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Surround Sound Formats Explained:

There are two ways to deliver a surround sound signal. The older analog matrix technology works by mixing (matrixing) multiple channels into two main channels; the matrixed information then has to be decoded into the original channel configuration. The newer digital discrete technology keeps all the channels separate, producing a more clearly isolated surround effect.

Most newer audio/video receivers contain multiple surround sound decoders, and handle all the format-switching automatically. But you still might need to know what you're dealing with, so see the description below for a basic guide to surround sound formats.

Matrixed Surround Sound Formats

Surround sound was first heard via a matrixed technology—the original Dolby Surround format. Matrix technology has become more effective over the years, but still isn't as realistic as discrete surround sound. We've seen three different matrix technologies in the home, all analog, and all from Dolby Laboratories.

Dolby Surround

The original surround sound format for home video was Dolby Surround. This is a simple three-channel format, with left, right, and surround channels. The surround channel is matrixed into the left and right channels; the Dolby Surround decoder separates the surround channel from the mix and runs it into one or two surround speakers. When you use two surround speakers, they both play the same surround channel.

Dolby Surround was first used in some prerecorded videotapes, and in some augmented stereo television broadcasts. It's not used much today.

Dolby Pro Logic

Dolby Pro Logic is a more sophisticated version of Dolby Surround. Pro Logic adds a center channel to the mix; it's a four-channel system with left, center, right, and surround channels. The center and surround channels are matrixed into the left and right channel information, and then separated out with a decoder. As with Dolby Surround, the single Dolby Pro Logic surround channel is often fed to two surround speakers, which both play the same track.

Dolby Pro Logic is the surround technology used in many television, cable, and satellite broadcasts, as well as in most prerecorded videotapes. It's still in use today.

Dolby Pro Logic II

Dolby Pro Logic II is a version of Dolby Pro Logic with a different purpose. This surround format is designed to simulate a surround effect from a two-channel source. You use Dolby Pro Logic II to play stereo soundtracks and CDs on a surround sound system. (The newer Dolby Pro Logic IIx format applies the same technology to 7.1-channel systems.)

The Discrete Surround Sound Formats

Discrete surround sound keeps each channel separate. 

Dolby Digital

The most popular discrete surround sound format today is Dolby Digital. As the name implies, it's a digital process, unlike the analog matrix technology used in the older Dolby Pro Logic format.

Dolby Digital is used in most commercial DVD soundtracks as well as some satellite TV and cable TV programming, and is the format specified for all HDTV broadcasts. In short, it's the surround sound format of choice today.

There are actually several variations of Dolby Digital, depending on how many channels are used. Dolby Digital is actually a flexible format that supports up to 5.1 channels. So mono films are often recorded in Dolby Digital 1.0 (one center channel, no subwoofer), and older stereo films in Dolby Digital 2.0 (left and right front channels, no subwoofer).

The most common format, however, is Dolby Digital 5.1. This format includes left front, center front, right front, left surround, and right surround channels, plus a separate low frequency effects (LFE) channel that is fed to your  subwoofer. Add that up and you get the 5 main channels plus the ".1" LFE channel. 

DTS

The Digital Theater Systems company markets a discrete surround format, called DTS, that competes head-to-head with Dolby Digital. Like Dolby Digital 5.1, the DTS format includes five main channels (left front, center front, right front, left surround, and right surround) plus a separate LFE channel. 

DTS works much like Dolby Digital, but with higher data rates. This results, to some ears, in slightly better sound quality. DTS is an optional format in the home theater environment. There are far more DVDs with Dolby Digital soundtracks than with DTS soundtracks. Most A/V receivers include both Dolby Digital and DTS decoders, however, and will automatically detect DVDs with DTS soundtracks.

Dolby Digital EX

Dolby Digital EX is a variation of the Dolby Digital format. EX adds a "surround rear" channel in addition to the regular left and right surrounds. This rear channel is placed behind the listener, while the left and right surrounds are placed to the sides. Note, however, that this surround channel is matrixed, not discrete, so it's not overly directional.

There are actually two variations of Dolby Digital EX—the original 6.1-channel format (with a single matrixed rear channel) and the newer 7.1-channel format (with matrixed rear left and rear right channels). 

Dolby Digital EX is sometimes referred to as THX Surround EX, as it has THX certification. Learn more about THX here.

