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Understanding Surround Sound Formats

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Surround sound

Because most people, from surround sound novices to A/V experts, find surround sound formats a little confusing, we've provided quick descriptions below of many common formats. You can also take a look at our surround formats chart for an even faster summary. If you're looking for tips on getting surround sound in your home theater, check out Dave's article on How to Avoid Common Home Theater Pitfalls.

Before we get started, there are a few general terms and concepts you should know.

5.1: 5.1 surround sound is the most common format. It includes a total of six channels — five full-bandwidth channels with 3-20,000 Hz frequency range for front left and right, center, and left and right surrounds, plus one "low frequency effects" (LFE) subwoofer channel for frequencies from 3-120 Hz. 6.1 and 7.1 surround sound systems are also available, and simply add another full-bandwidth channel to the mix. (To see how the speakers are arranged in a multichannel surround system, check out the diagrams in our guide to home theater speaker placement.)

Discrete: Some channels are considered "discrete" — that means that the sound information contained in each of the available channels is distinct and independent from the others.

Matrixed: Other channels are considered "matrixed" — that means that the sound information in those channels is extrapolated from information in other channels. Though you'll notice more precise surround effects from discrete channels, you can still expect engaging sound from matrixed channels.

Lossless: Most surround formats are compressed so that they're small enough to be stored or transmitted — on a DVD, for example, or in a satellite TV broadcast. But now, some higher-capacity Blu-ray Discs™ can hold lossless surround soundtracks that are identical to the original studio recording. The higher quality of these formats makes for more detailed audio.


5.1-channel surround

As we stated above, 5.1-channel surround sound is the most common home theater configuration today. And there are two main formats that deliver surround sound for 5.1-channel systems.

Dolby Digital
Dolby® Digital quickly established itself as a reigning surround format, largely thanks to DVDs. These days, it's also used in video games and HDTV programming. Although Dolby Digital, strictly speaking, is simply a method of encoding audio information digitally, the term is often used to refer to 5.1-channel audio — its most popular form. In discussing Dolby Digital surround sound, we'll be focusing on this multichannel format.

Unlike earlier forms of surround sound, Dolby Digital 5.1-channel audio is a "discrete" multichannel surround sound system. With six discrete channels, sounds can be placed very precisely, for improved dialogue clarity, imaging, spaciousness, and realism. You also get a dedicated subwoofer channel, for plenty of deep bass.

How Dolby Digital works

DTS
Like Dolby Digital, DTS® provides 5.1 channels of digital audio. However, DTS uses less compression than Dolby Digital. As a result, some say that the sound produced by DTS is slightly more accurate than the sound produced by Dolby Digital. While most audio/video receivers will have both Dolby Digital and DTS, fewer DVDs and video games are encoded with DTS, compared to the number encoded with Dolby Digital.


6.1-channel surround

Although 5.1-channel is still the most popular surround format, and many of the home theater systems being sold today are 5.1-channel systems, 6.1-channel audio is also a common option. A 6.1-channel system delivers an even more enveloping surround effect than a 5.1-channel system. Let's take a look at the main options. (To see a typical 6.1-channel setup, check out the diagrams in our guide to home theater speaker placement.)

DTS-ES
DTS-ES™ uses existing digital multichannel technology to deliver the "5.1" channels of regular DTS, plus it adds a discrete, full-bandwidth back surround channel. Although more movies are encoded for Dolby Digital EX (see below) than for DTS-ES, there are still plenty. And today's 6.1-channel receivers are likely to have both formats.

Dolby Digital EX and THX Surround EX
Dolby Labs and THX collaborated to come up with their own solution to 6.1-channel surround sound, too. They do essentially the same thing, both adding a matrixed back surround channel for use in a 6.1-channel speaker system, in order to provide an even more complete 360° soundstage. (If you have a 7.1 surround system, the same audio information will go to both of your back surround speakers.)

Many DVDs are encoded for Dolby Digital EX, and have that extra channel of surround information ready to go. Also, if you're playing a regular Dolby Digital 5.1-channel DVD, a Dolby Digital EX or THX Surround EX™ decoder will simulate 6.1-channel surround by processing the audio information in the discrete surround channels and sending the matrixed audio info to your back surround speaker(s) as well.

How DTS-ES 6.1 works

7.1-channel surround

In addition to HD video, today's Blu-ray disc formats can also support more detailed audio. Most Blu-ray disc players support 7.1 audio formats, and some even offer high-quality, lossless surround sound.

While your high-def disc player and receiver may be able to decode these new surround sound formats, it's important to note that not all discs you play will take advantage of them. Be sure to check out the details on the Blu-ray discs you watch to see which audio format they use. You'll also want to connect your player to a compatible receiver using your disc player's 7.1-analog audio outputs or a compatible HDMI output (version 1.3).

Blu-ray Logo

Lossless surround sound formats
The newest high-resolution surround formats, Dolby TrueHD and DTS-HD™ Master Audio, offer up to 7.1 discrete channels of lossless audio. Along with adding two extra rear channels to the standard Dolby Digital and DTS formats, Dolby TrueHD and DTS-HD Master Audio discs are encoded with more audio information per channel. In fact, it's identical to the movie studio's original master. That means the improved directionality and more precise effects makes it even closer to the experience of being in a movie theater.

Additional discrete 7.1-channel surround formats
You may find that some Blu-ray discs are also encoded with other discrete 7.1-channel surround formats. Dolby Laboratories and DTS developed Dolby Digital Plus and DTS-HD (High Resolution), respectively. These formats deliver 7.1 independent channels of sound. They provide more detailed surround effects than 5.1 Dolby Digital and DTS, though they aren't lossless like Dolby TrueHD and DTS-HD Master Audio. Receivers that support lossless 7.1-channel formats will also support Dolby Digital Plus and DTS-HD (High Resolution).


Matrixed surround decoding for older sources

If you use a stereo analog connection to your receiver, or are connecting older equipment like a VCR, your receiver may employ one of these types of processing to decode the signal. Dolby Pro Logic II includes two independent full-bandwidth surround channels, three matrixed surround channels, and a dedicated low-frequency channel for your subwoofer. Many recent home theater receivers also offer Pro Logic IIx processing, which can turn the same sources into even more enveloping 7.1-channel sound.

Receivers with Dolby Pro Logic II and IIx give extra intensity to the thousands of VHS movies and TV broadcasts recorded in either stereo or older 4-channel Dolby Surround. They also include special modes tailored to turning stereo music into realistic surround sound.

DTS Neo:6 is essentially identical to Pro Logic II — it's simply the processing DTS came up with to deliver 5.1 or 6.1 channels of sound from a two-channel stereo source. And just like Pro Logic II, it delivers a convincing surround sound experience.


"Height" channels for a taller soundstage

A few receivers offer a newer surround sound format called Dolby Pro Logic IIz. It adds two "height" channels to your front soundstage. These speakers typically mount on the wall above your existing front left and right speakers.

A receiver with Pro Logic IIz can divvy up the front soundstage audio. It sends directional sounds — like a car racing by — to your regular front speakers, and non-directional sounds — like the roar of a crowd at a stadium — to the height channels. The result is a larger, deeper front soundstage, and a more immersive experience.


"My receiver has other surround formats, too"

Sometimes manufacturers will put their own special processing in a receiver, often called Digital Signal Processing (DSP), in addition to the formats described above. Many home theater receivers use Digital Signal Processing to create soundfields — simulated acoustic environments, like a concert hall or stadium — and for precise steering of multichannel soundtrack information. This feature may go by different names, depending on the manufacturer. Check your owner's manual for details about your receiver's DSP modes.

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