XLR vs. DMX Cables

In this article, Peter Awad describes the difference between an XLR cable and a DMX cable. Are XLR and DMX cables the same thing? Can they be used interchangeably?

It’s the classic “why use a hammer when the back of a screwdriver will do?” We’re often quick to grab the next best thing instead of the right thing. But what’s the real difference between an XLR cable and a DMX cable?

The Science of XLR vs. DMX Cables

On the surface, they look exactly alike (assuming your DMX cables have 3 pin connectors) and aside from the labelling on the jacket, who would really know the difference? It really has everything to do with the electrical properties of the wire. It’s easy to assume that three strings of wire wrapped in a jacket are the same as any other bundle of wire, but when you look at the science of it, there are important things about those wires. When data is sent down the wire, it doesn’t get sent as ones and zeros exactly, the method of communication is based on the voltage going up and down. (eg. +5 volts = 1 and 0 volts = 0). Therefore all the rules of electricity come into play. You may remember in science class something called Ohm’s Law or the formula V=IR. Well, it defines the basic rules for how the signal is sent down any kind of wire, and different kinds of wire have different electrical properties.

The Practical Side of XLR vs. DMX Cables

OK! Enough of the sciency stuff. Let’s just say that there is a slight difference between the electrical properties of microphone wire and DMX wire, enough that microphone wire is not the right wire to use because we are altering the electrical relationship of the circuit.

So why do so many people say they are the same thing? Many people will even say it’s a marketing ploy to make you buy more cables. If we start by asking who originally defined DMX512, you discover special groups that love to use acronyms for their names. Groups like IEEE, EIA, TIA and ANSI were formed to make sure that different manufacturers followed the same set of rules so their devices would work properly with other devices. These rules define things like: maximum wire length, speed in which the data is sent, maximum voltages, and even things like the size, shape, and number of pins in a connector. Understanding these rules, we discover that DMX is not supposed to be a 3 pin connector, but actually a 5 pin. The DMX512 standard actually goes so far as to state that 3 pin connectors are prohibited. However many manufacturers disregarded this rule because DMX doesn’t use the other two pins on the connector or the other two wires that are supposed to be in the DMX cable (yes, DMX cables are supposed to have 5 wires) and most importantly, using 3 pin connectors allows customers to use microphone cables when in a pinch.

So if the manufacturers say it’s okay and user experience shows that a microphone cable will do exactly the same job, why do we bother making or using DMX cables? Well it goes back to what I said at the start: Why use a hammer when then back of a screwdriver will do? If we are honest with ourselves, we have all cheated like that at some point in our life. I remember in my younger days watching my step-father use a chisel as a flathead screwdriver to remove a light switch. The chisel slipped into the electrical box and sparks flew.

We could go on for pages more explaining the science behind DMX and microphone signals, but at this point I think you get the general idea that there is a difference between these circuits. The question you have to ask yourself is, “If I know this cable is not exactly a DMX cable but it usually works, am I willing to take the chance that by using this cable the electrical properties of the circuit are going to change and something may stop working?”

I am going to let you all in on a secret here, I have cheated to the point of using networking cable as DMX cable. I’m not proud of that choice. If I could go back and do it over again, knowing what I know now, I would only use DMX approved wire.

So if you are lighting for a church service, and the lights are low while the worship leader is wrapping up the worship set with prayer, and you can feel the Spirit moving; how much of a schmuck would you feel like if suddenly one or all of the lights started to strobe because you chose to cheat and use a microphone cable?

It’s the respect for the whole art of lighting that will always influence my choice to use the correct cable. I hope you will too.

More reading for the sciency people:
Wikipedia – https://en.wikipedia.org/wiki/DMX512
OpenDMX – http://www.opendmx.net/index.php/DMX512-A

 

Peter Awad is a husband to Trisha, Foster parent to many, Tech Guy for The Church and a lover of fine coffees. @ChurchTechGuy

50 responses to XLR vs. DMX Cables

  1. Actually, using network cable isn’t cheating at all. Cat5 is recognized as valid cable for DMX installs. It’s not intended as a flexibile cable though and is limited to fixed installs.

    Im in the midst of installing a new lighting system. Cat5 STP is actually recommended for the permanently installed DMX ceiling fixtures and I terminate it into a real 5 pin DMX jack for portable lighting use as well. The 2 unused pins are now starting to be useful. The lights I’m installing all support RDM which uses the other 2 wires to talk back.

