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Author Topic: DMX or Audio? how to ID unmarked cables?  (Read 17823 times)

dave moldover

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DMX or Audio? how to ID unmarked cables?
« on: April 03, 2012, 01:25:24 pm »

Trying to separate and ID many finished XLR cables we have from various sources. Some are marked "microphone" or "audio". Others are marked DMX so we assume high impedance digital signal cable.
What is the simplest, reliable way to determine the type of the unmarked cables? Once we get them all sorted out I'll mark them permanently so they won't get mixed up again.

thanks.
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James Feenstra

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Re: DMX or Audio? how to ID unmarked cables?
« Reply #1 on: April 04, 2012, 11:10:15 am »

get an ohm meter....if the resistance of the cable is ~60ohms, it's not for dmx
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Mac Kerr

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Re: DMX or Audio? how to ID unmarked cables?
« Reply #2 on: April 04, 2012, 11:26:33 am »

get an ohm meter....if the resistance of the cable is ~60ohms, it's not for dmx

How does that work? The characteristic impedance of DMX or digital audio cable is supposed to be 110Ω to 120Ω, but that is not resistance, which will be the same for either, based on wire gauge. The characteristic impedance is based on an infinite length of cable (no reflections) at the frequency in question, which is in the megahertz for DMX and digital audio.

Mac
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Nick Pignetti

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Re: DMX or Audio? how to ID unmarked cables?
« Reply #3 on: April 04, 2012, 01:10:53 pm »

I can't answer your specific questions regarding cables you already have, but what I do to keep it organized is this:

I buy bulk wire and roll my own.
DMX>Grey Cable
Audio>Black Cable

Then, I use color coded rings on the adapters to designate the length.
blue=15ft
red=25ft.
yellow=5ft.
etc. etc.
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Randall Hyde

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Re: DMX or Audio? how to ID unmarked cables?
« Reply #4 on: April 04, 2012, 03:19:37 pm »

Trying to separate and ID many finished XLR cables we have from various sources. Some are marked "microphone" or "audio". Others are marked DMX so we assume high impedance digital signal cable.
What is the simplest, reliable way to determine the type of the unmarked cables? Once we get them all sorted out I'll mark them permanently so they won't get mixed up again.

thanks.

Can't help you with unmarked cables in a bin.  I always stick red (audio) or green (DMX) shrink-wrap on the XLR connectors to identify the cables (also identifies them as *mine* at a venue, as I've never met anyone else who does this). Also, on my audio cables, I tie a quick knot in the cable next to the XLR connector and put the shrink-wrap over that (and on to the connector) to provide extra strain relief.

If I couldn't figure out a cable's type, it would likely become a DMX cable - mic cables seem to work better as DMX cables than DMX cables work as audio cables.
Cheers,
Randy Hyde
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Mac Kerr

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Re: DMX or Audio? how to ID unmarked cables?
« Reply #5 on: April 04, 2012, 05:14:05 pm »

If I couldn't figure out a cable's type, it would likely become a DMX cable - mic cables seem to work better as DMX cables than DMX cables work as audio cables.
Cheers,
Randy Hyde

DMX cables should work just fine as audio cables, audio cables may not work as DMX cables. Audio will be fine over any kind of twisted pair, DMX has very specific needs.

Mac
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Randall Hyde

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Re: DMX or Audio? how to ID unmarked cables?
« Reply #6 on: April 04, 2012, 07:41:31 pm »

DMX cables should work just fine as audio cables, audio cables may not work as DMX cables. Audio will be fine over any kind of twisted pair, DMX has very specific needs.

Mac

Good to know.
Though I've often used mic cables as DMX cables in a pinch. Never had any problems. Of course, I always use terminators, too.

I have used DMX for audio, but only for line level. Though I've long since purchased enough audio cables that I'd never find myself short, I'll have to keep this in mind if the unthinkable does occur and I have a mic and a DMX cable and that's it.
Cheers,
Randy Hyde
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James Feenstra

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Re: DMX or Audio? how to ID unmarked cables?
« Reply #7 on: April 05, 2012, 10:16:36 pm »

How does that work? The characteristic impedance of DMX or digital audio cable is supposed to be 110Ω to 120Ω, but that is not resistance, which will be the same for either, based on wire gauge. The characteristic impedance is based on an infinite length of cable (no reflections) at the frequency in question, which is in the megahertz for DMX and digital audio.

Mac
You can still measure the impedance of the cable with a quality meter...

