using two single screened cables for a balanced signal

[adam5365]

We have an amount of single core screened cable stored at work, and I am just about to install some balanced line inlets/outlets around the stage area.

Would I be able to use two runs of single core screened cable running together for each balanced signal or will the screen interfere with the canceling effect of the hot and cold side of the signal?

 

 

Adam

[paulears]

It's a bodge, but does in fact work fine. Balanced lines in the old days were bog standard telephone lines - no screen at all!

[bruce]

If you do this, make sure both cables follow exactly the same route. Splitting them - or (worse) running another cable between them - will cause problems.

 

My guess is that you'd probably get away with it for line-level signals, but not for mic-level.

 

Personally, I'd use "proper" cable - the cost of the cable is generally a relatively small part of the total cost.

[Shez]

Would I be able to use two runs of single core screened cable running together for each balanced signal

 

You'll get very little interference rejection that way. Balanced lines rely on the fact that the signal cores are twisted together so that any induced interference is induced in both cores equally and can then be rejected by the receiving circuitry. Using two separate wires will mean that either wire could pick up interference that the other one doesn't, which completely negates all the advantages of a balanced system. The screen is actually superfluous in a properly balanced system - unshielded CAT5 is fine for audio.

 

Admittedly I've never tried using two single cables with balanced signals so can't speak from experience, but the theory is solid... B-)

[Bobbsy]

Like Shez I've never tried this in real life but I think his analysis is right. The whole point of the common mode rejection that balanced wiring provides is that any interference affecting the run will be EXACTLY the same in both signal cores so, when one signal is inverted and mixed with the other, half the interference will then be anti-phase and cancel itself out. As Bruce says, if you manage to keep the two cables exactly adjacent to each other it might work "sort of" but I doubt it would be as effective as a single cable.

 

Bob

[Jivemaster]

For audio at line level it will send signal but the noise cancelling that balanced line is used for relies on the signal cores being close and twisted together so your config will not be as noisless as it could/should be. For and install the cost of the correct cable shouldnt be a factor as you will repeatedly curse the lesser cable,

 

For higher frequencies (DMX et al) only proper balanced cable works.

[Steve1812]

Hi,

 

I write as an enthusiatic amateur at a school in Lancashire.

 

I run 50 metres of permanent install cable from the stage to the sound desk.

The balanced cable is 2 core side by side with the screen/ground around the outside.

Noise is zero - including when recording kids to Cubase.

 

Question - does it matter if the 2 signal lines are not twisted - my experience says no.

 

Steve

[paulears]

The screening is only really effective at RF frequencies - additional immunity from external noise sources is provided by the twisting. This is why CAT5 cable works well.

 

 

edit

 

Jim Brown the acknowledged expert on all things audio said this, back in 2001 - when the theater-sound list discussed the topic.

 

In general, twisting is much more important than shielding for minimizing pickup of hum and buzz. Shielding starts getting important at radio frequencies, and you have to get into braid shields to get even halfway decent capacitance balance. Braid-shielded mic cable is well known to have much better rf rejection than foil/drain shielded cable.

 

Note also that CAT5 and CAT6 cables are not shielded at all. Although I haven't done it, I would feel quite comfortable running line level audio around on CAT5 or CAT6. BTW, CAT6 is simply a tighter spec version of CAT5.

 

The more twists per inch the better, the more uniform the twists the better. The smaller the loop area (between the two conductors) the better.

If anyone wishes to read more - here's a link to their archive.

http://brooklyn.com/theatre-sound/archive/...-2001/0160.html

[KevinE]

I agree with Shez, in fact many technical books are very particular about not using seperately screened wires. It should ONLY be a twisted pair with one OVERALL screen. The same goes for DMX.

 

Balanced cable isn't expensive, I think your method will be false economy.

 

The only thing about CAT5 is that it's meant for a digital signal surely?

[adam5365]

Thanks guys

I was afraid that it would be a bad idea. I just forgot to order some when I made up my order and was hoping to do the job on tuesday.

Guess I'll have to order some cable on monday and make a start on tuesday by do the mounting the boxes on the wall bit.

[Shez]

The only thing about CAT5 is that it's meant for a digital signal surely?

 

Can I ask where that assumption came from?

(I'm not trying to be funny or anything; I'm genuinely interested in how such a conclusion would be reached)

[KevinE]

Just because it was designed for Telephone and Cabled Data. (C.and T.) ie CAT. I dont think it was specifically designed for low-level balanced mic or hi-fi audio, though many people of course use it for that. Pro audio & DMX usually stipulates an overall screen.

[J Pearce]

Use STP (Shielded Twisted Pair) CAT5 if you're really worried about the shield.

[Shez]

I dont think it was specifically designed for low-level balanced mic or hi-fi audio

 

It's amazing what you can actually send over CAT5 - various type of analogue & digital audio & video are possible. There was a great presentation hosted by Canford a few months back where they went over all of this. There was a video of it online; not sure if it's still accessible though. Really interesting stuff.

[mervaka]

cat5 is fine for it, four twisted pairs of high quality copper!