Surround 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 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.)
We've put them roughly in order of their application to the encoding systems described on the previous page, though, as you will see, 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 synthesize stereo surround (rear) channels from Dolby Surround's surround channel. 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.
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.
THX: Strictly speaking, this is not a decoding system. (There is 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 theaters. Later it extended to home theater 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 codeveloped 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 decorrelates 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.
Writer Michael Riggs has been reviewing A/V gear for more years than he likes to admit.
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