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DMX cables

Discussion in 'Lighting and Electrics' started by Radman, Aug 16, 2004.

  1. Radman

    Radman Well-Known Member

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    What is the difference between a 3 pin DMX cable and an audio XLR cable? Will an XLR cable work for DMX without damaging or highly noticable side effects? Curious because my uncle in Minneapolis just got an AMDJ rig with a DMX Operator and 2 DP-DMX20-L dimmer packs. The DMX cables were very expensive, and he wanted to know if he could get by without them.
     
  2. Mayhem

    Mayhem Senior Team Emeritus Premium Member

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    The short answer is yes. Provided that the auido cable is of a decent quality there should be no significant problems.

    Also, ensure that he terminates his DMX signal at the last peice of equipment.

    Do a quick search of the forums here as there have been several posts related to this.
     
  3. ship

    ship Senior Team Emeritus Premium Member

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    DMX cable has been discussed many times on this and other forms. You might look into the other conversations on the subject for a more in-depth answer. The basic semi-agreement is that it will frequently work just fine especially in shorter lengths. The problem is that it won’t always work dependable especially dependant upon length. And another part of the answer is a definate that you will get a more reliable signal thru true DMX 512 Protocol cable.

    Note that three pin DMX grade cable is very much similar to Microphone cable, it’s just made to a different and better standard with a better isolation and grade of wire. The signal which is going thru it is very dependant upon the nature of the data going thru it and the resistance in the wire. Heck you can even send data at times down a extension cord - and in some instances you will be. A Color Blast for instance uses standard 16/3 SJT wire for both power and signal. This is getting complex but it's a question of data down the wire, signal loss of the wire and isolation of the data. Cat.5 wire due to it's twisting will also reliably pass a DMX signal. Given the equipment is constant refreshing and repeating the signal, and that all the data available is not used, it might work just fine at times in a very general assumption DMX Tools will probably stomp on my head about. What wire to use and works is a large debate in the industry as with the necessity to terminate. Terminating is not always necessary either.

    Beyond this, see what the American DJ manual says. While for instance my NSI dimmers speak Microplex instead of DMX, they are designed to work with at least microphone cable. The American DJ equipment in being also lower end gear might also be designed to work with lower end cable fairly reliably. Given the brand, this might be a possibility especially for less than 50' lengths but still dependant upon the grade of the microphone cable.

    As a minimum for dependability I would say digital quality microphone cable would be fine, so would single pair instead of dual pair DMX cable. Both will save you money over true dual pair DMX cable.


    Hokay, hope it helped a little because data even having read the Protocol book is still something I try not to think about much beyond making it function.

    For me, I get lots of XLR cable coming back from shows that is not ours. If it has a name and phone number on it, great. If not, it's inspected for sutibility for making ours. In my case the standard is that if the conductors are tinned, and have at least a braided or foil shield around them, than there is a good chance it's going to pass a signal adiquately. If the wires are copper, the shield wire twisted if not it's own conductor in not shielding the other two, it's trash or given away. For the most part I equate the above with better grades of mic cable that are going to be within the tolerances of reliably passing a DMX signal.

    If I remember correctly, the impudence is the major factor and it should be 120 ohms. Anything from 100 ohms to 130 ohms works decently and digital grade microphone cable is normally about 100 to 110 ohms.
     
  4. DMXtools

    DMXtools Active Member

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    No stomping, I promise... Both Ship and Mayhem have a good grasp of the situation. They've told you "what" and I'll try to explain "why."

    Decent microphone cable and true DMX cable are very similar. Given a small system (controller, two dimmer packs, one effect) and relatively short total cable length (under 200 feet total), your uncle could probably get away with using some pretty cheap mic. cables. In my own small system, for example, I'm running the DMX signal through a spare channel of my microphone snake, 100 feet to the stage, then through another 200 feet in five different mic. cables to get to four scanners and a DMX-to-microplex translator (so my older microplex dimmer packs can run from the same controller). It works reliably. As a system gets larger, however, errors add up. The longer the total length and the more devices on the circuit, the more you need to worry about using true DMX cables.

