Surround Decoding 101

Shopping for an AVR you're going to be confronted with sheer tonnage of surround sound decoding options. You don't really have to pick and choose among them since they're all included, but we thought that you might want to know what you're buying in all those little logos that appear on your AVR's front panel, and also get a basic primer on surround sound in general.

Two companies dominate the surround sound decoding business, offering all-digital, 5.1-channel surround sound in two primary formats- Dolby Digital and DTS. Both companies offer a variety of ways to decode 5.1-channel soundtracks, and both companies offer decoding features that can decode stereo and 5.1-channel soundtracks to 6.1- or 7.1-channels.

You've been hearing the term "5.1" applied to home theater for years now. If you ever wondered, here's what it means: The number refers to an array of five full-range channels capable of cleanly reproducing both the highs and lows of a typical movie soundtrack, plus a low-frequency effects (LFE) channel that operate within a fraction (".1") of the audio spectrum, producing deep bass only. The front soundstage is comprised of a left, right, and center channels, with left and right surround channels at the sides or rear as well. The LFE channel is (usually) reproduced by a subwoofer, which can be placed in a variety of locations. When done correctly, this combination can transport the listener to wonderful places.

Basic 5.1-Channel Surround Decoding
Dolby Digital
Dolby Digital first became available on Laserdiscs, but went on to become the official audio standard for DVD, meaning that all DVDs are required to carry audio in the Dolby Digital format. In addition to being ubiquitous on DVD, DD is currently found on HDTV broadcasts over-the-air, on cable and satellite, as well as gaming, next-gen HD media and many more applications.

Although Dolby Digital can, and most often does carry up to 5.1 channels, this does not mean that all programs carry a 5.1-channel soundtrack. Even a mono (single-channel) soundtrack can be encoded as Dolby Digital.

Dolby Digital is what's known as a "lossy" compression format because it employs massive amounts of data reduction. On broadcast and DVD it's common for DD soundtracks to max out at 384kbps for all six channels in a 5.1-channel program (though they occasionally go as high as 448kbps). On Blu-ray and HD DVD Dolby Digital maxes out at 640kbps.

DTS ("Digital Theater Systems")
Pepsi to Dolby's Coke, DTS is also a digital surround format that's most often encoded at 5.1-channels for program material. DTS too was born on Laserdisc first, and followed Dolby Digital to DVD, although as an optional codec it's been used on DVD quite sparingly. While DTS is also a "lossy" compression scheme, its claim to fame is operating at higher data rates, thus requiring less compression of the original signal.

While you can't really compare different codecs based on their data rates alone, DTS has developed a strong following over the years, and many enthusiasts do prefer it to Dolby Digital. But DVD and broadcast media have bandwidth and storage space limitations, and the data required for DTS' audio leaves less room for high quality video and, on DVDs, extra features. This has limited its use. It has been more prevalent so far on Blu-ray and HD DVD, which offer much higher data rates and far greater storage capacity.

Extended Surround
Dolby Digital Surround EX
Surround EX adds a third surround channel to traditional 5.1-channel Dolby Digital. This rear surround channel can be decoded and played back using a single rear surround speaker, or two. The idea is that the basic surround channels are placed on the sidewalls, with the rear surround speaker(s) placed along the back wall. This ostensibly allows more distinct and convincing sonic pans from front to back in the soundstage.

In a feat of electronic sleight-of-hand, the additional surround channel is "matrix" encoded into the existing left and right surround channels of a 5.1-channel soundtrack, and then extracted during playback.

There is very little program material officially encoded with this rear surround channel, but in theory if a soundtrack is mastered this way theatrically it would also carry over to the 5.1-channel soundtracks on DVD, broadcast HD, or next-gen HD media.

DTS-ES is DTS' answer to Dolby EX and works in essentially the same fashion as far as the speaker configuration is concerned, and comes in two flavors: DTS-ES Matrix and DTS-ES Discrete. The Matrix version of DTS-ES works much the same as Surround EX, extracting the rear surround information encoded in the left and right surround channels. With the Discrete variety however, the rear surround channel is encoded as a discrete, independent channel during mastering for home playback. DTS claims this results in more precise "steering" of rear surround information.

Although both varieties of DTS-ES are included in every AVR out there, DTS-ES program material is more scarce even than Surround EX.

DTS 96/24
A high-end breakthrough that allowed some DVD-Audio music discs and a very few DVDs to deliver 24-bit, 96-kilohertz quality for 5.1-channels. Other than a few DVD-A discs released on DTS' own label, not many discs support this format. In addition, both DTS and Dolby have developed fully "lossless" codecs for Blu-ray and HD DVD that obviate this codec. More on that below.

