by Pablo Bellinghausen –
Analogue audio connections are an aspect of pro audio to which many give little thought, but can quickly become a show-stopper. Every single lead in the signal path has the potential to degrade or even completely kill your sound, and in a system with dozens of channels and hundreds of metres of cable, a faulty connection can potentially ruin a golden take, or even a high-profile live show. It is however easy (although not always cheap) to get the basics right. Before we go into any specifics however, it is crucial to recap on a few basic but essential concepts of audio so we can explain certain types of wiring later on.
Sound is a vibration of the air around our ears, which oscillates back and forth from the average atmospheric pressure and causes our eardrum to move with it.
It can be helpful to think of audio as two forces constantly pushing and pulling back and forth in a sort of tug of war; it can be a pressure difference between the air inside and outside of the inner ear, or in analogue audio, an electric charge difference between the two ends of a circuit.
Here’s a little snippet of the a capella hit song “Tom’s Diner” by Suzanne Vega, which was used to test MP3 algorithms when the format was invented:
This is what happens if we “invert” the above recording, so that the air that was originally being pushed into our ears is now being pulled away from them:
In any normal testing conditions, these files sound identical, even though they are the exact opposite of each other; and although there are some rare occasions in which there might be a slight difference due to the inherent distortion in equipment and our ear, for most music the sound is the same.
The process of changing the signal in this way is called called reversing its polarity (sometimes mistakenly called phase) – whenever the signal in the first file is positive, the signal in the second one is negative, and vice versa.
Signals with opposite polarity have an interesting property: when added together, they cancel each other out.
Adding two signals of opposite polarity can be visualised as a stalemate or a tie in the aforementioned tug of war, with one force pulling as much as the other at any given point so that there is no signal left.
Balanced vs. Unbalanced Connections
Now that we know why signals of opposite polarity cancel each other out, we can talk about balanced leads, a very clever trick that is used in pro audio to remove most of the noise that is normally picked up by cables. A normal audio connection requires two conductors: the central conductor that carries the signal, and the ground, which surrounds the central connector. The ground acts both as a shield for the conductor, and as the link that completes the electrical circuit.
This works pretty well in many cases, particularly on short cable runs. If the signal strength is high, the impedance is low, the cable is of high quality, and there are no nearby sources of interference, an unbalanced connection is all you need. However, on long runs or in places with high electromagnetic interference, the cable will add a noticeable amount of noise to the signal.
A balanced connection sends the signal twice, on two separate conductors instead of only one. These are called “hot” and “cold”, with the former usually having red insulation for easy identification.
However, the polarity is reversed on the “cold” conductor, in the same way as explained above. When the cable is subjected to interference, noise is added to both conductors, which are so close together that said noise is basically the same on both.
Here is the clever bit: in the input of a balanced connection, the polarity of the “cold” signal is reversed again, and then added to the “hot” one. The original signal is added to itself, therefore getting louder, and the noise, which is now at opposite polarity, cancels itself out.
In our next instalment we will be taking a look at the types of cable to use for different applications, as well as all going through the connectors that are used in analog pro audio. Stay tuned!