stuff. Alas they're underpowered and priced well above novelty value.

That switch box looks like the perfect addition to a basic 2-input mixer! Great find applause