# Dolby Surround

Dolby Surround was the earliest consumer version of Dolby's multichannel analog film sound decoding format Dolby Stereo. It was introduced to the public in 1982 during the time home video recording formats (such as Betamax and VHS) were introducing Stereo and HiFi capability. The term Dolby Surround is used so as not to confuse theater surround (Dolby SR, which has four channels of audio) with home stereo, which has only two. Dolby Surround is the earliest domestic version of theatrical Dolby Stereo. The term also applies to the encoding of material in this sound format.

## Technical details

When a Dolby Stereo/Dolby Surround soundtrack is produced, five elements of audio information (left, center, right, sub and surround) are matrix-encoded into only two channels. The surround information is carried on stereo sources such as VHS TV, DVDs, 35mm theatrical print optical soundtracks, or television broadcasts from which it can be decoded by a processor to recreate the original Dolby Surround (left, center, right, sub and surround) mix. Without the decoder, the information still plays in standard stereo or mono.

As the technology of a Dolby Surround decoder is virtually the same as decoding the monaural surround soundtrack, many Dolby Stereo encoded films could be transferred with little change to the stereo soundtrack, lowering the costs of re-recording the audio of a film to video. In fact, most left/right/surround Dolby Surround decoders included a modified Dolby B decoder as part of their design.

The Dolby Surround decoding technology was updated during the mid-1980s and renamed Dolby Pro Logic in 1987. The terms Dolby Stereo, Dolby Surround and LtRt are used to describe soundtracks that are matrix-encoded using this technique.

Dolby Surround Matrix Left Right Center Surround
Left Total (Lt) $1$ $0$ ${\frac {{\sqrt 2}}{2}}$ $j{\frac {{\sqrt 2}}{2}}$
Right Total (Rt) $0$ $1$ ${\frac {{\sqrt 2}}{2}}$ $-j{\frac {{\sqrt 2}}{2}}$

Note that j represents a 90° (π2 radians) phase shift.