    I will reiterate what Mayhem said about termination. Any cable has a characteristic impedance, a function of the cable's resistance, capacitance and inductance. I'm not going to go into the mathematical proof here, but the characteristic impedance is constant regardless of the length of the cable: a foot of Belden 9841 DMX cable has a characteristic impedance of 120 ohms. So does a mile of Belden 9841.

    DMX signals are high-speed electrical pulses. Electrical pulses travel through Belden 9841 at about 2/3 the speed of light. But if the impedance changes, it can speed them up or slow them down, distorting them. More important than the specific characteristic impedance, you should try to keep the impedance constant: if you use mic. cables, try to use the same manufacturer's type throughout.

    Impedance changes can also reflect the pulses. When the pulses get to the end of the cable (without a terminator), the impedance goes up to near infinity and you get a big reflection - the pulses hit the end of the cable and bounce back toward the controller, interfering with the pulses the controller is still sending out.

    A terminator is just a resistor placed across the signal leads at the very end of the line. It's proper value matches the characteristic impedance of the cable - 120 ohms. To the pulses, that 120 ohm resistor looks like another chunk of Belden 9841... that goes on forever. The pulses go into the terminator and never come back. The signal stays clean and the system runs reliably.

    John
     
  5. Radman

    Radman Well-Known Member

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    Tried. Their manuals are crap.

    Thanks guys, I think I'll wire him up a terminator and tell him to either use good mic cables or spring for the DMX.
     
  6. cambo1000

    cambo1000 Member

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    One time I borrowed one of the youth dept. Coemar Ispots to use on the main sanct. stage. I ran your standard 3 pin XLR cable into a stage sub-snake, which showed up right belowed in the sound booth on the console, unplugged from the back of the sound board, then I just added on another cable, plugged into my controller (mcl 16) and away I went...

    I wonder if it could be run through a patchbay though..hmmm...
     
  7. cambo1000

    cambo1000 Member

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    oh, there are adapters you can buy to convert 3-pin to 5-pin and vice versa. But be aware that they will not work if you are trying to enter a non-dmx controller into an dmx universe.
     
  8. zac850

    zac850 Well-Known Member

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    I've done this once, use mic cable as DMX cable, and I must say that it worked fine. I ran cable from my American DJ board about, oh, 60 feet to a dimmer pack, and about another 30 to a secound dimmer pack. It worked fine, and I had no problem with any DMX type things. A fair bit of this run was next to power cables running to the booth with another light board/dimmer set-up (didn't have enough free channels on the American DJ dimmers).
     
  9. cambo1000

    cambo1000 Member

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    I've never been concerned with running DMX around power, it's just control isn't it?
    My TD didn't want me running my control cable next to the audio lines because fear of interference, but that wouldn't do any harm; it's just 1's and 0's.
     
  10. cambo1000

    cambo1000 Member

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    I've never been concerned with running DMX around power, it's just control isn't it?
    My TD didn't want me running my control cable next to the audio lines because fear of interference, but that wouldn't do any harm; it's just 1's and 0's.
     
  11. zac850

    zac850 Well-Known Member

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    Yea, DMX next to power lines shouldn't be a problem, its audio cables next to power lines that dim that give the hum. I'm not sure of the physics of it, but if the powerlines are at full, there won't be a hum, and if there at 0 there (clearly) will not be a hum. Anything from 1-99% on the dimmers will give a hum.

    I'm sure DMXtools or Ship can tell us why, but its been explained to me and I still don't understand it, so I just accept it as fact.
     
  12. cambo1000

    cambo1000 Member

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    no freaking way, that's what going on in a room here at church...geez I'm an idiot.

    Four fresnels on an American DJ dimmer and controller. 1%-99%=buzz, just like you said.
    Alright, that solves that. Thanks, gives me something to fix tomorrow.
     
