DMX though snake

[stvcmty]

On the "We're getting a nasty buzz from our lights" thread, there was mention of noise from ruining DMX though an audio snake.

 

What is the general view on running DMX though a spare snake channel?

 

I run a few wireless microphones, so I always have a free channel on the snake. Since I switched to a DMX lighting rig, I have used a snake channel to get DMX from the lighting board back by the mixer up to the lighting equipment on stage. I have not had any noise problems. Both of my snakes have individually shielded pairs with foil shields.

 

I have looked at cable specifications, and the audio twisted pairs are not the same as the cable specified for RS485 (The hardware underlying DMX). It worked the first time I did it, so I have done it since. In the long run, will using a snake channel for DMX negatively impact anything?

 

Along the same idea, I run video though the snake using a 75 ohm coax to UTP balun. Video is much higher frequency than DMX, but I have not noticed any problems. The tricky part is finding a laptop with a good composite video out. The balun takes the signal for 75 ohm coax cable and impedance matches it to 100 ohm impedance twisted pair cable. Another balun on the other end takes it back to 75 ohm to be terminated by the projector. The baluns are designed for cat 5 cable, but there is enough twist in the snake cable that video works. I tie the shield to ground at the sending balun, and I don't do anything with the ground on the projector end.

 

It is much simpler to run one snake and get everything though it, rather than to run a snake, a DMX cable, and a video cable.

 

A snake is made for audio, but what can be run though it safely?

 

Stv

[agedhorse]

All of this can work ok BUT, it works best on opto isolated DMX or if using individually INSULATED and shielded, twisted pairs. Not all snakes have individually insulated jackets as it's not normally a problem and it keeps the snake diameter more reasonable.

 

Same applies to the balun video transmission in that it must be pretty well isolated via the baluns.

 

characteristic impedance of the DMX may indeed be a problem in some environments, for some cable lengths above some nominal amount, etc due to reflected signal interfering with the data stream. DMX is pretty darned fast, so if it doesn;t work, be sure you have a true DMX back-up cable.

[abzurd]

I'm the one who posted that in the other thread. My experience is, you'll know right away if it's going to work. A) You'll either get noise or you won't and 2) The lights will generally "act up" immediately if they are unhappy with the signal. Our new EWI 100 ft snake works great. The 100 foot, no name, 30 year old snake didn't, nor did the 250 foot snake the band engineer has.

[milesdf]

the only real issue here is that the snake pairs are not equivilent to the specs for dmx cable. but people use audio cable for dmx all the time with no issues, so if it works, go for it.

[abzurd]

 

the only real issue here is that the snake pairs are not equivalent to the specs for dmx cable. but people use audio cable for dmx all the time with no issues, so if it works, go for it.

 

 

Yep, BTW, if you want reliable DMX cables you can't beat the EWI quad cable. It comes in blue so another benefit is it's distinguishable from your normal XLR's.

[agedhorse]

DMX cable is 110 ohms characteristic impedance and lower capacitance per foot IIRC, impedance mismatch can cause problems in some installations and for higher distances, and the capacitive loading is proportional to length and causes pulse rounding leading to errors if there's enough of it.

[W. M. Hellinger]

 

DMX cable is 110 ohms characteristic impedance and lower capacitance per foot IIRC,

 

FWIW: Although the EWI blue quads abzurd recommended might function for DMX applications, I do stock 110 ohm AES/EBU cable stock for DMX applications (which is also blue colored).

 

BTW: Thanks to all for the invite back... and it's good to see that a civilized demeanor has returned to the forum.

 

And thanks to the folks who are putting the efforts into maintaining a civilized standard here.

[tony k]

[agedhorse]

BTW: Thanks to all for the invite back... and it's good to see that a civilized demeanor has returned to the forum. And thanks to the folks who are putting the efforts into maintaining a civilized standard here.

 

It has been "quiet" the past few days with the new "oversight".

[milesdf]

 

Yep, BTW, if you want reliable DMX cables you can't beat the EWI quad cable. It comes in blue so another benefit is it's distinguishable from your normal XLR's.

 

 

Noooooo!!!!!!

 

DMX calls for a cable with a capacitance of 10-15 pF/foot. Regular mic cable has a capacitance of about 25 pF/foot. Because of the added dielectric in the quad cable the capacitance of that kind of cable is typically 50 pF/foot.

