Surround Sound: The Audio Side of Home Theater

Everything you need to know about surround sound formats and technologies

Ever since stereophonic sound became popular in the 1950s, the race has been on to create the ultimate home listening experience. Even as far back as the 1930s, experiments with surround sound were being conducted. In 1940, Walt Disney incorporated his innovative Fantasound surround sound technology to immerse the audience in the visual and audio sensations of his animation achievement, Fantasia.

Although Fantasound and other early experiments in surround sound technology could not be duplicated in the home environment, that didn't limit the quest by recording engineers for both music and film to develop processes that would eventually result in the surround sound formats that are enjoyed in home theaters all around the world today.

What Is Monophonic Sound?

Monophonic (mono) sound is a single-channel, unidirectional type of sound reproduction. All elements of the sound recording are directed using one amplifier and speaker combination. No matter where you stand in a room, you hear all the aspects of the sound equally (except for room acoustic variations).

To the ear, all the sound elements, voice, instruments, and effects appear to originate from the same point in space. It is as if everything is "funneled" to a single point "in front" of the listener. If you connect two speakers to a monophonic amplifier, the sound will appear to originate at a point equidistant between the two speakers, creating a "phantom" channel.

Speaker placement
Harman Kardon

What Is Stereophonic Sound?

Stereophonic (stereo) sound is a more open type of sound reproduction that lets the listener experience the correct sound staging of a performance. The left and right speaker channels give the impression of sound coming from all directions around the listener, rather than just a single point.

The Stereophonic Process

The main aspect of stereophonic sound is the division of sounds across two channels. The recorded sounds are mixed so that some elements are channeled to the left and right parts of the soundstage.

One positive result of stereo sound is that listeners experience the correct sound staging of symphony orchestra recordings, where various instruments naturally emanate from different parts of the stage. However, monophonic elements are often still included. By mixing the sound from a lead vocalist into both channels, the vocalist appears to be singing from the "phantom" center channel, between the left and right channels.

Limitations of Stereo Sound

Stereophonic Sound was a breakthrough for consumers of the 1950s and 1960s, but it does have limitations. Back then, some recordings resulted in a "ping-pong" effect in which the mixing emphasized the difference in the left and right channels too much, with not enough mixing of elements in the "phantom" center channel.

Also, even though the sound was more realistic, the lack of ambiance information, such as acoustics or other elements, left stereophonic sound with a "wall effect," in which everything hit you from the front and lacked the natural sound of back wall reflections or other acoustic elements.

Four-Channel Discrete Sound

Two developments occurred in the late 1960s and early 1970s that attempted to address limitations of stereo: four-channel discrete and quadraphonic sound.

With four-channel discrete, four identical amplifiers (or two stereo amps) are needed to reproduce a sound. While this made for rich and impressive sound reproduction, it was costly in the days of tubes and transistors rather than integrated circuits and chips.

Also, such sound reproduction was only available through broadcast means—two FM stations, each broadcasting two channels of a program simultaneously. That means you would need two tuners to receive it in full, as well as four-channel reel-to-reel audio decks, which were also expensive.

In addition, vinyl LPs and turntables could not handle playback of four-channel discrete recordings. Although several interesting musical performances were simulcast using this technology (with a co-operating TV station broadcasting the video portion), the whole set-up was too cumbersome for the average consumer.

Quadraphonic Sound: A More Realistic Approach

The quadraphonic format consisted of matrix encoding of four channels of information within a two-channel recording. The practical result is that ambient or effects sounds could be embedded in a two-channel recording that could be retrieved by a normal phono stylus and passed through to a receiver or amplifier with a quadraphonic decoder.

In essence, quad was the forerunner of today's Dolby Surround. If you own any old quad equipment, they can decode most analog Dolby Surround signals. Although quad promised to bring affordable surround sound to the home environment, the need to buy new amplifiers, receivers, and speakers—not to mention the lack of standardization among hardware and software makers—led to quad's decline before it could get a foothold.

The Emergence of Dolby Surround

In the mid-1970's Dolby Labs—with breakthrough film soundtracks such as Tommy, Star Wars, and Close Encounters of the Third Kind—unveiled a new surround sound process that was more easily adaptable for home use.

With the advent of the HiFi stereo VCR and stereo TV broadcasting of the 1980s, there was an additional avenue for surround sound to appeal: home theater. Up to that point, listening to the sound portion of a TV broadcast or VCR tape was like listening to a tabletop AM radio.

Dolby Surround Sound: Practical for the Home

With the ability to encode the same surround information into a two-channel signal encoded in the original movie or TV soundtrack, software and hardware manufacturers had a new incentive to make affordable surround sound components. Add-on Dolby Surround processors became available for those that already owned stereo-only receivers.

