DMX and sound down the same multicore

[S&L]

I have done a couple of shows with a touring band recently as dep sound engineer (what I know about lights you can write on the back of a stamp).

The lighting engineer is now in the habit of using one of the multicore cables for DMX lighting - the others are all carrying sound send and return. I see no apparent side effects and audio and digital signals seem unaffected - but are they? Will this work in all situations?

[beware]

Not best practice, but I've never had a problem.

[Shez]

It'll work fine most of the time, but it'll stop behaving at the worst possible moment. (Have a look at the countless discussions about DMX down mic cable elsewhere here.)

[Bobbsy]

As said, not great practice but I've also let it happen with no ill effects.

 

If your reference to "send and return" implies line level signals, obviously these are more robust than mic level and less prone to interference than mic level.

 

One other note is that is that the quality and build of the multicore will have a lot to do with how well you get on.

[S&L]

As said, not great practice but I've also let it happen with no ill effects.

 

If your reference to "send and return" implies line level signals, obviously these are more robust than mic level and less prone to interference than mic level.

 

One other note is that is that the quality and build of the multicore will have a lot to do with how well you get on.

 

sorry send and return was my own naive short hand - the stage signals are a mix of mostly line signals with some mic signals - but I was curious about the implications in all situations and not just the one I encountered.

the multicore in the case above is (I think) from loudest.com so not bargain basement but not really top grade either.

I take it from the above posts that in some circumstances:

a) mic signals may recieve interference particularly on lower grade cables and

b) DMX may fail once in a blue moon because it's not a proper DMX cable

but if pushed you can get away with it 9 times out of ten?

does that sum it up?

[timsabre]

I would say 99 times out of 100 you will have no problem.

A recipe for trouble is if the multi does not have independent earths/screens on every core. Then you can get some fun earth loop problems which can be hard to find.

I have experienced a couple of "DMX noise on the audio" problems caused by such earth loops. DMX makes a strange and quite characteristic chittering noise if it gets onto the audio.

I have never had a problem with the DMX failing.

[peza2010]

Ive done it countless times and not once had a problem. Our multicores have 4 ways of gender swappable lines for various things.

[MarkPAman]

Also done it loads of times without problems, in the past.

 

However, these days my multi is digital, so any lighting bod that's relying on me to provide a cable will be disappointed! It's happened once so far

[Wilflet]

Providing your using good professional gear- everythings balanced, high quality multi etc you should be fine, if theres 2 multis I prefer for it to go down the returns (all line level) to sends.

Lesser multicores it may become an issue more often.

I always make it quite clear to the lampys if I hear any noise from them doing this Im unplugging it straight away, whenever it happens- thus if they dont want to risk loosing all their DMX 5 seconds before the headliner goes on they need to weigh up the hassel of running their own line vs the risk of having to run their own line half way during the show.

 

As mark says these days the "oh you must have a spare line for DMX/Comms/pyro/composite video, you noise boys always have loads of spare lines" is met less with "Id really rather you didnt, I'll help you this time but bring the right cable next time" and more with "thats not how AES50 works mate"

[paulears]

Over the past few years I've often wondered if the DMX send receive circuits and components are better designed, because even though we have so much more devices working on the DMX universes, the actual faults we used to get seem to be going down. Is this what's happening - impedance and capacitance problems not so critical?

[timsabre]

Over the past few years I've often wondered if the DMX send receive circuits and components are better designed, because even though we have so much more devices working on the DMX universes, the actual faults we used to get seem to be going down. Is this what's happening - impedance and capacitance problems not so critical?

 

They are much better - the receivers are more sensitive and load the line less, so you can operate more devices on a line. The transmitters are more powerful and able to overcome cable capacitance. It's normally just one chip - 75176 or similar.

[indyld]

Over the past few years I've often wondered if the DMX send receive circuits and components are better designed, because even though we have so much more devices working on the DMX universes, the actual faults we used to get seem to be going down. Is this what's happening - impedance and capacitance problems not so critical?

 

I think that sometimes this is to do with the prevalence of optisplits at many more more points in the system (effectively retransmitting) which in my experience have become more common as a means of distributing DMX around even smaller rigs, possibly because splitters have become cheaper. I used to fault find DMX systems by taking any buffers out of the chain which had the effect of making the signal less robust and the faults more obvious. (When I say fault finding, I mean without a scope etc. but tracing DMX cable or fixture faults 'in the field' without diagnostic kit)

 

I've never had any issues sending DMX down the audio multi, nor have any of the sound boys I worked with. A colleague of mine did find a potential pitfall of doing this, however:

 

When plugging up to an audio stage box, do not, ever, mix up your comms and your DMX as the comms PSU blows every DMX card on the line!!!

[RichM]

I've seen this done in pretty much every small to medium scale corporate show I've ever done, not once had a problem.

[Brian]

...do not, ever, mix up your comms and your DMX as the comms PSU blows every DMX card on the line!!!

 

Serves people right for using non-standard compliant, and poorly designed, equipment. ;)

[timsabre]

...do not, ever, mix up your comms and your DMX as the comms PSU blows every DMX card on the line!!!

 

Serves people right for using non-standard compliant, and poorly designed, equipment. ;)

 

I presume you are referring to 3-pin DMX, but most current equipment I know of would tolerate that no problem. Going back 10 years, maybe not. The Pulsar Masterpiece is designed to survive 240V mains on the DMX.