Digital Surround Sound Systems: A Summary

Digital surround sound delivers audio across several loudspeakers so that sounds appear to come from specific directions around the listener. Where stereo uses two channels, surround formats define a set of discrete channels — commonly front left, centre, front right, left and right surround, and a separate low-frequency channel — that together produce the familiar 5.1 configuration. The aim is to match film and music mixes that were created with a particular speaker arrangement in mind.

Several competing systems carry these channels. Dolby Digital, technically called AC-3, is a compressed multichannel format used on DVD-Video and in digital broadcasting. DTS (Digital Theatre Systems) is a separate, incompatible format that also uses perceptual compression, typically at different data rates. THX sits apart from both: it is a certification programme that sets quality standards for cinemas, equipment and home rooms, rather than a method of encoding audio. Higher-resolution disc formats add their own dedicated audio streams alongside these surround tracks.

Getting good results depends on both the equipment and the room. At the production end an encoder compresses the channels into a single bitstream; at playback a decoder in an audio-video receiver reconstructs them and feeds the loudspeakers. Bass management gathers the low frequencies from each channel and routes them to a subwoofer, since deep bass cannot easily be located by ear. Correct speaker placement, listening position and attention to room reflections then determine how faithfully the original mix is reproduced.

Frequently asked questions

What channels make up a 5.1 system?

Five full-range channels — front left, centre, front right, and two surrounds — plus a separate low-frequency channel, usually played through a subwoofer.

Does the encoder or the decoder run during playback?

The decoder runs during playback, reconstructing the separate channels from the compressed bitstream. The encoder is used earlier, when the soundtrack is produced.

Is THX a rival to Dolby Digital and DTS?

No. THX is a set of quality-certification standards. The audio itself is still encoded in formats such as Dolby Digital or DTS.