DTS ES

Like Dolby Digital EX, DTS ES is a 6.1 technology with an added surround rear channel. There is also a 7.1-channel version, with two surround rear channels. Both versions use matrix technology to create and decode the rear channel(s).

DTS ES Discrete

If matrixed surround channels aren't ideal to your ears, check out the DTS ES Discrete system. Unlike both Dolby Digital EX and DTS ES, DTS ES Discrete uses discrete technology for its single rear channel. The rear channel in ES Discrete is thus more distinct from the side surrounds than with the other 6.1-channel formats.
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Discrete Surround Sound Formats... more details

Dolby Digital or AC3 – Dolby Digital is a compressed discrete surround sound format that is dominant on DVD-Video movies and is frequently found on DVD-Audio discs. Most receivers and A/V Preamps are capable of decoding Dolby Digital. Dolby Digital’s main advantage is that it can fit in places where uncompressed surround sound simply wouldn’t go. One of those places is on some satellite TV broadcasts. You do need a Dolby Digital-capable DSS (digital satellite) receiver, but it is nice to get movies from the dish in surround. Music enthusiasts argue that the 12:1 compression on Dolby Digital loses a little too much audio quality as compared to other “lossy” surround sound formats like DTS.

DTS – DTS was developed for “Jurassic Park” in 1993 and is the main competitor to Dolby Digital in the A/V market. DTS’ advantage over Dolby is its 3:1 compression versus 12:1. While the sound is often better, studio and record labels have to decide if they want to sacrifice supplemental material on a DVD in place of better surround sound. DTS has produced many of their own musical titles on CD, DVD and DVD-Audio that feature their surround sound format: DTS CDs can be played on any CD player with a digital output or on a DVD player. 

MLP – MLP stands for Meridian Lossless Packing. It is the “Lossless” compression scheme used on DVD-Audio to get the highest resolution surround sound and stereo music from a DVD disc. 

DSD – Found exclusively on SACDs (Super Audio CD), Sony and Philips’ DSD format uses a different recording and compression scheme than the PCM (Pulse Code Modulation) used by DVD-Audio and all other digital media. 



“5.1” – 5.1 consists of the five surround speakers (center, two fronts, two sides/rears), plus a subwoofer. Because in early systems the Low Frequency Effects (LFE) channel was limited only to low frequencies, it was referred to as “.1”. The LFE channel is often referred to as the “subwoofer” channel. The LFE channel on a DVD does not go to the subwoofer -- it goes into your system’s bass management. Similarly, the subwoofer doesn’t just receive signals from the LFE, but instead receives all the bass from all channels that your other speakers can’t reproduce, thanks to the bass management system.


DTS ES (6.1) – DTS has a format that most frequently adds a rear center speaker to the surround sound package found on selected music and movie discs.

THX EX (7.1) –  if you have the discs and a preamp that can play 7.1, you can add more speakers to your system to increase your system’s ability to resolve detailed and complex surround effects from more modern films. THX EX suffers from the same problem as DTS ES, people are hard-pressed to name any movies in the format. 
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Quadraphonic – While quad was commercially not a huge success, mainly because of the limitations of vinyl as a surround sound medium, many recordings were made in quad during the 1970s. Some of these have been released on DTS CDs. Others are making their way to SACD or DVD-Audio. 


Matrix Surround

In the days before multi-channel digital audio systems like DVD, surround (originally quad, but later other systems as well) had to be squeezed into two channels for compatibility with stereo broadcasts and albums. This was never as good as having extra “real” channels, but as few people actually had four-channel tape recorders or record players, it was the best that could be done. The process of squeezing four or more channels into two and then trying to get them back again at the receiving end is called “matrixing” and the systems that do this are called “Matrix Surround” systems. Dolby, because of their prominence in the film world, became the leader at this with Dolby Pro Logic and Pro Logic II.

Many TV broadcasts are actually transmitted in surround using this system – look for messages at the beginning saying, “in surround where available” – and if you switch your receiver to “Pro Logic” or “Pro Logic II,” you will be able to listen in surround. The Pro Logic decoder in your receiver or preamp will also do a fair job of creating a surround effect from ordinary stereo material.

Pro Logic II is leaps and bounds ahead of the old Dolby Pro Logic. Pro Logic II can add significant information to your rear speakers, which really adds to watching satellite TV, digital cable, watching movies from a VCR and more.
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