About Home Theater Surround Sound Formats

Home Theater Surround Sound Formats

When people talk about surround sound, you hear a lot about Dolby® Digital — and it is, without a doubt, one of the most popular formats out there. Yet there a few other surround formats on the market, too — from Dolby Pro Logic™ (Dolby Digital's simple but effective predecessor), to Dolby Pro Logic II (a recent advance on the technology of Pro Logic), to DTS®, THX Surround EX™, and more. Read on to understand how each format works, and what they have to offer.

Dolby Pro Logic

Dolby Pro Logic was the home theater sound standard for years, and is still a very common form of Dolby Surround. It provides four channels of sound: three full-range channels which play through the front right, front left, and center speakers, and a fourth channel of limited-bandwidth sound which is shared by two surround speakers.

Dolby Pro Logic sound starts as four channels and is encoded as two channels for transfer. This encoded signal is easily carried via videotape, satellite, or stereo TV broadcast. During playback, your receiver converts the Dolby-encoded signal back to 4 channels.

Dolby Pro Logic is a matrixed multichannel system, which means that some channels are actually derived from other channels. In this case, center channel sounds are created from parts of the front right and left channels. The single matrixed surround channel is split and sent to the surround speakers, so both surround speakers play the same sounds.

Dolby Digital

Dolby® Digital has quickly established itself as today's preferred audio format. Although Dolby Digital, strictly speaking, is a particular 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 Dolby Pro Logic, Dolby Digital 5.1-channel audio is a discrete multichannel surround sound system. Discrete means that the sound information contained in each of the six available channels is distinct and independent from the others. These six channels are described as a "5.1-channel" system, because there are five full-bandwidth channels with 3-20,000 Hz frequency range for front, center, and surround, plus one "low frequency effects" (LFE) subwoofer channel devoted to frequencies from 3-120 Hz.

Dolby Digital features stereo surround channels: you can hear separate off-screen sounds to the left rear and right rear simultaneously! And with five full-bandwidth channels and a dedicated subwoofer channel, Dolby Digital is capable of delivering much more deep bass than Pro Logic. The discrete nature of Dolby Digital means that sounds can be placed much more precisely, for improved dialogue clarity, soundstage size, spaciousness, and realism.

Dolby Digital uses data compression to reduce the amount of disc and broadcast space required by 6 discrete channels of sound. Because of its efficiency and high sound quality, Dolby Digital has been chosen as the multichannel digital audio format for DVDs, high-end laserdiscs, and HDTV (High Definition Television) broadcasts.

There are already nearly 2,000 "Region 1" coded DVD titles which feature "5.1-channel" Dolby Digital audio (check the disc packaging for "Dolby Digital 5.1"). To experience the Dolby Digital sound on these discs you need a DVD player, a Dolby Digital receiver (or preamp/amp/decoder combination — another option is a DVD player with Dolby Digital decoding built in), and speakers.

Every Dolby Digital receiver also includes Dolby Pro Logic decoding, so you'll still be able to enjoy those thousands of VHS movies and broadcast TV shows with Dolby Surround sound!

THX Surround EX

THX Surround EX isn't actually a new format. It is instead a collaboration between THX and Dolby Labs designed to give surround sound improved directionality. THX adds another one or two "back surround" speakers to the speakers already present in a 5.1-channel system. However, those added channels, rather than being discrete, simply share a matrixed channel of sound composed of information from the regular surround channels.

Currently, only a few DVDs are encoded in THX Surround EX, and have that extra channel of surround information ready to go. However, you can play a regular Dolby Digital 5.1-channel DVD, and a THX Surround EX decoder will simulate 6.1- or 7.1-channel surround by processing the audio information in the regular surround channels and sending it to your back surround speaker(s).

Dolby Pro Logic II

Dolby Pro Logic II, on the other hand, improves on original Dolby Pro Logic with special circuitry that creates a convincing 5.1-channel experience with stereo, full-bandwidth surround channels! And yet Dolby Pro Logic II is using the same Dolby Surround-encoded and stereo sources that Dolby Pro Logic uses. Receivers with Dolby Pro Logic II will give extra intensity to the 12,000 VHS movies with Dolby Surround and the limitless TV broadcasts and music available as stereo sound. Yet even this format is only a supplement to the impressive 5.1-, 6.1- and 7.1-channel surround sound available from digital sources.

DTS (Digital Theater Systems)

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. The down side is that most DTS DVDs don't have as much room for extra features (like commentaries, foreign languages, and multiple versions of the movie). Also, there are very few DTS DVDs or CDs available, compared to the number of Dolby Digital discs. The good news? Most Dolby Digital receivers also include a DTS decoder. As a result, you can enjoy Dolby Digital and DTS DVDs, without spending a lot of extra money.


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. That additional channel may be played through one or two speakers. Because that sixth channel of full-bandwidth sound is discrete, rather than matrixed, some consider DTS ES an improvement on THX Surround EX. Currently, only a few DVDs are encoded with 6.1-channel DTS ES sound.