    Standards…. Turns out someone was thinking ahead :)

  2. Agreed – network cabling is fine (and good) as long as the electrical properties are the same as “official DMX cable”. In fact, the wikipedia article referred to includes this phrase regarding the DMX 512A standard:
    “DMX512-A (ANSI E1.11-2008) allows the use of eight-pin modular (8P8C, or “RJ-45″) connectors for fixed installations where regular plugging and unplugging of equipment is not required.”

    The real issue here since it’s a digital signalling protocol is the capacitance between conductors & the shield of the cable, which is what you could pretty much boil this article down to.
    The USITT FAQ (http://www.usitt.org/content.asp?contentid=373) on DMX states this:
    A common question is “can we use a good microphone cable?” The answer here is no. While there is some tolerance allowing for 100 ohm to 120 ohm cable, simply put, microphone cable is not at all suitable because of its high capacitance and incorrect characteristic impedance.

    Since there are several rules to the DMX standard, you could use official DMX cable with 5 conductors and still have a problem. Some important “DMX rules” not mentioned that can cause problems:
    1) only 32 loads per cable run
    2) terminate all runs to prevent reflections (some instruments auto-terminate, some don’t)
    3) don’t split the signal without an opto-splitter or distribution amp

    Ignore these at risk to your lighting reputation

    • The reason why network and DMX cable are different from XLR is purely twist rate. Without the proper twist rate of internal conductors the XLR cable is will not break the induction loops causing crosstalk and the data to fail. Cat5/6 has a higher twist rate than DMX and so is as reliable as DMX. It’s also recommended for installs because it is much easier (and cheaper) to get shielded, plenum Cat6. Although, those bright blue network cables look awful across a stage. ;)

      • DrummerCody, you’re on the right lines, but not quite correct when you say “The reason why network and DMX cable are different from XLR is purely twist rate.”
        Read my post further down the thread – there are quite a number of other factors which make them different.
        In a simple data transmission system, where one cable carries one signal, and is not in tight proximity to other cables, the twist rate is fairly insignificant. In systems such as Ethernet where all four pairs in a cable may be used simultaneously, and cables are packed in bundles, it becomes much more of an issue! (Which is why the four pairs in an Ethernet cable are twisted at different rates!)
        Twist rate alone is not a good indicator of what makes a good cable for DMX (if it was, we’d use StarQuad). In the DMX/Mic cable debate, the most significant factor is characteristic impedance. It becomes very significant if cables of different impedance are mixed up, and this is typically the most likely cause of data transmission errors.
        On an install, I wouldn’t pay the extra for Cat6 over Cat5e – the performance will be no better, and Cat5e is easier to work with without damaging the cable.

  3. It’s probably good advice to use dmx cable. If you are a new lighting guy, here’s practical life experience. If you are using a professional console, or software with a professional dongle, and you are using professional lights, (the ones that don’t say Shenzen on them), then do whatever you want. A Mac 250 will work with twine! Other than that, proceed with caution, but you won’t break anything. Oh, and beware of Chauvet.

    • I am new and have to use Xlr to save $ , just a fact of like. But chauvet does not like Xlr!!!

      • Bill, fact is DMX cable isn’t really much more than mic cable, at least not if you’re buying it from a good vendor. There are many manufacturers who sell economy DMX cables. But just like mic cable, there is cheap stuff, medium grade and high end cable options out there.

        And I will 2nd (or 3rd or 4th as it were) using Cat5e in permanent installs. I would never use it where I have to plug or unplug things regularly, but our company uses Cat5e for DMX runs to wall plates or splitters all the time. But for cables from the plates to the fixtures or between fixtures, I would totally use shielded, heat shrunk DMX cable. Physically it will take the wear and tear much better while still providing good, clean data to your fixtures.

      • Chauvet just doesn’t like to work in general. Their fail rate at the moment is obscene, over 10% from the factory are DOA. If I was you, I’d switch to Elation…

    • lighting reality October 28, 2016 at 2:43 am

      chauvet is nothing to beware of

  4. Just curious here… Would 3 pin XLR AES/EBU 110 ohm cable work for DMX signals/wiring? It is becoming a lot more common these days.

