There was discussion about “balanced cables” on the IRC. So I thought to explain what it exactly means to those who haven’t had the time or interest to check it out themselves.
The idea is pretty simple. Balanced audio cables have two wires to transfer the signal. In one wire the signal is in inverse phase. Basically the waveform is turned upside down. 
Now, when some external interference enters the cable it modifies the signal on both wires the same way.
After the signal has ended up in the receiving device, the second, inverse, signal gets inversed again, and it’s mixed together. What happens now? Both wires have exactly the same signal. BUT, the interference was same in the wire, so the interference itself gets inversed, and it cancels itself out on mix. Check out the sketch I drew for you:
I also made XRNS (2.0) demo for you how it works:
http://sva.ee/phaseNoiseCancel.xrns
How I made it?
- Load a clip of music and make two copies of it.
- Reverse the phase of one copy (Inverse in Audacity)
- Mix in the same sample of pink noise on both samples
- Reverse the phase of the second copy again.
- Now the music is in same phase, but the noise in second sample is in reverse phase.
- Play both of those samples back together: Clear music!
What is balanced audio good for?
To transmit audio data on long cables, you are likely to get some interference in the cable. Balanced audio cancels this out, so for example on concerts, the connection between the stage -> sound technician -> PA doesn’t get excessive amounts of noise in it.
Is there such thing as balanced audio cable?
No! If someone tries to sell you one for extended price. It’s not going to help you! “Balanced audio cable” is just a shielded audio cable with two wires inside for signal. XLR is designed for stuff like that. But you can actually use any cable which has shielding and two wires inside close enough together.
What about TRS cables?
TRS is a type of connector. It has nothing to do with balanced or unbalanced audio. Cables with TRS connectors are mostly used for transmitting stereo signal. Yes the 3.5mm jack on your headphones that goes into your MP3 player is TRS connector. So is the bigger 6.3mm one. You can transfer balanced audio over those cables. Some devices have balanced 6.3mm TRS sockets. It’s recommended that you get a cable which has twisted pair inside and one shielding for those connections. Not the common flat cables with two cables welded together, which is most commonly used on stereo connectors.
What should I consider when purchasing “balanced cables”?
First of all. Make sure that the devices you are going to connect actually support balanced audio. As explained, the “magic” is not inside the cables, but inside the devices. If both devices have XLR input/output you can pretty safely assume that you can make balanced connection between them using XLR cable.
If there are TRS plugs on both devices that have writing “Balanced” on them, and you connect them like that, you are also taking the advantage of balanced audio connection. But be aware. When connecting balanced audio, it takes one TRS cable to transfer one mono channel. You need TRS cable not TS cable. TRS has tip-ring-sleeve configuration (three wires), while TS cable has two wires. (Sometimes referred to as, stereo and mono jack).
If you are making a connection between your sound card and speakers using one stereo TRS cable and you are going to buy “Balanced TRS Cable” from shop for extra money, you are not making any benefit at all. The connection IS NOT balanced. It uses the two wires to transfer different stereo channels, but not the reversed phase signal, thus you just wasted your money.
Any questions?