Another way would involve reading what's printed on the cable (model number) and sticking it into google...95% of the time you'll get some kind of conclusive result
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Mac Kerr

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Re: DMX or Audio? how to ID unmarked cables?
« Reply #8 on: April 05, 2012, 10:22:18 pm »

You can still measure the impedance of the cable with a quality meter...

So what is the impedance of a 25' piece of cable? How about a 1000' piece? Which one is 110Ω?

Mac
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duane massey

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Re: DMX or Audio? how to ID unmarked cables?
« Reply #9 on: April 05, 2012, 11:50:25 pm »

Rarely had a problem using audio cable for DMX, never had a problem using DMX cable for audio (other than having to cut off two of the 5 pins and bending the other ones and filing them down...).
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Duane Massey
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James Feenstra

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Re: DMX or Audio? how to ID unmarked cables?
« Reply #10 on: April 06, 2012, 12:38:17 pm »

So what is the impedance of a 25' piece of cable? How about a 1000' piece? Which one is 110Ω?

Mac
One crude way to measure characteristic impedance: unroll a good length of line (say 50 feet or more), solder a 120 ohm resistor to the far end, at the close end hook up a pulse generator and and oscilloscope. See if there is any reflection of the pulse rising edge that is observed on the scope after a round trip time delay. If you can see any voltage spike, solder a different value, 110, 130, etc ohm resistor at the far end. When you have soldered the resistor that results in the smallest reflection back to the source, read that resistor value and that IS your characteristic impedance.

this would be a very quick test to determine if a cable is 60 or 110 ohms as you only need a couple resistors

there's also a way to do it with a standard ohm meter, but it escapes me at the moment

googling product id #s will still likely be the fastest, most accurate, way to figure it out, as the manufacturer *should* know what their cable is
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Tony Mitchell

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Re: DMX or Audio? how to ID unmarked cables?
« Reply #11 on: April 07, 2012, 03:44:41 pm »

Rarely had a problem using audio cable for DMX, never had a problem using DMX cable for audio (other than having to cut off two of the 5 pins and bending the other ones and filing them down...).

...which leans to answering the OP's question re how to ID unmarked cables (count the pins) - assuming you don't have any 5pin audio cables nor any 3pin dmx.
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James Feenstra

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Re: DMX or Audio? how to ID unmarked cables?
« Reply #12 on: April 08, 2012, 12:40:42 am »

...which leans to answering the OP's question re how to ID unmarked cables (count the pins) - assuming you don't have any 5pin audio cables nor any 3pin dmx.
Although not technically dmx standard, there are plenty of fixtures that only take 3 pin data, which is likely the reason of the ops question

Also, there are a few older/stereo mics with a 5 pin connection that I've seen
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Micky Basiliere

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Re: DMX or Audio? how to ID unmarked cables?
« Reply #13 on: April 08, 2012, 08:27:14 pm »

Rarely had a problem using audio cable for DMX, never had a problem using DMX cable for audio (other than having to cut off two of the 5 pins and bending the other ones and filing them down...).

+110 !!! me either... the audio not for DMX is BS... never had any sort of an issue
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dave moldover

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Re: DMX or Audio? how to ID unmarked cables?
« Reply #14 on: April 11, 2012, 11:02:14 am »

All cables in question have 3 pin XLR connectors. ADJ told me the "ACCUCABLE" branding was used for both DMX and Audio cables but those with Black ends connectors are DMX and Silver Connectors are audio. That helps with a few of them. Many do not have any marks or branding on them.

As far as using the audio for DMX, we have done this successfully too but for the future I want to create separate  groups of cables. Our shows are getting more complex, cable runs longer, each fixture is a critical part of a show. Much of it is going op in the air so cable replacement after it's flown is not possible. I don't want to take extra risks and have a signal cable fail during a show. This is why we want to ID all the cables in the current inventory.