  5. Hey guys, as you can see, we sponsored this article. Just a few thoughts to clear things up.

    1. Something always works until it doesn’t. We have used XLR mic cables for DMX before. We have even run DMX through audio snakes in emergencies in the past. You CAN run DMX though barbed wire. That doesn’t make it a good idea.

    2. Any assertion that high end units will function perfectly with XLR is just wrong. While they may be less susceptible to the sort of noise that an XLR cable can induce, that doesn’t mean it is okay. For example, it may work in a chain with 5-10 units, but it probably won’t work with 20 or 25 unit chains. It may work over 50 or 75 meters, but probably won’t work over 200 or 250 meters.

    3. CAT5e is acceptable for permanent installs (in fact, we do all our installations with CAT5e backbones now) but keep in mind that it must be STP or run in grounded EMT. In fact we use STP AND run it in grounded EMT. It is NEVER acceptable to make DMX extensions out of CAT5e cable.

    4. No, 3 pin XLR AES/EBU cable even at 110 ohms is not acceptable.

    Thanks guys

    Mike

    • so if i wanna buy speaker cables with XLR end, to have a good quality in sound, what should I look for??

      • lighting reality October 28, 2016 at 2:50 am

        why would you want to do that? it is stupid. xlr cables are for mics,DI’s etc. the size wore you should be using for speaker cable wont even fit into a xlr connector. clearly you should not be going anywhere NEAR audio. be a box pusher as it seems that is what you are qualified to do.

    • lighting reality October 28, 2016 at 2:48 am

      thanks for the propaganda but once again you are selling nonsense. yes there is a slight difference between the cables but that has no practical effect on the lights. either works fine. the nonsense about 3/5 pin is equally nonsense. 5 pins were used because of the idea for future use of the other pins which ultimately was never done. trying to tell someone that the two, non used wires in a official dmx cable change something is selling sand to an arab. its bullshit and anyone doing it or telling them they need separate 3 pin cables for lighting and audio is taking advantage of the users ignorance and the trust they put in you. 3 pin/5 pin….dmx/mic cable….ZERO difference as far as functionality goes with the lights

      • So the science says the cables will behave differently, practical measurements using electronic instruments demonstrate that they do behave differently, and engineers have experienced problems when using mixed cables confirming that they do behave differently, but we’re still supposed to believe that there is no difference? On what basis can we reach that conclusion?

  6. I had to go back and reread this again. It read more like a matter of opinion rather than a white paper or even a comparison of XLR vs DMX. No science just take my word for it on faith. Ironic. As for using DMX over CAT5? Actually it is pretty much a good application for the use of CAT5. Twisted Pair offers it’s own shielding and can be used for audio, video as well as Data applications over extremely long distances. Now using a DMX cable or even a mic cable would be better for portable setups, drop and tack, CAT5 would be a more fragile solution in comparison. To be completely honest here, simply stating “We could go on for pages more explaining the science behind DMX and microphone signals”is more of a filler to the rest of the paragraphs. Even worse is glossing over Ohms Law instead detailing how DMX signals are transported voltage differences (eg. +5 volts = 1 and 0 volts = 0) while a good explanation of DMX signal it really doesn’t help explain the differences between the 2 signals. e.g. sound travels from the microphone as much smaller voltage fluctuations that are then amplified in a mixer for recording and sound reinforcement. So what you’re telling the reader is you don’t know or there is no real difference other than the say so?

    I actually learned more by reading the comments and appreciate the input there.

  7. Dmx-512 standard design from rs-485 and update
    Dmx-512 only “0” or”1”
    And work @256k….
    When cable very long….
    The cable’s cap between conductors is a trouble maker!!
    You send “0” but cable have last bit is”1”
    The cable’s electrical from “1” to “0” need time!
    So you send” 01110001” will become ”01111001”

    You should select low capacitance cable,and 110 ohm type “twisted” cable!
    Network cable is a standard” 110 ohm””twisted” cable
    for dmx-512 buliding trunk is a good select!

  8. Thank you guys. i have found my problem thanks to this post.

  9. we use cat5 (stranded) cable exclusively around the stage for DMX and it performs exceptionally well.

