http://www.dfd.com/whyterm.html
This article is a good start, although it deals mostly with termination. (another bag of worms I suspect)
This article is a good start, although it deals mostly with termination. (another bag of worms I suspect)
The Ongoing Debate of Mic Cable for DMX
- Thread starterbishopthomas
- Start date
Well, actually a number of people do know why. Quite simply, it's capacitance. The more capacitive your cable path, the more slewing in the transitions between low and high bits. If there is enough slewing, the receiver chip now sees the transition time at a different time and then the data is no longer recognizable as DMX by the UART.
If you have high quality audio cable, the capacitance will be less. Low quality and the capacitance is typically more. Since audio frequencies are a lot lower than DMX and almost never have the sharp transitions in signal that DMX has, the higher level of capacitance (as compared to DMX cable) does not affect the audio signal. I do have scope shots but unfortunately they are at work and I'm not.
I can't begin to tell you how many times I've had people call in with control issues to find out they used mic cable. Every once in a while they say they checked all out in the shop before they brought it out on site. I then ask if the shop mock up used all the cable being used in the rig and the answer is always no.
The longer the mic cable run with DMX in it, the greater the possibility of loss of control.
David
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bishopthomas
Well-Known Member
You're right, I misspoke. There are certainly people who do, in fact, know the why behind it all. It's just that that is never presented in the DON'T USE MIC CABLE! threads that I have seen. It's never questioned, just accepted as fact.
How do you feel about quad mic cable used for DMX? The capacitance is probably still too high, but is it approaching tolerable levels? And if so what is the relationship between DMX data cable, single twisted pair audio, and quad core audio cable?
I'll be honest, as a long time proponent of using quality cable, I have never even researched quad mic cable. So, to give it a quick once over, I looked up the specs from Canare here: Canare Corp.: Star Quad Series: Star Quad Microphone cable(L-4E6S / L-4E5C). The typical DMX cable spec for capacitance is around 13pF/ft and the Canare quad is four times that. In addition, it has quite a low nominal impedance.
There are a couple of factors that affect distance which include capacitance, cable impedance and the number of unit loads on the data chain. In an ideal world DMX can reach 1600'. I would guess that quad cable might get you 300' or so, but that is a complete guess. Cable runs are only going to get longer with the prevalence of LED fixtures and propensity to movers.
The more important question is: what is the loss of a show or the hours troubleshooting worth to you and does it cost anywhere near what a set of good quality DMX cables cost.
Using good quality DMX cable always works. Using mic cable might work. Beyond that, I leave it to you.
Oh funny, I just looked at the spec sheet I posted above and at the very bottom there is an interesting frequency chart for 100m (328ft). Yes, it shows a fall off to very low levels at frequencies just under the 250kHz mark. Now you can't exactly relate DMX bit rate (4uS) to a regular frequency, but no doubt the quad cable is not efficient at transmitting effective DMX data.
bishopthomas
Well-Known Member
Thanks for your insight, David, very good stuff.
I can assure that our Engineers understand transmission line theory very well - George has been working in the industry for over 30 years and was responsible for the electronics in many well known lighting products, including one of the Hog front panels and the Icon control desk - so he knows a bit electronics and DMX. We will use parts and techniques according to the particular product we are making - therefore when designing our professional range of consoles and DMX interfaces we would use more expensive parts then when we design a low cost device.
We sell the MagicDMX at £10 ($16) - that is how much it costs us to make it - we don't make a profit on it. The idea was to make a DMX device that was very low cost so that young people, students and people new to the industry could start off without spending much money. Even $16 is a lot of money in many parts of the world. MagicQ software is free to use and unlike our competitors all the universes are unlocked and can be used to output ArtNet/Pathport or ACN streaming DMX without purchasing anything.
By using standard 3 pin XLR cable pre-made we cut several $ off the price thus making it an even better bargain. 3 pin XLRs are much more common than 5pin XLR due to their usage in the audio and other industries.
It is interesting to note that the MagicDMX has been one of the most succesful products we have ever made in terms of reliability and customer feedback - we get very few complaints - it just works. Sometimes the simplest things in life are the best!
Chris Kennedy
Software Director
ChamSys
Actually it's far worse than that. Audio caps out around 20kHz sinusoidal waveform however DMX512 is 250kHz square wave (worst case) which means if you want a decent approximation of a square wave at the other end of your cable you'll be needing some higher order harmonics, for arguments sake up to 10x 250kHz. High capacitance cable needs more current from the RS485 driver and they simply current limit and round off the square wave to the point where on a long cable run it's pure hope the near sinewave DMX512 signal you've created will be squared up within the bit sampling tolerance of the receiving UART.
Worse still, and probably a good source of many failures, audio cables typically have the shield connected to both pin 1 and the connector case with can nullify your DMX isolator/splitter depending on it's design.
I'll bet that most know that dmx cable is better for running dmx applications. And yet there have been times when, for whatever reason, we have been forced to use something else. Sometimes it was fine, and sometimes it wasn't.
