The value of the capacitor determines how quickly the capacitor will charge and how fast the voltage across it will rise to match the applied voltage. The higher the capacitance, the slower the voltage will rise.

When you apply a data signal like DMX to a cable, the source sees a load that looks like a resistor in series and a capacitor across the output. The value of the capacitance depends on the construction of the cable.

There are all sorts of factors that influence the characteristic capacitance of the cable, such as the type and size of the insulation and the type of shield it has.

A DMX signal is a series of square waves, or simply a voltage quickly switching on and off across the cable. If the cable has a high characteristic capacitance it resists the change in voltage and badly distorts the square wave.

It makes it look more like a sawtooth wave than a square wave. Low capacitance cable, on the other hand, helps maintain the integrity of high frequency signals.

At first glance microphone cable looks just like data transmission cable. It comes in twisted pairs and has a nicely braided shield. But if you examine the specs you’ll see that it has a relatively high value of capacitance.

It makes little difference to an audio signal because the frequency of the audible range is far below that of the DMX baud rate. Low frequencies are relatively unaffected by high capacitance. The voltage isn’t changing as quickly, so the capacitor doesn’t resist the change as much. Audio signals don’t care much about capacitance. DMX signals live and die by capacitance.

Data cable has a low capacitance and is made specifically for data transmission. It might cost more than low impedence cable and it’s not as readily available. You might have to source it somewhere other than the Radio Shack down the street.

In “Recommended Practice for DMX512 – A Guide For Users and Installers” by Adam Bennette (published by PLASA and USITT) there are a variety of recommended cables made by Belden, Alpha Wire, and Proplex (distributed by TMB Associates, AC Lighting, and Creative Stage Lighting) that also distribute excellent quality data cable under the trade names Tourplex and Duraflex, respectively.

Buy data cable for data transmission. All it takes is time and money. But think about how much time and money you save when you avoid problems with inferior data cables. I’ve seen countless “mysterious” problems that were caused by nothing more than high capacitance cable. The frustrating thing is that you can’t see, hear, taste, touch or feel the problem.

After all, the cable looks like it is working. It tests okay with a continuity checker. It provides a good solid connection and there are no cold solder joints. Yet it will make automated lights wig out.

The cable will probably even work okay if the system is small with short runs and few lights. But when you start adding fixtures to the system, they misbehave. They act strangely, moving for no reason, doing things they shouldn’t do.

The only way you can see the problem is to use an oscilloscope to look at the distortion in the signal. Or you could avoid the problem altogether and buy the proper data cable. All it takes is time and money.

There’s enough stray capacitance in this world without inviting the more domesticated variety to live in your data system. Buy and use data cable that is designed to work correctly with DMX data and you won’t have to explain your bad results.

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