  10. DMX isn’t a type of cable or connector at all – it’s a protocol. Technically, any cable with an XLR-style connector is an “XLR cable.” Even 4, 5, or 6-pin. Of course, most people refer to a 5-pin XLR cable as a “DMX cable” since that’s what it’s most often used for. But in actuality, it’s a 5 conductor cable with 5-pin XLR connectors.

  11. THERE IS NO SUCH THING AS “XLR CABLE”.

    I assume you guys are talking about mic cable. XLR is a connector type. Mic cable uses 3-pin XLR connectors. USITT-Compliant DMX cable uses 5-pin XLR connectors. Clear-Coms use 4-pin XLR connectors.

  12. To go back to the original article, some of the science is inaccurate – ohms law is actually much too over simplistic and does not explain the problems of using the wrong cable at all. If we were only concerned about resistance, then a good low resistance mic cable would out-perform a cheap higher resistance DMX cable, since the voltage drop in the lower resistance cable would result in lower voltage drop. The real problem is impedance, which although measured in ohms, is very different from resistance. Impedance is a combination of resistance, capacitance, and inductance. At high frequencies, the reactive components of the impedance are much more critical than the resistance. In short, ohms law does not “define the basic rules for how the signal is sent down any kind of wire” – it doesn’t even come close!

    The second mistake in the science is the reference to zero volts and 5 volts – in DMX, the actual voltage doesn’t matter – it’s the differential between the two wires – so when wire A is at a higher voltage than wire B, that’s a ‘1’, and when A is at a lower voltage than B, it’s a ‘0’. The actual voltage doesn’t matter.

    The problem arises because as the cable gets longer, and there is more “loss”, then the differential becomes less, and the receiving circuits become more sensitive to interference. That interference may be an external air-borne source, or if the DMX circuit is improperly terminated, the reflected signal from the end of the DMX line itself acts as an interference signal.

    In reality then, a long run of low-loss microphone cable, terminated at 110 ohms (if that’s its characteristic impedance), would work every bit as well as “proper” DMX cable. (But you’d need to know the characteristic impedance, and you’d need to properly terminate!)

    On short runs, using mic cable will not create problems, particularly if the same cable is used for everything (using slightly different cables, even if they are nominally 120 ohms, may actually be worse than using a single cable type throughout, even if that single cable is the wrong impedance)). The signalling speed of DMX is relatively low in data transmission terms, and as such, the cable impedance is not critical, which is borne out by the fact that on short runs, you can get away with no termination at all!

    On long runs, termination becomes critical, as does using cable of consistent impedance. Even then, it depends on how the equipment is arranged in the circuit – for example, if a set of lights were only a few meters apart, but located say 500m from the lighting desk, it’s unlikely that using a long run of microphone cable would be any different than using “proper” DMX cable. On the other hand, if the lights were distributed along the 500m length of cable, it would be a lot more likely that the system would have problems.

    So the answer to the question “can microphone cables be used interchangeably with DMX cables”?, the answer is “it depends on the circumstances!”! :)

  13. Jerry Silverthorn January 19, 2015 at 9:25 am

    No I am really confused ! I am an electrical engineer and I do understand the principles here … so why when I cut open some professional DMX cable do I find that it is 5 core screened with no twisted pairs ? So that’s 6 conductors in total ..

    I need a drink.

    • LOL – well it depends….. within the spec, when you cut it open, it might look like a mic cable (2 conductors twisted together with a screen), it might look like STP (2 twisted pairs with an overall screen and drain wire – that’s your fifth connection), other data cables (2 twisted pairs, each with its own foil screen, and a drain wire) – the one thing that’s unlikely is that there will be six wires.
      Is it OK to look like mic cable? Well yes, because I’ve seen mic cables which have capacitance (both conductor to conductor, and conductor to shield) which exceed the requirements of the DMX standards.

  14. This article is really not useful. You used a bunch of words just to say, “You’ll just have to try it to see if it works.”

