AfterDawn: Glossary

Discrete Surround

Discrete surround describes any surround sound encoding where the additional channels, beyond the standard left and right for stereo audio, are encoded in separate streams rather than being matrixed from a stereo signal. The most common discrete surround formats are the various versions of Dolby Digital and DTS.

Matrix Surround

Early home theater surround sound systems had to allow surround sound and standard stereo to co-exist in a singal signal. Center and surround channels are literally created by combining elements of the left and right channels. Since any sound to be included in the surround channel(s) must also be present in either the left or right channel as well, this can't produce sounds isolated to the rear only. This type of surround encoding, known as matrix surround, was superceded by (digital) discrete surround formats which allow five or more distinct (discrete) channels to be encoded.

DVD and Discrete Surround

With the introduction of the DVD player, digital surround sound encoding became standard. Digital surround formats aren't limited to storing only two channels as a downmix to stereo, or even matrix surround, can be performed by the decoder used for digital to analog conversion. In fact one of the requirements of the DVD-Video standard is for playes to do just that for legacy home theater equipment and TVs. This is also a standard feature for software media players designed to handle surround sound.

Beyond 5 Channels

Although 5 channels are standard for discrete surround, there are newer formats that introduce up to 2 additional surround channels. However, these extra channels are often matrix encoded in the standard left surround and right surround channels, and are therefore not discrete. Some 7 channel surround encodes include a discrete (mono) rear surround channel that's used for both rear surround speakers.

Discrete Surround Playback

Depending on the surround format and the capabillities of your home theater equipment, there are different configurations you might use for playback of discrete surround. There are two basic steps, transmission and decoding (digital to analog). Decoding may be handled by either a media player (DVD player, HTPC, Set-top box, etc,...) or a home theater receiver or digital signal processor (DSP).

Media Player Decoding

If decoding is handled by your media player you'll need a separate analog connection to your receiver for each discrete channel (6 for 5.1 surround). This method is typically used with high resolution audio with a bitrate that exceeds older digital connections. Alternatively, surround sound can be decoded to (still digital) LPCM and transmitted digitally. Once again this may exceed the capabilities of the older S/PDIF interface. For losslessly compressed or uncompressed discrete surround HDMI is required for digital connections.

Home Theater Receiver Decoding

For older (lossy) surround formats, such as Dolby Digital, the signal can be sent to a receiver without decoding beforehand. This is generally accomplished using a S/PDIF connection. It can also be done with HDMI, assuming your receiver supports it. Likewise, newer lossless formats can also be passed across HDMI due to its improved bandwidth over S/PDIF. Any time you connect a device using analog connections no decoding by the receiver will be required.

Encoding Discrete Surround

Most discrete surround encoding tools are aimed at the professional market, with consumer options often limited to only two channel encoding. Sony's Vegas is a notable exception. Rather than being a professional tool costing thousands of dollars it's aimed at the prosumer market, selling for only a few hundred dollars. However, for the most part there's no need to create your own surround audio, so this isn't a problem for most people.

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