 

Probablly not a worthwhile risk, especially since that variety is more expensive.

 

As for AES cable, its almost identical to the dmx standard, tried to buy up 200' from a closeout a couple months ago but unfortunatley it was all gone by the time my order went through. If you can find it cheaper (unlikely) go for it.

 

Edit: Woah i missed audiopile aes/dmx cable in blue? you'll be getting a call tommorrow! why isnt this on the website, or did i miss it?

[Gregidon]

My question is... if people tend to like to run DMX through their snake, why don't manufacturers put a "DMX" channel in snakes that matches the DMX standard? Seems like a good move

 

Good to see you again mark

[milesdf]

I think whirlwind might have a product that does this, touring grade stuff though.

 

Edit: nah im wrong turns out i was thinking of a strictly dmx snake i saw once, i cant find it from whirlwind. in most modern touring situations they need as many dmx universes as possible to facilitate lights and to keep timings low on the lines, as well as the full 2 pairs for certain high level pro grade stuff. so this probablly means 6 channels with 2 pairs each, thats 12 pairs right there, which is a little too much to take up on a primarily audio run. As for the lower level markets, its almost pointless because everyone here is doing different stuff and the market is probablly too small. Also alot of the medium sized providers tend to do only lighting or sound, worthless for either to have such a product. But at the same time, a dmx channel or two could still be used for audio, so personally i would try to get funds to purchase this.

[abzurd]

 

Noooooo!!!!!! DMX calls for a cable with a capacitance of 10-15 pF/foot. Regular mic cable has a capacitance of about 25 pF/foot. Because of the added dielectric in the quad cable the capacitance of that kind of cable is typically 50 pF/foot. Probablly not a worthwhile risk, especially since that variety is more expensive. As for AES cable, its almost identical to the dmx standard, tried to buy up 200' from a closeout a couple months ago but unfortunatley it was all gone by the time my order went through. If you can find it cheaper (unlikely) go for it. Edit: Woah i missed audiopile aes/dmx cable in blue? you'll be getting a call tommorrow! why isnt this on the website, or did i miss it?

 

 

I didn't realize this. Interesting. We've had good luck with this cable . That may be because I bought some colored cables from Pulsar initially and they just flat out suck. I think I bought 10 or so in various lengths. All had a jumper connecting a pin to the ground that I had to cut out. In the 4 months I've had the cables something like 8 of the 10 have failed and the rubber strain relief has completely cracked all the way around in 2.

[W. M. Hellinger]

 

My question is... if people tend to like to run DMX through their snake, why don't manufacturers put a "DMX" channel in snakes that matches the DMX standard? Seems like a good move.

 

I did that once about 3 years ago. The product flopped... basically for the same reasons that I don't actively promote the EWI AES/EBU cable... wayyy too many customer service issues due to a general lack of understanding by a significant percent of the users. DMX seems to be one of those things where one thing wrong and the system won't work properly... but two things wrong and it might work properly.

[milesdf]

 

I didn't realize this. Interesting. We've had good luck with this cable . That may be because I bought some colored cables from Pulsar initially and they just flat out suck. I think I bought 10 or so in various lengths. All had a jumper connecting a pin to the ground that I had to cut out. In the 4 months I've had the cables something like 8 of the 10 have failed and the rubber strain relief has completely cracked all the way around in 2.

 

 

Well I mean if it works for you, it works for you. For jumping lights you'll probablly never have an issue, and the kind of issue you might see would probablly be hardly noticeable. And I agree, most of the dmx cable i've seen out there is lacking in quality but very very expensive. Probablly gonna end up buying a roll of bare cable and making my own one of these days, assuming the ewi product doesnt work out.

[agedhorse]

Generally, when DMX doesn't quite work out, it's a total disaster. Rarely is it barely noticeable given the protocol.

[abzurd]

 

I did that once about 3 years ago. The product flopped... basically for the same reasons that I don't actively promote the EWI AES/EBU cable... wayyy too many customer service issues due to a general lack of understanding by a significant percent of the users. DMX seems to be one of those things where one thing wrong and the system won't work properly... but two things wrong and it might work properly.

 

 

Posted this earlier, but for what it's worth Mark, the EWI snakes handle the DMX with no perceptible noise at all.