    • The point is Eustace, when it doesn’t work, at least you’ll know why! :)
      Having said that, the incorrect use of terminology (e.g. “XLR cable” – there’s no such thing!) is likely to mean that you’re just more confused than when you started reading!
      Come to think of it, yours is probably the most comprehensive and accurate comment in the whole thread! :)

  15. Thanks, Brian Adams, for your comments. I found the original article rather infuriating. I wanted information on the differences. Saying one is a hammer and the other is a screw driver is not explanation unless they actually are very much like hammers and screw drivers (which they are not: I am neither hammering nor screwing with these cables).
    I had heard all sorts of false rumours of resisters being used or different wiring to the ends. After opening them up, I see that, just like mic cables, pin 1 is wired to the shield while pins 2 and 3 each are insulated wires. The outer shell of the dmx cables generally feel stiffer but that shouldn’t affect anything electronically. So, what causes there to be a possible slight electronic difference with DMX cables? I’m tempted to get out my DMM and starting to measure resistance and capacitance, etc…
    Do DMX use a thicker gauge of wire possibly? Are the internal wires twisted around each other more? What is the difference? The lack of information is what makes people so suspicious. Having a company that sells DMX cables write an article saying to buy more DMX cables with no solid explanation.
    I had a 6 Chauvet LED slimpars connected by mic cables in a single run with one DMX cable in there… of course it was the DMX cable that caused problems…. which helps verify that there is a difference and you can’t jump back and forth easily between types (especially with Chauvet).

    • Matt – the difference is in the make up of the cables – very slight differences in insulation material, thickness of insulation etc. will affect capacitance and inductance. These two factors combined with resistance and (usually unimportant) conductance, determine the characteristic impedance.
      Characteristic impedance is not the same as resistance, and really is only an important issue at high frequencies (which is why we can be much less fussy about the impedance of mic cables).
      The important thing to remember is that changes in impedance have a much more profound effect on signals than simply having the wrong impedance. Lets say you had 200m of 100 ohm cable – chances are DMX signals would travel down it with no problem. Now lets cut it in the middle, and add in a couple of metres of 120 ohm cable. This “mismatch” will cause signal reflection, which appears as digital noise, and this noise will corrupt the DMX signal.
      You have seen this in action – your single DMX cable was probably not faulty – it was just different from the others, so the mismatch gave you problems. When you used microphone cable consistently, you had no problem.
      In short, if you are going out to buy cables for your lights, buy 120 ohm DMX cables. (Use them as spare mic cables if you want – the mics will not care!) If on the other hand, you have a stack of 100 ohm mic cables, all made using the same cable type, there is every chance these will be perfectly satisfactory in all but the very largest DMX cable runs.

  16. Hi Brian,

    Thanks for all your input! I was just as frustrated with the details of the article and some of the comments in here. I am looking to get 25 LED Pars to string up approximately 10ft. apart from each other and all the light guys I’ve spoken with said to just go with MIC cable, just not the cheaply made ones. Given the costs I’m looking at to get the lights, plus all the additional hardware and cabling required, I could benefit for having extra MIC cable for when I DJ.

  17. Not all lighting technicians are as confused as this. Some have learned enough electronics to know what terms like “impedance” and “bandwidth” actually mean. DMX digital signals are transmitted at 250 Kbits/sec, a bandwidth easily handled by low-impedance microphone cables. What’s more important is how resistant the cable is to microphonics, which are static electricity disturbances caused by internal friction within the cable. To test an XLR cable for microphonics, connect it to a dynamic mic and mixing board, and turn the gain up high enough to hear noise in the channel over headphones. Wrap the mic in a pillow to keep it quiet, and whip the middle of the cable like a jump rope. If you hear crackling, that’s microphonics and you’ll want to ditch that cable for a higher quality brand, regardless of whether you’re using it for audio or DMX signals.

    • “What’s more important is how resistant the cable is to microphonics,” – I think you’ve been badly misinformed. The trobolectric effect commonly referred to as being “microphonic” certainly does have an effect on audio signals – but that’s because typical microphone signals are in millivolts, so a noise signal of a few microvolts may be audible. DMX signals on the other hand, are typically 100 to 1000 times larger than microphone signals, so these miniscule voltages would not even be detected by the DMX electronics. It’s been said numerous times throughout the discussion, but I’ll repeat it again – the most significant difference between microphone cables and DMX cables is impedance, and that is the most significant factor in choosing any cable for data transmission, whether it’s DMX, Ethernet, or anything else. As has already been mentioned, it’s impedance mismatch which is the big problem – where (for example) a 100 ohm cable meets a 120 ohm cable, there is signal reflection – these reflections (unlike microphonic noise) really do have significant voltages, and will affect the ability of the electronics to discriminate between the zero and one voltage levels. There’s actually a good chance that a quality DMX cable WILL be microphonic, because (a) the cables don’t get any significant movement when in use, and (b) the signal levels are insignificant, so there’s no need to engineer the cable to avoid them.

