The Dolby Surround format, engineered by Dolby Labs in 1982, was the first digital audio coding system to add an additional channel to the two channels used in stereo sound. This additional channel, called "surround", is used for adding an extra dimension to the sound.
The three channels encoded in Dolby Surround travel through two traditional stereo channels, in order to stay compatible with existing audio equipment. A decoder is needed to use the third channel, which is discretely mixed with the first two.
The surround channel has an upper frequency limit of 7 kHz and, in order to make the effect more realiztic, a delay time of about 20 ms.
Devices with a Dolby Surround decoder normally carry the following logo:
The Dolby Surround Pro Logic format (usually called Dolby Pro Logic), which appeared in 1987, is an improvement to Dolby Surround, adding a fourth audio channel to Dolby Surround, a central channel for reproducing the actors' voices.
All together, a Dolby Pro Logic system includes:
It also adds improved sound quality and adjustment capabilities.
Devices which can decode a Dolby Pro Logic-encoded audio source usually bear the following logo:
Dolby Pro Logic II, created in August 2000, can artificially reconstruct a 5.1 sound environment, by applying computer processing to a stereo source (2.0) or Dolby Surround (3.0/4.0/4.1).
Devices which can decode a Dolby Pro Logic II-encoded audio source usually bear the following logo:
The Dolby Pro Logic IIx standard, introduced in 2003, can artificially reconstruct a 6.1 or 7.1 sound environment from a stereophonic source. It offers several sound ambiance settings depending on how the speakers are being used:
Devices which can decode a Dolby Pro Logic IIx-encoded audio source usually bear the following logo: