Sound around

You know the drill: you have your popcorn ready, and you're ready to plop onto the couch and watch a movie on your home theatre system or media PC. So you put in a DVD and click through to find the setup screen, where several surround sound options are available. But which setting should you select?

Before you choose, you should know how to select a surround mode suitable for your home theatre equipment, and to do that you should understand how the formats work. To help make sense of surround sound terminology, I've defined all the major surround sound encoding and decoding formats for you.

Surround sound encoding repackages multiple audio channels in a format that is more convenient for storage or transmission. Decoding reverses the process to play the signal. The audio is encoded at its source - when a DVD is produced, for example. It gets decoded by whatever you use to play it - in the case of a DVD movie, your DVD player or your A/V (audio/video) receiver. The decoding applied to a signal must match the encoding scheme that was used to create it, or else you may wind up with no sound at all.

DVD movies and some digital TV programs (high-definition content, in particular) use special techniques to encode and store soundtracks containing more than two channels of audio. An A/V receiver, which sits at the heart of most home theatre systems, handles the signal routing from the DVD player, VCR, or TV; the amplification to the loudspeakers; and, critically, the surround decoding. A digital connection between components is generally preferable, and I'll be assuming that this is what you're using from here on in.

Encoding systems

Here's an explanation of the basic surround encoding systems in general use today.

Dolby Surround: This is a "matrix" encoding scheme, which means that it folds four channels (front-left, front-right, centre, and surround) down into two channels. With the proper decoder, it can be reproduced as four-channel surround sound; without decoding, it will play as ordinary stereo. As a result, Dolby Surround is extremely versatile, since it can be applied to any analog or digital audio transmission or storage medium (such as a CD) or to any device that supports ordinary two-channel stereo. The downside is that the four original channels can never be extracted perfectly, although the latest decoding techniques (Dolby Pro Logic II, for example) come amazingly close.

Dolby Digital (AC-3): This is tech­nically a digital data reduction, or com­pression, system. Although similar in concept to MP3 encoding, Dolby Digital (or AC-3 as it's sometimes known) is more efficient and can carry up to six channels of high-quality audio. The most common formats are 5.1-channel (five full-range channels - front-left, front-right, centre, left-surround, and right-surround - plus a bass-only channel for low-frequency effects, or LFE) and 2.0-channel, which is simply good old stereo. And, like any other stereo soundtrack, Dolby Digital 2.0 may be Dolby Surround encoded.

Dolby Digital is the audio format used in HDTV broadcasts and the standard format for DVDs with surround sound. To decode it using your A/V receiver, you will need an optical or coaxial digital connection to the source.

Dolby Digital EX: This format, an extension of Dolby Digital, matrix-encodes a back surround channel into the regular left and right surround channels; the additional channel can be extracted on playback to create 6.1-channel surround.

DTS: Short for Digital Theatre Systems, DTS is like Dolby Digital but less efficient. DTS is an optional soundtrack format on some DVDs. As with Dolby Digital, a digital connection is required for decoding in your A/V receiver.

DTS-ES: This is the DTS equivalent of Dolby Digital EX, though it allows inclusion of a fully discrete (as opposed to matrix-encoded) back surround channel.

Decoding systems

Listed below are explanations of the main surround decoding options you're likely to encounter on current A/V receivers. (All DVD players and software incorporate at least a two-channel Dolby Digital decoder so that they can provide stereo analog audio output in setups that don't include an A/V receiver.)

I've put them roughly in order of their application to the encoding systems described in the previous section, though, as you'll find, the correspondence is not always exactly one-to-one.

Dolby Pro Logic: This is the original standard for decoding Dolby Surround soundtracks. Because of the way matrix decoding works, Pro Logic attempts to generate surround sound from ordinary stereo (non-surround encoded) sources - such as a CD - but the quality tends to be hit-and-miss, depending on the characteristics of the particular mix.

Dolby Pro Logic II: An advanced matrix decoding system, Dolby Pro Logic II does a better job of unpacking Dolby Surround soundtracks than Dolby Pro Logic. It uses information in the front channels to synthesise stereo surround (rear) channels from Dolby Surround's surround channel - see screen shot. But Pro Logic II's most appealing feature may be its music mode, which generates natural and pleasing surround from conventional stereo recordings.

Dolby Pro Logic IIx: In addition to the regular left and right surround channels, Dolby Pro Logic IIx, another matrix-decoding format, creates one or two back surround channels, for 6.1- or 7.1-channel playback. Pro Logic IIx can be applied not only to stereo and Dolby Surround soundtracks, but also to Dolby Digital 5.1 tracks. Depending on the receiver, you may also be able to use it with DTS soundtracks.

DTS Neo:6: This matrix-decoding format generates six-channel surround sound from Dolby Surround and stereo (non-surround encoded) sources. The sixth channel is a back surround, akin to Dolby Digital EX and DTS-ES.

Dolby Digital: You must use the Dolby Digital format to decode Dolby Digital soundtracks and get discrete 5.1-channel surround sound - that is, sound encoded for the left surround channel comes out of the left surround speaker, with no audio "bleeding" out of the other speakers. And because Dolby Digital is the audio standard for HDTV and DVD, it's on its way to supplanting Dolby Pro Logic as the base level for surround decoding - see this picture.

Dolby Digital EX: Use this format to decode discrete 6.1-channel surround from Dolby Digital EX soundtracks.

DTS: This format works to decode discrete 5.1-channel surround sound from DTS soundtracks.

DTS-ES: Use this mode to decode discrete 6.1-channel surround from DTS-ES soundtracks.

The others

THX: Strictly speaking, this is not a decoding system. There's no such thing as THX encoding, either. THX originated at Lucasfilm, under then technical director Tomlinson Holman (its initials stand for Tom Holman's eXperiment), as a quality-assurance program for movie theatres. Later, it extended to home theatre products as a quality-certification program, combined with certain post-processing technologies applied to the outputs of surround decoders to improve the correlation between the sound heard at home and the sound in properly calibrated cinemas. Later still, came the THX DVD-certification program, which is strictly quality-assurance. The THX logo can mean somewhat different things, depending on where you see it.

THX Surround EX: Dolby Digital EX was co-developed by Dolby Laboratories and THX (then a division of Lucasfilm). THX Surround EX is THX's own implementation of the decoding, which incorporates THX enhancements. Probably the most important difference is that THX Surround EX decoding requires two back surround speakers (merely an option for Dolby Digital EX) and de-correlates the signals feeding them so that they are no longer exactly identical, to make the sound seem bigger and more spacious.

Dolby Virtual Speaker: This is a special matrix signal processing decoder that provides a good impression of surround sound from just two front speakers.

Dolby Headphone: Dolby Head­phone is Dolby Virtual Speaker for headphones. This tends to work better with headphones than with speakers and can be quite impressive.

DTS 96/24: Digital Theatre Systems' top of the range 5.1 surround sound standard, upsamples existing content to six channels, or delivers native 96kHz/24-bit audio via DVD movie or audio content - see screen shot

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