2.2 Types of Audio Signal

Whilst the nature of an audio signal is very simple, there are many applications of audio, each requiring different bandwidth and dynamic range. Like video signals, audio signals exist in a variety of formats. Audio can mean a single monophonic signal, or several signals can be required to deliver sound with spatial attributes. In general timing and level errors between these signals will be detrimental. In stereo, two channels are required to give a spread of virtual sound sources between a pair of loudspeakers. With more channels, loudspeakers can be provided at the rear and sides to give a feeling of ambience.

A common surround sound format has five channels, where one of these feeds a front centre loudspeaker. This centre channel is not strictly necessary but exists because of the rather different circumstances in which cinema sound has traditionally been produced. In some cases a sixth channel is used for low frequency effects.

In the digital domain it is straightforward to deliver several discrete audio channels, whereas in analog systems it is not. As a result systems such as Dolby Surround were developed to allow some surround effect to be encoded into only two analog channels.