  13. zac850

    zac850 Well-Known Member

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    Yea, if your mic cables need to cross power cables, do it at a 90 degree angle. Other then that, depending on how much power is flowing through the wires, just move them a foot and a half away from each other or so. Or further then that, depending on if there is still hum
     
  14. cambo1000

    cambo1000 Member

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    Yeah, before I installed the sound and lights, the electrician ran me a nice dual circuit quad box right along my lighting bar. Good spot for the box, except for that hum problem. It's going to be tricky to find another spot for the DMX line...
     
  15. ship

    ship Senior Team Emeritus Premium Member

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    Don’t look at me on this. With the shielding on DMX cable, much less balanced circuits, there to the best of my knowledge not much of a field. I remember this also being discussed elsewhere but because I don’t do shows anymore, nor do I do electronics, it’s not at the top of my head.

    I remember this was a problem with analog signals, but also know that on things like a Hog Remote, there is both power and data going to the RFU. Also on a scroller, there is normally both power and data going thru the same cable. In both instances it’s no doubt low voltage (not known for sure) but in any case because both are inside of the shield, this if there were a problem would have more effect than something on the outside.

    I do also know that it’s somewhat common practice to loom data cable in with Soco cable.
     
  16. cambo1000

    cambo1000 Member

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    well in my case, I just used a standard xlr.
     
  17. DMXtools

    DMXtools Active Member

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    Actually, it's audio (your snake) next to power, especially the power to a dimmer, that's nasty.

    Start with the relationship between magnetism and electricity. When you move electricity through a wire, it generates a magnetic field. We use this concept in motors and speakers, working the magnetic field of a coil of wire against the magnetic field of a permanent magnet to produce physical movement.

    When you move a magnetic field through a wire (or a wire through a magnetic field), it generates electricity. We use this principle in microphones, where sound vibrates a coil of wire in a permanent magnet's field. The amount of electricity generated depends on many things, including the speed of the movement.

    When you put an audio cable next to a power cable, you're creating a transformer of sorts. The power is AC: alternating current. The electricity starts from zero, builds up to a maximum, falls back to zero, then switches directions and does the same thing. Well, the magnetic field caused by the electricity moving through the wire does the same thing. This magnetic field, as it builds up and collapses, moves through the audio cable. This can generate a small voltage in the audio cable. Fortunately, with 50 or 60 Hz power, the movement is so slow that the voltage generated is almost negligible - until you add a dimmer.

    Solid-state dimmers use an electronic switch to turn the power on part-way through the AC cycle. The current flowing through the wire doesn't follow the voltage during it's gentle rise, but snaps on suddenly. This sudden turn-on means that the magnetic field doesn't grow gradually, it grows very quickly, moving through the audio cable at a much higher speed and, as a result, generating a much higher voltage - one that can create a noticeable buzz. Because it occurs as a pulse, it contains a lot of high frequency components - you're more likely to hear it in your mids or even your tweeters than in your subs.

    The size of this sudden change also contributes to determining the voltage. When the dimmer is at zero, it never turns on, or if there is a keep-warm setting, waits until the voltage has fallen back to near zero before turning on, so the pulse is very small. Similarly, when the dimmer is at 100%, the electronic switch turns on just as the voltage is beginning its gradual rise, so the current follows the voltage smoothly and slowly. When a dimmer is at about 50%, the electronic switch is turning on just as the voltage hits maximum, resulting in the largest sudden change... and the loudest buzz.

    Most dimmers have filters to try to eliminate this problem, but they're not always successful. The filter components are some of the most expensive parts of a good dimmer... and one of the areas where certain manufacturers cut costs to deliver unbelievably low-priced dimmers.

    John
     
  18. zac850

    zac850 Well-Known Member

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    Woops, yea, thats what I meant. I meant DMX next to power shouldn't be a problem, and that its audio next to power that gave problems. That was just a typeo, sorry.

    I'll edit my post to correct that.
     

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