DMX through audio snake (I know its a stupid question)

[strandcentury]

My schools main DMX line has been somehow cut (rodents, whatever, tension). It runs under the ground and we cannot fix it until the school boards electricians make an appt. and diagnose the situation which will take a while. We have a show coming up and we do not wish to run it above surface nor do we have the funds to rent a wireless DMX combo.

We checked both ends of the DMX line and the pins are correctly connected. It is not the console or dimming equipment either since we ran it back stage without any problems (and no I will NOT run the show back stage!.)

However... our audio crew does have a Radial RDX-24 audio snake with 4 returns and 24 inputs (3 of the 4 returns are ALWAYS in use). They are willing to let us use it as long as it does not cause any damage to their equipment or present any undesirable effects to their sound quality.

I was thinking of taking a standard 25 foot DMX cable... cut it in half, solder in standard XLR-3 connector on each of the halfs.

So what I would end up with is one XLR3 female to XLR5 male (for the console side of the snake, booth) and one XLR3 male to XLR5 female (for the amp/backstage side of the snake).

I would be running the DMX line through the snake inputs (use 24 since we never go past 20 inputs) and not the returns.

Our dimmers are stage R and our amps are stage L. We would run a 50 foot XLR5 cable from our dimmers to the amps (and snake) via our catwalk. It would then run into snake input #24 via the XLR3 male to XLR5 female cable and continue its way up under ground through the snake up to our tech booth. The #24 input up in the booth would then meet the XLR3 female to XLR5 male cable and be connected to our lighting console.

Theoretically this should be possible since the snake does not have any internal electronic/processing components and should just be simple plug to outlet type connection.


Any ideas on this issue?

-LX Specs-
Console: ETC SmartFade 24/96
Dimmers: 18 ch Electro-Controls @ 2.4k - 100A 230V (Retrofitted to support DMX 5 years ago)

-Sound specs-
Allen Heath sound console
3 x QSC amps
6 x EV speakers

(Mics/processing equip. should not be important

Radial Engineering RDX-24 24 ch audio snake w/4 returns

 

[Grog12]

Impedance on the snake could be the downfall to your bandaid situation.

 

[mstaylor]

It is possible but it could also go south on you. As grog12 says the impedance is going to be the problem. Is there a way to finish the run from the catwalk to the booth above the ceiling instead of through the snake?

 

[jeffsw6]

They are willing to let us use it as long as it does not cause any damage to their equipment or present any undesirable effects to their sound quality.

It will not cause any adverse effect on their equipment or audio signals.

It might work, or it might work during rehearsal and then some or all of your instruments might freak out during a performance, or you might try it out and have no success right from the start. You do have a small advantage since you only have one device on your DMX network (other than the console, obviously) but there is no guarantee it won't act up. Having the DMX segment properly terminated will most likely improve your chances, so if you aren't sure about that, check your termination.

FYI I do this routinely over ~250 feet of audio multi-core and ordinary mic cable, not proper DMX cable. It works fine for me, but I don't plan on doing it this way for much longer, because I know that it might not always work fine.

 

[derekleffew]

DMX down a mic snake is a controversial and often-debated topic. http://www.controlbooth.com/forums/...371-trouble-dmx-signal-could-you-help-me.html Most have done it at one time or another, seldom with any issues, but few would recommend it as standard practice.

 

[len]

Most have done it at one time or another, seldom with any issues, but few would recommend it as standard practice.

Ditto. My standard response to any dmx cable issues is: It'll work. Until it doesn't. And you don't have any control over when that will be.

 

[JD]

Board to snake ~~~~~ (stage end) opto splitter > rest of system.
This layout will help a bit. The real problems are often echos as the cable changes impedance from being an audio cable (~50 ohm) to dmx cable (~120 ohm.) By putting an opto splitter at the stage end of the run, you are using the splitter to buffer the signal before driving the legitimate DMX cable(s).

 

[mstaylor]

Ditto. My standard response to any dmx cable issues is: It'll work. Until it doesn't. And you don't have any control over when that will be.

Well put len. I tell guys that it may work 1000 times in a row then go south, it may never work, but I like the way you say it.

 

[PadawanGeek]

Board to snake ~~~~~ (stage end) opto splitter > rest of system.
This layout will help a bit. The real problems are often echos as the cable changes impedance from being an audio cable (~50 ohm) to dmx cable (~120 ohm.) By putting an opto splitter at the stage end of the run, you are using the splitter to buffer the signal before driving the legitimate DMX cable(s).

I like this idea a lot. Something like this Enttec would be good. Also, you could get two Art-Net nodes, one at the console and one at the dimmer rack, but that's a little more expensive.

 

[ship]

Well put len. I tell guys that it may work 1000 times in a row then go south, it may never work, but I like the way you say it.

Been using 400' digital grade audio snakes for years for 8-16 and more pair FOH runs. Literally since DMX came out in pre-dating me as a buyer.

Agreed with the above observations, but it also depends on the grade of audio snake in use and it's imputence in resistance. If your snake is at like 110 Ohms, you should be fine - the opto splitter will also be useful for this. If less than that... yes you will probably have problems. You don't need all five pins working just drain and pins two and three. Potentially, if you are short a drain but pins two and three work, and you have a working drain or possibly even ground/drain from your pair you might be able to make it work. Just a concept in some times the index drain in parallel when shielded with the other pair and away from conductors could work. (See next in concept of it perhaps working.)


Anyone able to say how to test this beyond cable specification for actual use? Given the DMX lines were cut or what ever happened to them, might be also added resistance to the sound snake over it's specification.

I have dealt with this problem before - concrete shifts, buildings settle and conduit in them break and score or sheer conductors. ?This a warranty thing that can get done in replacement before the show?