It sounds like there may not be a simple, reliable way to sort the unmarked cables. We'll have to replace the questionable ones. They are mostly shorter pieces.

thanks for the input and suggestions.
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James Feenstra

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Re: DMX or Audio? how to ID unmarked cables?
« Reply #15 on: April 11, 2012, 11:10:45 am »

+110 !!! me either... the audio not for DMX is BS... never had any sort of an issue
if you haven't had issues with it you haven't had enough of it in your rig yet.
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DanGlass

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Re: DMX or Audio? how to ID unmarked cables?
« Reply #16 on: April 12, 2012, 06:58:09 am »

James, you are 100% correct.  Anyone who hasn't had an issue with using audio cables for DMX simply hasn't used enough of it on a show or is using so much flashing and movement that they haven't noticed. I spent quite a few early years with the mindset that audio cable was just fine until that one bigger show when everything went crazy. 

it can be either a bad cable that causes the issue or simply to much resistance build up, either way you cant find the exact source and your whole show is down the drain.  The cable standards were not setup to sell more cable to simply help prevent issues on shows.  The fact is that if you use audio cable you are rolling the dice every time and probably will be fine 99% of the time but is that 1% percent worth the risk?  The more cable you use the more resistance you create and then next time you add that new light will that be the time it all goes crazy.  Yes,DMX cables are more expensive and most sound companies have more audio cable then they need on a show so it is easy to just dig into that same case and pull out another cable, but for me the cost does not outweigh the benefits.  I know that unless my DMX cable is broken it will work every time, no worries, no guessing, no hesitation.   You cant say that with an audio cable.

If a lighting guy compares all the info and does his research and still decides to use audio cable then I salute you Sir for you are a braver man than I.  Also know that when you finally get the call to step up and do a large scale show for a nice payday and during the middle of your show half of your lighting rig begins to strobe, twitch, and do everything you dont want it to do, the following year when you are at that show please come up to where the lighting console is and introduce yourself to the new guy supplying lights for the event.  He will thank you for the oppourtunity and for making the choices you did, then you can kindly find your seat in the audience with the rest of the paying crowd.
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Mac Kerr

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Re: DMX or Audio? how to ID unmarked cables?
« Reply #17 on: April 12, 2012, 12:57:53 pm »

it can be either a bad cable that causes the issue or simply to much resistance build up, either way you cant find the exact source and your whole show is down the drain.

The issue is not the resistance in the cable, it is the characteristic impedance of the cable. This is the characteristic of the cable that makes all the interfaces between different lengths of cable, and equipment I/O, impedance matching so there are no reflections off a mismatched impedance. At the very high frequencies inherent in DMX data and digital audio this is very important in order to eliminate corrupted data.

Mac
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duane massey

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Re: DMX or Audio? how to ID unmarked cables?
« Reply #18 on: April 12, 2012, 07:06:12 pm »

(other than having to cut off two of the 5 pins and bending the other ones and filing them down...).

Hopefully it was apparent that this was meant to be humorous?
There is no argument on my part that using the proper cables is a proper thing to do, but in smaller systems where you only have 6-12 fixtures (and cable lengths are short) it becomes less of an issue. Just got back from re-working an existing system with 40-something fixtures on every type of XLR cable imaginable, and changing out the cables was not an option. Non-critical application (flashing lights on a dance floor, basic movers/scanners), but I would have preferred starting from scratch with all new DMX cables.
You can "feel" the difference on the Accucables, the DMX cables have a stiffer shield (maybe braided as opposed to spiral wrap on the audio cables?).
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Duane Massey
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Micky Basiliere

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Re: DMX or Audio? how to ID unmarked cables?
« Reply #19 on: April 12, 2012, 10:39:30 pm »

Hopefully it was apparent that this was meant to be humorous?
There is no argument on my part that using the proper cables is a proper thing to do, but in smaller systems where you only have 6-12 fixtures (and cable lengths are short) it becomes less of an issue. Just got back from re-working an existing system with 40-something fixtures on every type of XLR cable imaginable, and changing out the cables was not an option. Non-critical application (flashing lights on a dance floor, basic movers/scanners), but I would have preferred starting from scratch with all new DMX cables.
You can "feel" the difference on the Accucables, the DMX cables have a stiffer shield (maybe braided as opposed to spiral wrap on the audio cables?).

Accu Cable ??? that's American DJ crap... if i was to spend stupid amounts of money on buying "DMX" cables i would not buy that... the only difference you feel is cheap.
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duane massey

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Re: DMX or Audio? how to ID unmarked cables?
« Reply #20 on: April 12, 2012, 11:36:48 pm »

Mickey, to each his/her own. I use them for installs, and have yet to have any problems. For touring or rentals, maybe not, as the connectors are not very robust. As far as rolling your own, I used to make all my own cables until the pre-made cables became cheaper than I could buy the connectors for.
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Duane Massey
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Re: DMX or Audio? how to ID unmarked cables?
« Reply #20 on: April 12, 2012, 11:36:48 pm »


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