  18. Can anybody tell me how I connect the wires in a cat6 cables to a 3 pins xlr connector. I want to use cat6 cable in a fixed installation in my church.

    • Per – I don’t know why you’d do that to be honest! Cat6 will perform no better than Cat5e, it’s more expensive, and more difficult to work with. In any case, the wiring convention is
      white/orange – Data1+ (Equivalent XLR pin 3)
      orange – Data2- (Equivalent XLR pin 2)
      white/green – Data2+ (Equivalent XLR pin 5)
      green – Data2- (Equivalent XLR pin 4)
      white/brown – Ground for Data pair 1 (Equivalent XLR pin 1)
      brown – Ground for Data pair 2 ( (Equivalent XLR pin 1)
      Blue pair not used.

  19. can you run DMX through an audio mixer and even through a wireless audio device (in ear monitors)? my IEM have a 32ohm out that i thought maybe could be made to 110 with attenuation.

    • Caleb, I really don’t want to cause offence here, but to be honest, I’m trying to figure out whether this was intended as a joke, or you really haven’t read much of the thread! DMX is a lighting protocol – even if it were possible to run it through an audio mixer (and it absolutely is not!), why on earth would you want to do this? I can only think that you are looking for a wireless DMX system but even if you could use your IEM transmitter (and no, you can’t!!), you’d connect directly rather than go through a mixer. If you want wireless DMX, there are quite a few manufacturers who supply DMX transmitters and receivers – any I have used have been 100% reliable, even in environments where there is a lot of other “radio” transmission in use. If you want to know more about wireless DMX, here is a useful intro – http://shop.bmisupply.com/docs/vendor-documents/city-theatrical—what-you-need-to-know-about-wireless-dmx-.pdf?sfvrsn=0

      • thanks for your answer Brian. I guess I’d like to know why it doesn’t work (though I didn’t ask that originally.)

        I have read all of this thread and actually read the link you posted before finding this discussion.

        I’m curios what the wireless DMX systems (which you can get 1 transmitter and and 1 receiver for only $40 on Amazon) do to the single that gets lost through a mixer. is DMX a frequency that audio devices can’t handle because they are too high or too low?
        don’t they need to be transformed into a digital signal to be sent wirelessly and them back to analog for the lights?

        (there’s no such thing as a stupid question!)

  20. Caleb – my apologies – sometimes when you’ve been doing something for years you presume “everyone knows that” – but we’ve all had to learn somewhere! You are on the right lines with the frequency being the issue, but it’s not quite that simple. The DMX signal is transmitted at 250 kbps – that is way higher than audio frequencies – in fact a good mixer input circuit will deliberately cut out frequencies this high (by using a low pass filter) as there is never anything useful for audio this high. In the digital world, typically the highest sampling frequency used for digital audio is 96kHz (i.e. 96,000 samples per second – usually 16 bit or 24 bit samples), but the sample rate is this high so that it is well outside of the audio range, and so that the highest useful frequencies can be faithfully reproduced. In the analogue world, the generally stated “ballpark” range for hearing is 20Hz to 20kHz (though there is no real “audio” as low as 20kHz, and although some may dispute it, most people’s hearing range goes nowhere near 20kHz).
    Audio mixers are therefore designed to operate on this bandwidth, and anything significantly higher is considered to be “noise” and is deliberately filtered out. It is vitally important that such filtering occurs, otherwise inaudible high frequency signals could be sent to amplifiers, which without filtering would feed them to speaker transducers which although they can’t reproduce high frequencies CAN be damaged by high signal levels above audio frequencies.
    So we have established that high frequencies in audio equipment are undesirable and deliberately removed (so your mixer will not pass DMX). But what about the wireless equipment?
    In an audio wireless system, the audio signal is typically compressed, and used to modulate a radio signal, which in the receiver is demodulated and decompressed (expanded) to restore the original signal. In high quality systems, the signal may be digitised, and then converted back to analogue, but this is only a method of reducing the effect of interference in the RF signal.
    In DMX however, it’s digital all the way the signal going into the transmitter from the lighting desk is digital, so there is no need for expensive and complex treatment of the signal going in. at the receiver end, again the output is digital, so the electronics is relatively cheap, and is not complex.
    For the lights, no, the signal is not transformed to analogue – the control circuits within the lights are totally digital. Dimming, colour control etc are all performed in the digital domain – but that’s another story!

