AfterDawn: Glossary

6.1 Surround

7.1 Surround refers to a Surround Sound format which includes four surround channels. The two standard surround channels from 5.1 Surround, Left Surround and Right Surround, are positioned directly to the sides of the listener, with two additional surround channels directly behind. Although the signals for the two rear speakers are delivered separately, theyre generally both from the same mono Back Surround Channel.

Matrixed and Discrete 7.1
Just as Surround Sound itself can be achieved using either matrix or discrete encoding, the additional surround Channels can either be included as a separate discrete channel or matrixed from the standard Left Surround and Right Surround channels.

The .1 Channel
There are varying opinions on the usefulness of including a separate LFE channel encoded into the audio stream, rather than simply relying on the listener's equipment for to create it from the 7 full frequency channels. Many feel that it's wasted information of no use to those without a subwoofer, and redundant to those who have one because they should already to be able to separate the appropriate frequencies with their existing equpment. For DVD-Audio there isn't even a requirement that the sixth channel be frequency Limited, meaning there's no guarantee frequencies higher than a subwoofer is designed to handle won't be included. Although this isn't an issue in practice, it's a good argument for simply dropping the sixth channel from production and leaving that to the consumer.

Lossy 7.1 Surround Formats
6.1 Surround can be found in both lossy and Losslessly Compressed formats. While it's also possible to use LPCM encoding, this is generally reserved for transmission rather than actual storage on a DVD or other consumer format. Dolby Digital EX and Dolby Digital Surround EX are both backward compatible with Dolby Digital 5.1 systems and also with DVD-Video, allowing them to be used on standard DVD-Video movie releases. Higher Bitrate 7.1 Surround can be encoded in Dolby Digital Plus format, which is still lossy and not compatible with DVD-Video.

Lossless Formats
7.1 Surround can be encoded losslessly in both Dolby Digital TrueHD and DTS-HD Master Audio. The Dolby format is mandatory for HD DVD players, meaning all players must be able to handle it, while it's optional for Blu-ray players. DTS-HD support is optional for both. Like the lossy alternatives, audio is often encoded using only full frequency channels, leaving the listener's home theater equipment to divert the appropriate frequencies to an LFE channel when appropriate.

Analog and Digital Connections
For more information on connecting 7.1 Surround equpment, see the relevant portions in the description of 5.1 Surround.

Additional Reading
Dolby Pro Logic II (Matrix Surround)

Dolby Digital


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