I can recall one episode where we DID use all dmx cable and still had an issue with one type of fixture. It took hours to rectify and to this day I still don't know why it happened. But if at all possible, I'm using dmx cable.
As for termination, I don't do that too often (actually never) and it hasn't been an issue. But as I am fond of saying: whenever someone says "we've never had a problem" my standard response is "there's always a first time."
I can recall one episode where we DID use all dmx cable and still had an issue with one type of fixture. It took hours to rectify and to this day I still don't know why it happened. But if at all possible, I'm using dmx cable.
As for termination, I don't do that too often (actually never) and it hasn't been an issue. But as I am fond of saying: whenever someone says "we've never had a problem" my standard response is "there's always a first time."
I agree.
I have had a terminator cause a chain of dmx fixtures fail.
However, the problem was not the terminator itself. There was a bad cable in the system that had pin 3 (IIRC) disconnected.
DMX will often work with one side disconnected, but in this case, the terminator caused the signal to drop too far for the fixtures to read.
I have had a terminator cause a chain of dmx fixtures fail.
However, the problem was not the terminator itself. There was a bad cable in the system that had pin 3 (IIRC) disconnected.
DMX will often work with one side disconnected, but in this case, the terminator caused the signal to drop too far for the fixtures to read.
I have had the same type of failure using a terminator due to a bad cable (once it was a cold solder joint inside a fixture that caused the same problem. I learned once on a show dmx cable cannot be substituted for mic cable. It creates all sorts of weird buzzes. I have also seen companies sell dmx cable but when looking at the cable itself it is actually mic cable. Personally I wish 3pin dmx never existed. I have had to deal with lots of strange cable failures in clubs due to mic cable being used but I have not sat down and done any tests in the shop with a scope. It's on the thing to do list.
bishopthomas
Well-Known Member
bishopthomas
Well-Known Member
On the flip side, however, DMX cable works just fine for microphones. I'm doing an audio gig on Sunday and will probably be using all DMX cable, only because it's more convenient at the moment. What is the difference in construction between the two varieties? The DMX cable I have just seems like twisted pair mic cable with a braided shield. I know there's something in there causing all that data goodness, but can't tell from the stripped 1/2" on the end.
It's an interesting argument where EMC comes into play. Without pin 1 to shield you can't properly protect the data/audio pair from interference (ESD, RF, direct EM coupling...) since there's a discontinuity in the shielding. The E1.11 standard should address this but it doesn't at the moment however it is given consideration in audio standards AES Standard » AES48-2005 (r2010): AES standard on interconnections - Grounding and EMC practices - Shields of connectors in audio equipment containing active circuitry
The EMC vs Electrical Safety vs Practicality debate will go on forever. Personally I feel the addition of a small capacitor between pin 1 and connector chassis is probably a good compromise providing RF coupling without DC/LF problems.
Think about a room with a really bad echo, that's what happens to your DMX512 signal when there's no termination. Signals hit the "open" end and reflect back again then mix with wanted signals potentially causing data corruption. OK with slow data rates like DMX512 this effect will only be an issue with long cable runs I'd never run a system without line termination.
Well, the insulation on the individual conductors will be both a different material, and in the case of the DMX cable, much thinner. Most of the time the DMX cable is also a smaller guage wire. It is the combination of thinner and different insulation, tighter twists per inch and smaller guage that defines the cable's capacitance and impedance. It may be hard to see the difference but it is indeed there. Don't forget that capacitance is defined as being two conducting plates [or conductors] seperated by a dielectric [insulation].
For comparison, look at the construction of a cable, CAT5E, which is designed for even higher data rates. The conductors are very small, tightly twisted and with a very thin insulation. What they couldn't solve in cable construction is handled by legally limiting usable distance to 100m.
Make sense?
David
I want to take a second and right some things that were said in this thread. You can argue all you want about Mic Cable vs DMX Cable, whatever. However, no one should judge ChamSys as a company based on the build quality of a $16 product which is clearly not intended for any professional use. Their product page clearly says:
Further more the product FAQ adds:
This product is somewhere between a toy and a test tool, and is clearly not inteded to be relied upon in any way by professionals in a show situation. Therefore, complaining about build quality is extremely unfair to ChamSys. Anyone who's been around CB long knows how generous ChamSys is about giving away their software for free and the quality of their consoles has never been questioned in these forums. The fact that they give these dongles away at cost in part to help young technicians learn, makes this whole thing even worse. Sorry ChamSys, you deserve better.
True. Except that I typically don't have "long" cable runs. Event lighting is a different ballpark than, say, concerts or theater. It's very unusual for someone in my segment to make a cable run between fixtures of more than 50'. Most of the time I have 15' - 25' between the controller and the first fixture, if that. My total cable on most jobs, even with 36 - 48 fixtures, is less than 750' total. Stage work is obviously different. You may have 300 - 500' of run before you get to the first fixture.
We had a show in January where we had mics farther from the console than we had mic cables. We also have made several cables from cat5e (3 pin xlr connectors) which we generally use for DMX, but I used them for the mics this time. Granted, it may work until it doesn't, but we had no issues with phantom powering mics on this cable.