  21. XLR is a connector type. DMX is a a communication protocol.
    Please research your articles before you release them. Cables Audio vs Lighting would have been a more accurate title.

    • Dan – your a bit late to the party here! That’s been pointed out two years ago! To be fair to the original author, it’s a common mistake – many people refer to microphone cables as XLR cables. (And as has already been noted, XLR’s are used for a lot more than microphones and DMX signaling!). If you want to be pedantic, “Cables Audio vs Lighting” isn’t a great description either – that could be a discussion about speaker cables compared to Socapex cables! :)

  22. Everyone knows the difference between hi impedance and lo impedance mics. If the cable has 25 picofarads per ft of capacitance, and the low pass filter frequency is defines a f=1/twopi*RC, and you calc the c of the cable for as many feet as you have, and use 20,000 as the R, you get the freq you can send down the cable. Should be 20,000 Hz for audio. That’s why mics use low impedance (150 to 600 ohms). Long cable runs. I guess the DMX cables obey the same laws of physics. freq should be 250K, R should be 120 ohms, and the total C for the run can be calculated. The operative concept is ‘low capactance’ and ‘low pf per ft’. Hope this helps.

    • Bob – it’s actually quite a bit more complex than that unfortunately! With high impedance mics, the source resistance (the mic itself) combines with the cable capacitance to form a low pass filter, resulting in loss of high frequencies, which gets worse with distance. With low impedance, this is not such a problem. DMX drivers, like low impedance mics, have low source resistance, so capacitance is not such a big deal. (The other reason for favouring low impedance mics is that mixers and amps have correspondingly lower input impedances, which (compared to Hi Z) reduce noise sensitivity). In any case, at high frequencies, cables need to be treated as transmission lines where resistance, capacitance and inductance all combine to give a constant characteristic impedance.

      The real enemy when transmitting digital signals (or high frequency analogue), is CHANGE in impedance. Lets say you have loads of 120 cables for your lighting – you accidentally introduce 100 ohm cable half way down the line. That’s two mismatches, and at both joints, where the impedance changes, there will be loss in the signal. That effect is likely to be worse than if you wired the entire system in 100 ohm cable, because low impedance line drivers will barely notice the difference, so the only place you have a problem is at the end-of-line terminator.

      250kbps is not the same as 250 kHz, and impedance is not the same as resistance, so the simple RC filter equations don’t really help in this situation.

  23. If DMX is 5 conductors why do all the lights come with 3 pin for in and out type plugs. Manufacturers need to get their act together. The other thing that grinds my gears is the speaker cable issue. Bought Yamaha 15″ speakers a few months ago and cane with jack sockets only. Some speakers have jack some have speakons and some have both. I wish they would standardise. I like the fact that speakons lock now you come to powered speakers and we are back to sleep. Grrrrrr am i the only one who thinks like this. I am still getting my head around lighting as fancy using DMX and a controller rather than the on board auto or dip switches.

    • John, I think the answer to your 5 pin / 3 pin question is probably already covered! I suspect asking manufacturers to “get their act together” is a bit of wishful thinking – they will do whatever they think sells most product! To be fair, some of them have realised the problem this has created, and fit both 3 and 5 pin sockets, so you have the best of both worlds.
      Speakers and connectors is a bit off topic, and you might be better starting a new debate about that one! To be honest, jacks on speakers nowadays is a fair indication that they are aimed at the “budget” market – even sub $100 pairs are available now with Speakons.

  24. So Brian, at the risk of provoking a “deeper than my small brain can handle” response (I appreciate your knowledge!), why would mixing Cat 5e (110 ohm) with true 120ohm cable be more “acceptable” than mixing 110 ohm single twisted pair wire with 120 ohm cable? (I’m rewiring our church lights with mostly Belden 1420A cable, but purchased some (cost driven) accu-cable 3-pin conductor dmx cable (110 ohm) for building patch cables (no more than 25′) between hanging fixtures). You’re saying this is NOT a good idea?