Taking Stereo To 5.1, And Beyond
Dolby Pro Logic II, Pro Logic IIx and DTS Neo:6
Variations on a theme, these are surround sound "processing" modes that can expand two-channel stereo sources to 5.1-channels, or, in the case of Neo:6, expand 5.1-channel sources to 6.1-channnels in extended surround setups. Each includes sub-modes optimized music and movies. While the result is not the same as a source that was recorded in 5.1-channels to begin with, these modes can process a stereo recording in a way that often convincingly emulates the clarity, depth, and directionality, if not the pinpoint accuracy, of a true 5.1 presentation.

Dolby Pro Logic IIx is Dolby latest take on the technology, and expands the decoding from 5.1-channels to allow for either 6.1- or 7.1-channels.

Next-Gen Surround Decoding
High-definition movie discs bring with them a whole new generation of audio codecs to do justice to the amazing, state-of-the-art video. Blu-ray and HD DVD specifications allow for "up to 7.1 channels" of discrete—not matrixed—audio, but here again, 5.1-channels tracks are ubiquitous so far. The following codecs are prevalent on Blu-ray and HD DVD.

Uncompressed PCM
Because of the vast storage capacity of Blu-ray Disc, Sony and Buena vista have released their Blu-ray titles using uncompressed, multichannl linear PCM. This is essentially a higher resolution multichannel version of the coding used on the Compact Disc. Although uncompressed PCM takes up a lot of space, the results are astounding, and leagues beyond the quality of any of the lossy compression schemes.

Dolby TrueHD And DTS-HD Master Audio
Both of these are "lossless" audio codecs. Although the signals are compressed on encode, when decoded the signal is restored bit-for-bit identical to the original master. Both codecs are capable of encoding up to eight full range channels of audio, and resolutions as high as 24-bits at 192kHz.

TrueHD decoding is mandatory for HD DVD players and optional for Blu-ray Disc players. All of Toshiba's HD DVD players are capable of decoding TrueHD at 5.1-channels, and the number of BD players that can make the same claim is growing. Many HD DVD discs are being encoded with TrueHD, and a couple of Blu-ray music titles have also used TrueHD.

DTS-HD MA is optional on both Blu-ray and HD DVD. Thus far, no players from either camp offer DTS-HD MA decoding. The only next-gen software supporting DTS-HD MA are the Blu-ray Discs released by Fox. DTS-HD MA is encoded as "extensions" of the original DTS codec, and every DTS-HD MA track is backward compatible with every DTS decoder in the market today. A DTS-HD MA track will playback over existing equipment as a 1.5Mbps DTS track.

The Dolby and DTS lossless codecs can only be carried in their native digital form over HDMI 1.3 connections, and the first AVRs that decode these formats are shipping this summer. Most next-gen players will convert TrueHD tracks to multichannel PCM, which has broader compatibility with a variety of HDMI 1.1 and HDMI 1.2 spec'd AVRs and pre/pros.

Dolby Digital Plus
The apparent successor to Dolby Digital, this versatile codec is also a "lossy" compression scheme that is capable of operating at data rates both higher and lower than standard Dolby Digital. It is regarded as both more sophisticated and more efficient than its predecessor, and Dolby believes that it outperforms DD in lower data rate applications, such as broadcast HDTV, as well.

For high-definition packaged media applications (Blu-ray and HD DVD, in other words), Dolby Digital Plus tracks are typically encoded at either 640kbps or 1.5Mbps. DD+ tracks at 1.5Mbps are frequently used on HD DVDs from Universal and Paramount, and are often spectacular, offering a clear and obvious upgrade over standard Dolby Digital.

DD+ is backward compatible with existing DD decoders, and plays back at a data rate of 640kbps, which is a notably higher data rate than the 384-448kbps that was prevalent on DVD. So, even if you're using that DD decoder you've had for years, you'll hear upgraded audio from a DD+ track.

For broadcast HDTV applications DD+ is slated to be used in lower bandwidth applications. Thus far, we've not seen any cable or satellite receivers capable of decoding DD+.

THX Post Processing
THX developed and licenses a variety of post-processing features to manufacturers of THX certified AVRs. These are referred to as "post-processing" because these are enhancements that are engaged after standard Dolby Digital and DTS decoding occurs. A movie on DVD is not encoded "in THX," in other words, it is encoded in DD or DTS and then THX adds some features once the soundtrack is decoded in the AVR.

Prominent among the THX feature set are RE-EQ, Timbre Matching, and Adaptive Decorrelation. Movie theaters and home theaters are very different as venues, and as a result the high frequency balance that is most appropriate for a large venue full of live bodies would be excessively bright in a home theater environment. RE-EQ tames this brightness, providing a smoother, more reasonable balance for home theater playback.

However, more and more movie soundtracks are purposely remastered for the home theater environment upon release to home video. Typically all of the THX features are engaged under the umbrella of THX Cinema mode, although sometimes RE-EQ can be engaged or disengaged individually within THX Cinema for this reason.

Timbre matching contours the frequency response to make the front and surround speakers sound more integrated and coherent. Adaptive decorrelation alters the time and phase relationships between the surround speakers to provide a more spacious, movie theater-like sound in your home theater.