What is balanced audio all about?

Question

What is balanced audio? What is a balanced cable? What is the advantage of using balanced audio? How does balanced audio work?

Answer 1

A balanced setup prevents electro-magnetic interference from corrupting an audio signal, such as one going between a microphone and a preamp, for example.

Cables themselves are not balanced, but an audio signal carried by a cable might be balanced. Cables which carry such signals normally have three conductors. Microphone cables are a typical example. The conductors are called hot or positive, cold or negative, and ground or earth. (The ground is not necessary, but is usually present.)

A balanced signal is resistant to interference because of an error-correcting feature built into its design. Note that the ground conductor is not necessary for the error-correcting feature to work.

To understand it, let us first consider a normal, unbalanced audio signal. Let's say it leaves its source looking like this:

The wire acts as an antenna which attracts noise that interferes with the signal. It arrives at its destination corrupted. The orange line show the portion of the signal in error. (The original is seen in a darker red.) Real-life interference is more random than regular, but this example helps to illustrate the effect.

Thus a corrupted version of the original signal arrives at the receiving equipment. What you see is, unfortunately, what you get.

In a balanced audio signal, the original signal is sent as-is through the hot conductor. A phase-inverted copy of it is sent in the cold conductor. The light blue wave shows the cold signal.

If the same interference occurs here, both signals are corrupted, but the interference affects both signals in the same way: they are forced downward by the same amount. The new signals are shown in orange.

If the receiving equipment summed the signals directly, we'd see the original signal gone and the corrupting noise doubled. These are represented in yellow and gray, respectively.

But that's not what happens. The cold signal is subtracted from the hot. The result is the shape of original signal, error free. (The corrected portion is shown in white.) This happens because the noise, which is in phase on both lines, is subtracted from itself.

Note that the amplitude (i.e. strength) of the original signal is actually doubled in the process. (This is not shown in the diagram.)

Further points:

• Typical connectors for balanced lines are XLR and TRS.
• Using a three conductor cable to connect two pieces of equipment does not necessarily mean the signal is balanced. The connected equipment needs to provide balanced connections, though a balanced input retains its interference-rejecting benefits, even if the source is unbalanced, as long as cold is not connected to ground at the input end.
• There are other uses for three conductor cables. For example, stereo analog audio can be carried on a three conductor cable. That's how headphones work.
• People often associate balanced audio with analog equipment, but it applies to wired digital connections, also. For example, AES3 (AES/EBU) is typically carried on three conductor cables with XLR connectors and is balanced, whereas S/PDIF is typically carried on two conductor cables with RCA connectors and is unbalanced. (Strictly speaking, AES3 and S/PDIF are data formats and can be carried over any digital medium. Fiber optic cables are one example. However, when wired, it is standard to use the cables and connectors described.)