Surround Sound Formats - Dolby Digital, dts, THX Surround EX, dts Extended Surround

Surround sound refers to the use of multiple audio tracks to envelop the movie watching or music listening audience, making them feel like they're in the middle of the action or concert. The surround sound movie soundtrack allows the audience to hear sounds coming from all around them, and plays a large part in realizing what movie makers call "suspended disbelief". "Suspended disbelief" is when the audience is completely captivated by the movie experience and is no longer aware of their real-world surroundings.

True surround sound formats rely on dedicated speakers that literally and physically surround the audience. There is one center speaker which carries most of the dialog (since the actors usually speak while making their on-screen appearance), and part of the soundtrack. There are left and right front speakers that carry most of the soundtrack (music and sound effects), and may carry parts of the dialog (when the director wants to intentionally off-set the source of the dialog to either side, from its default dead-center screen location). There is a pair of surround sound speakers that is placed to the side (and slightly above) of the audience to provide the surround sound and ambient effects. Finally, a subwoofer can be used to reproduce the low and very low frequency effects (LFE) that come with certain movies (e.g., the foot-stomping bass effects in "Jurassic Park" and "Godzilla").

A typical surround sound home theater system
(picture courtesy of Dolby Laboratories)

There are virtual surround sound algorithms (e.g., Sound Retrieval System [SRS] and other proprietary algorithms) that make use of only two left and right speakers and psycho-acoustics effects to emulate true surround sound formats. While we think the result is a more expansive soundstage with better ambiance, we have not heard a virtual surround sound implementation that comes anywhere close to resembling a true surround sound system. For the purposes of this discussion, we shall focus only on the true surround sound formats (that is, those that rely of multiple dedicated speakers).

Dolby Digital (formerly known as Dolby AC-3, where AC-3 is short for audio coding 3) is the de facto surround sound standard in today's home theaters. It is the surround sound format used in thousands of movie theaters today. And, since about the mid-1990's, it has become available for home theater use by consumers. Today, a large percentage of the DVD-Video titles come with Dolby Digital surround sound. Dolby Digital content first appeared on LaserDisc, since DVDs only emerged in the Spring of 1997. (Incidentally, Hi-Fi VHS still only supports up to Dolby Surround Pro-Logic.)

Not only is Dolby Digital the standard for DVD-Video, but it is also part of the new High Definition TV (HDTV) standard. It is used in pay-per-view movies and digital TV channels of digital satellite broadcasting (e.g., DIRECTV system). Dolby Digital is the successor to Dolby Surround Pro-Logic. The Dolby Digital surround sound format provides up to five discrete (independent) channels (center, left, right, surround left, surround right; giving it the "5" designation) of full frequency effects (from 20 Hz to 20,000 Hz), plus an optional sixth channel dedicated for low frequency effects (LFE), usually reserved for the subwoofer speaker. The low frequency effects channel gives Dolby Digital the ".1" designation. The ".1" signifies that the sixth channel is not full frequency, as it contains only deep bass frequencies (3 Hz to 120 Hz).

Readers should note that not all Dolby Digital soundtracks have 5.1 channels of audio. Those that are have the designation "Dolby Digital 5.1". Since Dolby Digital is a flexible surround sound format that supports up to 5.1 channels, Dolby Digital soundtracks could have one channel of audio (mono, designated as "Dolby Digital 1.0"), two channels of audio (stereo or Dolby Surround Pro-Logic, designated as "Dolby Digital 2.0"), or five channels of audio (designated as "Dolby Digital 5.0"). In fact, the Dolby Digital 2.0 soundtrack is required for all Region 1 (U.S. and Canada) DVDs. To learn more about Dolby Digital, read this Dolby Digital reference page.

An alternative and competing format to Dolby Digital is DTS Digital Surround, or just "DTS". Like Dolby Digital, DTS is another 5.1-channel surround sound format that is available in movie theaters, and as an optional soundtrack on some DVD-Video movies for home theater viewing. But unlike Dolby Digital, DTS is not a standard soundtrack format for DVD-Video, and is not used by HDTV or digital satellite broadcasting.

The primary advantage of DTS is that it offers higher data rates than Dolby Digital, leading many home theater enthusiasts to claim that DTS is better than Dolby Digital in sound quality. The down side is that a DTS soundtrack uses more of the disc's data capacity due to its higher data rate. This fact plus the fact that DTS is not a standard soundtrack format for DVD-Video makes DTS an optional 5.1-channel surround format that is actually available on few DVD-Video movies. There are far more DVD-Video titles with Dolby Digital soundtracks than there are those with the DTS surround sound format. For additional information about DTS, read this DTS reference page.

Dolby Surround Pro-Logic emerged in home theater systems in the early 1990's. It became the surround sound standard for Hi-Fi VHS, and is still the standard for today's analog TV broadcasts, since the Dolby Surround Pro-Logic signal can be encoded in a stereo analog signal. If you have an "older" Dolby Surround Pro-Logic receiver, you can still enjoy movies from DVD-Video, since all DVD-Video players down-mixes the Dolby Digital information to the Dolby Surround Pro-Logic format, and outputs the signal as a stereo audio pair.

