# Dolby Surround

Dolby Surround is 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, four channels of audio information (left, center, right, and surround) are matrix-encoded into only two channels. The surround information is carried on stereo sources such as VHS-HiFi, TV, DVDs, 35mm theatrical print optical soundtracks, or television broadcasts from which it can be decoded by a processor to recreate the original Dolby Stereo/Dolby Surround (left, center, right, and surround) mix. Without the decoder, the information still plays in standard stereo or mono.[1]

Due to the compatibility of the Dolby Stereo matrix with mono and stereo playback, when films originally made in Dolby Stereo were released on stereo domestic video formats or broadcast on stereo TV the original two-channel Dolby Stereo soundtrack could be used. A simplified decoder suitable for low-cost consumer equipment was licensed and produced that dispensed with a center speaker output and the logic circuitry found on the professional decoder, but did include the surround delay. To distinguish these decoders from the professional units found in cinemas they were given the name "Dolby Surround" decoders. The term "Dolby Surround" was also licensed by Dolby for use on TV programs or straight-to-video movies recorded through the Dolby Stereo matrix.[1]

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 Lt/Rt are used to describe soundtracks that are matrix-encoded using this technique.[1]

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

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