Originally featured on BroadcastEngineering.com
12/4/2009 10:41 AM
By Steve Lampen, Multimedia Technology Manager, Belden (www.belden.com)
Many people don’t know that you can send audio, real professional balanced-line audio, down category cables. In fact, not only is this possible, but those pairs make the greatest audio cable ever!
How can this be true? It’s really quite simple. In standard shielded twisted pairs, most of the noise rejection comes from the fact that the pair is twisted. Rejection of noise by the twisted pair is called CMRR, Common Mode Rejection Ratio. And, since the pairs in category cable are extremely precise, they have the best CMRR ever.
So why is there a shield on audio cable? Well, a foil shield is for RF protection. Foil shields don’t begin to be effective until 10 MHz or so. Braid shields go down to 1,000 Hz. Below 1,000 Hz, there is NO shield that has any effect on noise or interference. Then you’re entirely dependent on the twisted pairs, and CMRR, to get rid of noise.
In addition, the best category cables have Bonded-Pairs, where the two wires are affixed together along their longitudinal axis. Then, no matter how you bend the cables, the two wires stay right next to each other, maintaining consistent concentricity to give you the world’s best CMRR. It’s almost as if foil or braid shields are a Band-Aid on a poorly constructed twisted pair to help it resist noise and interference. Instead, Bonded-Pair construction fixes the problem: the pair itself. Then the limit of noise reduction on that pair is the balance (CMRR) of the source and destination devices.
The only times that a category pair won’t help you is if you need a ground wire to deliver something besides just shielding, such as phantom power in microphones, or some intercom systems that use the shield as the return path for the audio. Then unshielded twisted pairs (UTP) won’t work. But if all you have is audio, then you only need two connections (pins 2 and 3 in an XLR) for the signal. Put in a pair of Category 5e or 6 or 6A wires and you’re good to go!