    • Terry – sorry if I’ve caused any confusion with earlier comments – let me try and clarify the problem. The “big issue” is impedance. Many people regard impedance and resistance as being the same thing, and for some calculations, that’s fine, but in practice, there’s often a significant difference. Take your Belden cable as an example – according to the spec sheet – http://www.belden.com/techdatas/metric/1420A.pdf?ip=false it is 100 ohm cable – that’s it’s impedance. It’s resistance on the other hand, is 78 ohms per kilometre.
      If you have 100 metres of this cable, the impedance is 100 ohms, and it’s resistance is 7.8 ohms. If you have 10 metres, the impedance is 100 ohms, and the resistance is 0.78 ohms. In other words, resistance changes with length, and impedance does not.

      If you have an electronic circuit designed to use 100 ohm cable, and you connect it with this Belden cable to a receiver with a load of 100 ohms (which might be purely resistive – just so long as it’s 100 ohms!), you have a perfect transmission line – the length is completely irrelevant. (Length affects attenuation, which is a reduction in signal level, but it does not impact on signal quality, which is what we’re concerned about).

      Now suppose somewhere in the middle of our long 100 ohm cable, we introduce a section of say 120 ohm cable. We now have two impedance mismatches, and it’s these mismatches which do the damage. When our electrical signal hits these mismatches, some of the signal literally bounces back down the cable. This has two effects – it reduces the level of our “good” signal, but also has now introduced a spurious “noise” signal travelling in the wrong direction. Every impedance mismatch causes more of this “noise”, and once you have enough of it, it mixes with the good signals, corrupting them to the point where the electronics can no longer differentiate between “signal” and “noise”.

      So to go back to my original point – suppose you stick to 100 ohm cable, even though the spec calls for 120 ohm? Well there will be some signal loss at the “launch” point where the signal is applied to the cable, but no noise introduced. As you’ve consistently used 100 ohm, there are no mismatches, so again, no noise introduced as your signal travels down the cable. DMX devices on the line should all be high impedance, except for the very last ones at the cable ends, which should be terminated at 120 ohms. Mid-chain devices have no affect, but when the signal reaches the end termination points, there IS a mismatch, so some signal will be reflected back into the cable, but you’ve only got one reflection point at each end of the line.

      Now suppose you’d done what I suggested is a bad idea – used a few 120 ohm cables in the middle of the chain, and let’s make it really bad – they are not all in the one place, but are interspersed with 100 ohm cables. They are not “good” cables because they are 120 ohm – they are impedance mismatches, each of which produces two reflections, causing “noise”.
      Now let’s look at your specific scenario. If you use the Belden cable to go from a DMX splitter out to where the lights are, and then you use your accu-cable to do the final drop, you’ll have introduced one impedance mismatch. Will it work? Probably. It doesn’t really matter what length the accu-cable is – the important thing is not to keep changing impedance!

      On the other hand, if you went Belden from your desk to your first truss, then ran a few accu-cables, then went back into the installed wiring on Belden to get to another truss, then back to accu-cable, etc., this would be a REALLY bad design, and is asking for trouble!

      In summary – a big long transmission line which is consistent in impedance (even if it is the WRONG impedance), is preferable to a short transmission line with multiple impedance changes, (even if most of this line is the CORRECT impedance). If you MUST have an impedance mismatch, it’s best for it to be at the end termination point, rather than having multiple mismatches along the cable.

      Finally, accu-cable is being sold as “DMX” but with an impedance of 110 ohms, even though the spec calls for 120 ohm. I suspect the only difference between their “DMX” cable and their “microphone” cable is the label!! :)

  25. Thanks for the fast response Brian, you’ve helped in making my decision to stay with the 1420A (I’m thinking about making a 100 ohm terminator instead of using the store bought 120 ohm terminator) until we can afford to replace the entire room with “true” DMX. The downside will be any further expansion, we will have to take in consideration the existing “poor choice” cabling. We are currently using 14 moving head fixtures on one universe and approx. 750 feet of cabling (most already the Belden cable…with a mixture of ethernet/true DMX cables tossed in!!) Currently, it’s too cost/labor prohibitive to replace it all now. With all that said, I haven’t had any issues until I added a 100 foot cat 5e ethernet cable to the end of my last fixture to expand to some portable leprecon dimmer packs (very strange results!). Once I replace that cable with DMX everything “appears” to be fine. Thanks again for your input.

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