[milesdf]

 

Posted this earlier, but for what it's worth Mark, the EWI snakes handle the DMX with no perceptible noise at all.

 

 

While this may be true for your specific application, this isnt going to be universally true. For example, adding another 50' of dmx compliant cable after the snake might cause issues or perhaps one day you get the snake too close to a power supply somewhere and dmx stops working.

[agedhorse]

My experience is when there's a DMX problem, it's a big problem. There's no real error handling and often, if errors are encountered the first thing the fixture will try to do is reboot and initialize... or hold it's last valid command. I suppose it's possible for a fixture to react intermittently to invalid data depending on error handling, but I don't have experience with this.

[abzurd]

From my BE -

 

I know about the cap issue in cable with data also about reflections using long runs, with the stuff I currently have were ok up to 500 feet, I don

[bassred]

I believe Markertek can make snakes with almost any number of cable types and various options, if you are wlling to pay for it...

 

I've grabbed audi cables to hook lights together and not had a problem, and I've grabbed a DMX jumper to patch an effects unit in a rack with no poblems, I bet you'd be able to tell right away (as stated bythe other responders) if there was going to be an issue.

 

Interesting observation, though, I never paid attention to the actual difference etween the two cables, just figured DMX - lights, XLR-mic- audio....

[stvcmty]

 

DMX calls for a cable with a capacitance of 10-15 pF/foot. Regular mic cable has a capacitance of about 25 pF/foot.

 

 

If the capacitance per unit length for audio cable is about two to three times higher than DMX cable, does that mean the amount of cable able to be used is 1/2 to 1/3 as long?

 

On the mixer fan end of the snake, the channel I use for DMX is plugged directly into the lighting board. (Yay for large fan outs.) If I put an active splitter at the stage box end and use DMX cable from there, making sure all grounds are proper, would the active splitter keep things from getting crazy?

 

I am getting an active splitter soon because it would be easier to run a separate line to each lighting tree rather than to have to chain one to the other. (It would make more sense to the people who help setup.) I need to fined a DMX splitter that can terminate the feed and starts 2 isolated branches.

 

Do XLR connectors have a characteristic impedance?

 

Stv

[agedhorse]

Good question... XLR's do not really have a characteristic impedance but in the context of the protocol work just fine.

 

There are many possibilities for splitters available, Doug Fleenor Designs is a good place to start. You would probably want to optically split at the stage end as it's easier to control potential ground loops between the various fixtures and your audio world that way. The more fixture manufacturers involved, the more likely for problems as not everyone does things in the approved way.

 

Yes, as far as capacitance goes, the longer the canble the more important the cap/ft becomes. The characteristic impedance of the cable also becomes more problematic as the cable grows longer but also the degree of cable mismatch and how the loads are terminated and distributed can make for additional variables.

[milesdf]

Review of EWI (audiopile) 110 ohm cable (suitable for DMX):

 

I just recieved 200' of this cable the other day for our lighting rig, just got a chance to play with it today. This stuff is thick!! I don't know if I should be using this for DMX or to control my bomb killing robot. It barely fits into EWI xlr connectors, but it does make it with a little effort. It seems to coil and lay really nicely though, so this seems perfect from a reliability standpoint. The cable itself is blue, and by blue I mean almost neon blue. Perfect for the help to be able to tell what cable is which, but not so perfect for jumping lights on a truss. The cable is also marked with somewhat flaming gold lettering and reversed lettering. Kinda takes away from its looks overall but eh whatever. When I stripped it I found thick outer insulation, then a standard braided shield, and then an additional foil shield with drain wire! Double shielding! Then some pretty hefty 20 or 22 gauge signal wires wire plastic filler to maintain shape. This is some awesome cable! And as usual Liz and Mark were a dream to do business with.

[Dogoth]

 

That may be because I bought some colored cables from Pulsar initially and they just flat out suck. I think I bought 10 or so in various lengths. All had a jumper connecting a pin to the ground that I had to cut out. In the 4 months I've had the cables something like 8 of the 10 have failed and the rubber strain relief has completely cracked all the way around in 2.

 

 

Sorry I'm going a little off topic.

 

Isn't it amazing how cheap they can build those things. I bought a small snakeharness from them for a non critical application (knowing that it would be plugged in and left there) and yes it does work but WOW. The only thing metal in the whole XLR is the connection pins (I think the housing may even be plated plastic).