Extended Surround formats:
Dolby Digital EX™, THX Surround EX™ & DTS Extended Surround™ (DTS-ES™)

Just when you thought 5.1-channel Dolby Digital and DTS surround sound were enough, at the leading edge today are two new "Extended Surround" formats, namely THX Surround EX™ and DTS Extended Surround™ (or DTS-ES™ for short).

The THX Surround EX format is jointly developed by Lucasfilm THX and Dolby Laboratories, and is the home theater version of "Dolby Digital Surround EX™", an Extended Surround sound format used by state-of-the-art movie theaters. Lucasfilm THX licenses the THX Surround EX format for use in receivers and preamplifiers. And as of November 2001, Dolby Laboratories has begun to license what is THX Surround EX under its own name, Dolby Digital EX™, for consumer home theater equipment. (Since THX Surround EX and Dolby Digital EX are equivalent, we will refer to THX Surround EX and Dolby Digital EX interchangeably, with preference for the former since that name has been around longer.)

THX Surround EX is the Extended Surround version of Dolby Digital 5.1, while DTS-ES is that of DTS 5.1. The difference between the new Extended Surround formats and their 5.1-channel surround sound counterparts is the addition of a surround back channel, whose corresponding speaker is placed behind the audience. This allows certain soundtrack effects to be presented behind the audience, thereby achieving more enveloping and complete 360° surround sound. (Remember that in the 5.1-channel surround sound formats, the surround speakers are placed one on each side of the audience - not behind them.) Additionally, while the Extended Surround sound format calls for one surround back channel, two surround back speakers are generally recommended for better envelopment. Acknowledging this widely accepted industry position, some high-end receiver manufacturers have introduced "7.1-channel" capable receivers, with decoding and sometimes amplification for the two extra surround back channels.

Both THX Surround EX and DTS-ES Matrix surround sound encode the surround back channel information into the surround left and surround right channels (similar to the way the center channel is encoded for Dolby Surround Pro-Logic). This cross-channel encoding is referred to as matrix encoding, since the surround back channel is encoded and later decoded (or derived) from those of the surround left and surround right channels. Because of this matrix encoding scheme, the surround back channel is not a true discrete channel and is technically considered a 5.1- channel format. And for this reason, they are sometimes referred to as "Dolby Digital 5.1 EX" or "DTS 5.1 ES". To refer to these matrix encoded Extended Surround formats as 6.1-channel would be wrong. (When we use quotes, as in the "7.1-channel" reference above, we are recognizing that it may not be a true discrete 7.1-channel system.)

DTS-ES can optionally support a fully discrete surround back channel. That is, the surround back channel has it own data stream and is truly independent from those of the surround left and surround right channels. This true 6.1-channel format is appropriately called DTS-ES Discrete 6.1 (in contrast to its matrix counterpart, DTS-ES Matrix). And as with DTS-ES Matrix, this discrete format is better realized with two surround back speakers. So our comment above about high-end manufacturers implementing "7.1-channel" receivers and preamplifiers for this purpose still holds true.

The Extended Surround formats are completely backwards-compatible with their 5.1-channel counterparts. That is, THX Surround EX is backwards compatible with Dolby Digital 5.1, and DTS-ES Matrix and DTS-ES Discrete 6.1 are backwards compatible with DTS 5.1. Additionally, DTS-ES Discrete 6.1 is backwards compatible with DTS-ES Matrix. In order to hear the matrix Extended Surround formats, you will need a THX Surround EX, DTS-ES Matrix, or a generic "6.1-channel" decoder in your receiver or preamplifier and use the digital audio output of your DVD player. To hear DTS-ES Discrete 6.1, you will need a DTS-ES Discrete 6.1 decoder in your receiver or preamplifier. In any case, you will also need six or seven channels of amplification, and one or two extra speakers for the surround back channel. Rest assured, you can still use your existing (or a soon-to-be-purchased) DVD-Video player, as long as it features Dolby Digital and DTS digital output.

"Star Wars: Episode 1 The Phantom Menace" is the very first movie to feature the new Dolby Digital Surround EX format (though Dolby Digital Surround EX playback is offered only in the finest state-of-the-art movie theaters). Even up until now, only a handful of movies have been released with the new Dolby Digital Surround EX format. For lists of theatrical movie titles with Dolby Digital Surround EX soundtrack, click here for previous releases and here for upcoming releases.

Likewise, only a few DVDs released to date have either THX Surround EX or DTS-ES. The first DVD with THX Surround EX is "Austin Powers: The Spy Who Shagged Me", while the first with DTS-ES 6.1 Discrete is "The Haunting". "Terminator 2: Judgment Day (Ultimate Edition)" DVD features both THX Surround EX and DTS-ES Discrete 6.1. If you're interested in DVDs with Extended Surround sound, our DVD New Releases and Upcoming Releases pages designate those DVD titles that are available in either THX Surround EX or DTS-ES.

Additional Info: Proper use and placement of surround sound speakers are key to getting the most out of surround sound systems. For a more detailed discussion of proper use and placement of speakers for a home theater environment, read this authoritative guide from Dolby Laboratories.

Did you find this surround sound primer helpful? Let us know your thoughts, send an e-mail to us at Feedback@TimeForDVD.com.

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