Another option, do you have any ethernet running to the stage? Even phone lines - me noting my new construction house phone lines are ethernet not typical phone line. Might be another option for DMX connection. I have been play testing TMB's various Ethernet cable to DMX options over the past few months without complaints. Not cheap but the concepts seem to work so far. Not on large tours so far but for small area shows, this stuff works.

As for running lines to FOH from stage... there is fire doors that cannot be fed thru at best, and a 18" thick fire wall at worst which cannot be penetrated even by way of a little hole for wiring short of some work done. After that, it has to be Pleneum rated DMX cable for the run. The first I knew of, the second I was reminded the details of during this week on a quote.

 

[shiben]

(and no I will NOT run the show back stage!.)

Why not? Seems like the easiest solution to me. SM can still call cues from the booth, but you can sit anywhere you like within reach of a DMX cable and push go, probably could do it in a closet if you really wanted to (not saying this is a good idea). Unless your taking your own cues (and even if you are, a lot of SMs I know stand backstage to call the show) you dont need to be in FoH. Is it better, yes. Does it need to happen? No.

 

[bishopthomas]

I run DMX down audio snakes all the time. Big ones, small ones, reel ones, red fish, blue fish... Give it a go, what's the worst that could happen?

 

[Grog12]

I run DMX down audio snakes all the time. Big ones, small ones, reel ones, red fish, blue fish... Give it a go, what's the worst that could happen?

Uh beyond the board losing all control of lights during the middle of the show? Meh.

 

[mstaylor]

I run DMX down audio snakes all the time. Big ones, small ones, reel ones, red fish, blue fish... Give it a go, what's the worst that could happen?

I was doing a show with a national company, which I will leave unnamed, and they routinely run their DMX through their snake, it always works, I'm told by the tech and that night it didn't. Luckily I had my own DMX cables. As stated above, it will work until it doesn't.

 

[DuckJordan]

I'm going to have to agree with shiben on this one, Why not move backstage? your job as board op is to hit GO when the SM calls "GO", even if something isn't right on stage you can't really fix it without going up to cats anyway so whats the purpose of being behind the audience?

 

[FMEng]

Been using 400' digital grade audio snakes for years for 8-16 and more pair FOH runs. Literally since DMX came out in pre-dating me as a buyer.

Digital is the operative word. A cable designed to carry AES-3 digital audio has an impedance of 110 ohms, which is close enough to the DMX impedance of 120 ohms for reliable performance. Of course, digital capable snakes tend to be rare. Cat 5 cable, at 100 ohms, can also work better than audio cable in a pinch.

Before we condemn the existing cable for replacement, it would be wise to examine the terminations at both ends to make sure it isn't something simple, like a broken wire on a solder terminal. That is more likely than a broken cable. After that, check the continuity of the cable. Some DMX cables have two twisted pairs, but only the primary pair on pins 2 and 3 is actually used. Most systems don't use the secondary pair on pins 4 and 5. If the primary pair has a break, rewire both ends to put the remaining, good pair on 2 and 3.

 

[len]

I was doing a show with a national company, which I will leave unnamed, and they routinely run their DMX through their snake, it always works, I'm told by the tech and that night it didn't. Luckily I had my own DMX cables. As stated above, it will work until it doesn't.

You're not the only one. One tour I was on there were no spares for anything and no plan to replace if something failed. I asked about it at the beginning of the tour and was told "nothing bad ever happens." One afternoon the desk lost all show data somehow, either through operator error or just desk failure. The operator had to re-program everything from scratch. The next few days were not pleasant.

 

[hitchhiker]

I've run DMX through audio, and haven't had a problem - though it was only "conventional lighting" - 1 board to one dimmer pack.

 

[shiben]

My schools main DMX line has been somehow cut (rodents, whatever, tension). It runs under the ground and we cannot fix it until the school boards electricians make an appt. and diagnose the situation which will take a while. We have a show coming up and we do not wish to run it above surface nor do we have the funds to rent a wireless DMX combo.

Just thought of this. If the cable is actually broken due to rodents, then there is most likely enough room in there for you to just pull in a new cable (and even fish it if the spark wasnt kind enough to leave a rope in place for you), because as flexible as rodents are, they still cant get into a 1/2" conduit full of other stuff. Pulling a new cable is easy, not terribly dangerous or even risking damage to other things if the pipe is not too full or some nitwit filled a 3/4" pipe with 4 rubber sheathed multi channel sound cables. An easy way to figure out if your cable is really broken is to just undo both wall plates and tug on one. If one is moving and the other isnt, then your cable is broke. If neither is moving, check your ends and if nothing else find a new path. If both ends move, its not broken and its more likely an end or something. If you determine that the cable is connected but its the cable not an end, and you can pull the cable, attach the DMX cable of your choice to one end, lube it up, and start pulling smoothly and with consistant pressure thru the pipe until its all in. Make innuendo as often as possible, until a supervisor comes by and puts a stop to the dirty terminology. If its not possible to do an easy run thru, fish it, pull a pull rope through with the fish tape, and then pull the line through with the pull rope. Terminate the ends properly, and your good to go.

If its just buried, then start finding a new place to run the cable, because jackhammers will never be an option to fix it (also calls into question how it worked for so long). I also would tend to believe that its just a bad end on something, and that its probably fixable with a solder gun. Im still of the opinion that running from backstage is preferable to running through an audio snake, but if you must, it will probably work. As mentioned above, probably being the operative word.

 

[JD]

If you are lucky, you can use the old cable to pull the new cable through. You will then be able to inspect the old cable and find out what happened.

One other thought- If each end does not want to move, there is always the possibility that there may be a splice or patch block that no one knows about, and may be where the problem is.