Thursday, September 6, 2012

Cables and Interference

We got another email from our friend Michael on the subject of cables. We wound up discussing it a bit at the podcast that will air this weekend (9/9/12) and thought a written article might be of use as well. 

While reading your "Cables on stage" post, I remembered a few cable-questions I've had for a while now:

1) I read somewhere that some audio cables (very dim memory says "speaker cables") should not be kept in nice/tight loops/coils with signals running through them because of interference/signal loss.

2) What types of cables (if any) need to be kept apart/kept away from power cords to avoid interference?

3) Do certain types of cables react to things like refrigerators, generators, lighting, etc.?

As you know, there's a lot of mythology about this stuff out there, especially among guitarists ;)

While we're on cables: there's a lot of debate on the web about proper shielding, esp. in instrument cables, but also on XLRs. A word on that from the pros may be interesting imho. May save certain people a lot of money (or maybe not).

Cheers
Well for starters, a coil of cable can act as a choke which is a device that will frequency limit the signal. Fortunately for audio people the frequencies involved and the size of the coils tend to make this less of an issue. You can actually put this phenomenon to good use sometimes. A noisy channel can sometimes be helped by coiling the cable tightly around a piece of metal like a steel table leg. 
 
Of course in a well designed system that relies as much as possible on balanced connections, noise getting in is less of a problem. Take a look back on this blog or do a search for balanced audio and it should start to make sense.
 
Shielding is important for any kind of cable but especially for unbalanced connections like instrument cables. Wires of any kind will act as antennas. Having a good shield, either a braided ground wire or in some cases a layer of metal foil as well will take any noise being captured by that "antenna" and shunt it safely to ground. 
 
Any cable will be picking up noise from electric motors, vehicles, radio stations, cell phones and anything else you can think of. Balanced cables of any design and expense will do better at keeping this noise out of your system. On that count it really comes down to the quality of the insulation, strain relief and connectors when you're buying cables. There are a few situations where more expensive cables are preferred, but it's just as likely to be for their ability to hold up under heavy use than anything to do with the way they sound. In a case like the pristine realms of mastering studios you might be able to hear a difference between a $30 Whirlwind mic cable and a $100 hand made Mogami.  In the local club or high school theatre it's not likely to be apparent.
 
With instrument cables the connections tend to be of the unbalanced variety and here's where it's worth shelling out a little more for well shielded ones. Keeping the lengths to the absolute minimum will help too. The shorter the "antenna" the less noise will be picked up.
 
It's also worth noting that how you plug your system in can have a lot more to do with hum that many people realize. If you amps, instruments and PA are all plugged into outlets on the stage and your mixer is plugged in way out in the house on a different circuit, if those outlets aren't all on the same leg of the electrical service you're going to get some hum. That's why it's common practice to run power out to front of house along with the snake. (An example of audio running side by side with power and generally having no ill effects because the lines are all balanced.)
 
The last thing I'll cover is speaker cables. These are less likely to have issues because of the magnitude of the signal traveling on them. A mic level signal of a few millivolts is much more easily influenced by some stray RF than the output of an amp feeding subwoofers. If there's hum it's more than likely coming from somewhere earlier in the signal chain.
 
That's it for now Brethren of the Knob and Fader. This could easily turn into several more posts, each dealing with one small element discussed above. Keep your eyes peeled.

3 comments:

  1. In high RF areas whether wireless mics or a plethora of nearby radio/tv stations I would always recommend braided sheild quad mic cables (Where 2 helixed lines are connected to your positive and negative XLR terminals), especially if recording the content. I remember years ago being able to pic up a local AM radio station with some RCA cables connected to an AMP. While most of the time that might be overkill, when it comes down to the one take recording having to be the final take don't take the chance. As Jon said also make sure as much as you can that all devices be on the same leg of power or you're gonna have something humming some ugly tune somewhere. There's a whole engineering section on this called electromagnetics if you feel like making your head explode.

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  2. Great comment Audio Analyzer! I was going to mention the quad conductor cable as well. Here is a link to a manufacture's data sheet that has some interesting noise comparison plots.

    http://www.canare.com/UploadedDocuments/Cat11_p35.pdf

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  3. Ham radio guys are always talking about using that method for balanced antenna feed lines. I completely forgot about it in the audio realm.

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