Wiring a Studio: THE GROUND RULES

1.  Audio Signals

1.1  Electrical Circuits

Let's start by looking a simple electrical circuit - a battery connected to a light bulb.  The battery is the source.  The light bulb is the destination.

Electrical circuit

Note that two wires are needed to complete the circuit.  Two connections are always required in order for an electric current to flow through an electrical device.

1.2  Audio Circuits

An analogue audio signal is an AC voltage representing a sound wave.  In a similar manner to above, when we want to connect an audio source to an audio destination, two connections are required.

Audio circuit

The source might be a microphone which generates a small AC voltage in response to a sound wave.  The destination might be an amplifier input.

1.3  Balanced Audio Wiring

Audio signal wiring is referred to as being balanced when it is wired in the way shown above, that is, two wires are used to connect the audio signal.  These two wires are used only for this purpose, and do not come into contact with any other wiring.  In practice, these wires are combined into a single cable.

1.4  Unbalanced Audio Wiring

With unbalanced wiring, when multiple audio signals need to be connected, instead of using individual pairs of wires for each audio signal, one side of each audio signal is shared.  This common connection is usually also connected to ground and to the metal case of the equipment.

Unbalanced wiring

Note that there are still two connections to each audio device - it's just that a single wire is used to connect one side of the audio signal, whilst the other side is uses the common connection.  The symbol is often used to represent this common connection.

Inside a piece of audio equipment there is usually a number of separate audio stages, each with at least one input and one output.  In most cases, the audio signals between each stage are unbalanced.

An analogy to unbalanced audio wiring is the wiring methods often used in cars.  A single wire is run to each light bulb or other electrical device, and the second connection is the metal chassis.  One side of the battery is also connected to the metal chassis.

1.5  Pops, Hum and Crosstalk

Having discussed the two basic methods of wiring audio signals, let's now turn our attention to what can go wrong.

Electrical interference can superimpose itself on audio signals.  It should be pointed out that this problem is not unique to analogue audio signals.  It is just that since an analogue audio signal is supposed to represent a sound waveform, any corruption of an analogue audio signal degrades its quality.  Digital wiring is also affected by electrical interference, but digital signals do not depend on accurate reproduction of the waveform.  Instead, the destination relies on the signal being either above or below a threshold, which is interpreted as being either "1" or "0".  Unless the electrical interference is extreme, digital signals are virtually immune to this problem.

Electrical interference can be divided into three categories:

In the next few chapters, we will look at some of the mechanisms by which electrical interference finds its way into audio signals and what we can do about it.