As the popularity of this experience reached into more and more homes, more affordable Dolby Surround sound receivers and amplifiers became available. Eventually, this made surround sound a permanent fixture of the home entertainment experience.

Dolby Surround Basics

The Dolby Surround process involves encoding four channels of information: front left, center, front right, and rear surround into a two-channel signal. A decoding chip decodes the four channels and sends them to the corresponding destination. (The center channel is derived from the equal balance of the left/right channels.)

The result of Dolby Surround mixing is a more balanced listening environment. The main sounds derive from the left and right channels, the vocal or dialogue emanates from the center phantom channel and the ambiance or effects information from behind the listener.

In musical recordings encoded with this process, the sound has a more natural feel, with better acoustical cues. In movie soundtracks, the sensation of sounds moving from front to rear and left to right adds more realism to the experience by placing the viewer in the middle of the action.

The Limits of Dolby Surround

Dolby Surround has its limitations, however. With the rear channel being basically passive, it lacks precise directionality. Also, the overall separation between channels is less than a typical stereo recording.

Dolby Pro Logic

Dolby Pro Logic addresses the limitations of standard Dolby Surround by adding firmware and hardware elements to the decoding chip that emphasizes directional cues. In other words, the decoding chip adds emphasis to directional sounds by increasing the output of the directional sounds in their respective channels.

Although not as important in music recordings, this process is effective for film soundtracks. With greater separation between channels, it adds more accuracy to sound effects such as explosions, gunfire, airplanes, and other sounds. In addition, Dolby Pro Logic extracts a dedicated center channel that more accurately centers the dialogue. (For full effect, this requires a center channel speaker.)

The Limits of Dolby Pro-logic

Although Dolby Pro-Logic is an excellent refinement of Dolby Surround, its effects are derived strictly from the reproduction process. Even though the rear surround channel employs two speakers, they still pass a mono signal, limiting rear-to-front and side-to-front motion and sound placement cues.

Dolby Digital

Dolby Digital is often referred to as a 5.1 channel system. However, it must be noted that the term "Dolby Digital" refers to the digital encoding of the audio signal, not how many channels it has. In other words, Dolby Digital can be monophonic, 2-channel, 4-channel, 5.1 channels, or 6.1 channels. However, in its most common applications, Dolby Digital 5.1 and 6.1 are simply referred to as "Dolby Digital."

The Benefits of Dolby Digital 5.1

Dolby Digital 5.1 adds accuracy and flexibility by adding stereo rear surround channels. It allows sounds to emanate from more directions, as well as a dedicated subwoofer channel to provide more emphasis on bass-heavy or low-frequency effects. The subwoofer channel is where the .1 designation comes from.

Also, unlike Dolby Pro Logic, which requires a rear channel of only minimal power and limited frequency response, Dolby Digital encoding and decoding require the same power output and frequency range as the main channels.

Dolby Digital encoding began on laserdiscs and migrated to DVD and satellite programming, solidifying this format in the marketplace. Since Dolby Digital involves its own encoding process, you need a Dolby Digital receiver or amplifier to decode the signal accurately. The signal is transferred from a component, such as a DVD player, through a digital optical or coaxial connection.

Dolby Digital EX

Dolby Digital EX is based on the technology already developed for Dolby Digital 5.1. This process adds a third surround channel that is placed directly behind the listener.

In other words, the listener has both a front center channel and, with Dolby Digital EX, a rear center channel. If you are losing count, the channels are labeled: left front, center, right front, surround left, surround right, subwoofer, with a surround back center (6.1) or surround back left and surround back right. This requires another amplifier and a special decoder in the A/V surround receiver.

What Is the Benefit of Dolby Digital EX?

In Dolby Digital, much of the surround sound effects move toward the listener from the front or sides. However, the sound loses some directionality as it moves along the sides to the rear, making it difficult to impart a precise directional sense of sounds moving or panning across the room.

By placing a new channel directly behind the listener, panning and positioning of sounds emanating from the sides to the rear are much more precise. Also, it is possible to situate rear sounds and effects more precisely with the additional rear channel. This has the effect of placing the listener in the center of the action.

Dolby Digital EX Compatibility

Dolby Digital EX is completely compatible with Dolby Digital 5.1. Since the Surround EX signals are matrixed within the Dolby Digital 5.1 signal, software titles encoded with EX can still be played on existing DVD players with Dolby Digital outputs and decoded in 5.1 on existing Dolby Digital receivers.

Although you may end up buying new EX-encoded versions of the films in your collection, you can still play your current DVDs through a 6.1 channel receiver. You will also be able to play your new EX-encoded discs through a 5.1 channel receiver, which will retain the new information with the current 5.1 surround scheme.

Dolby Pro Logic II and Dolby Pro Logic IIx

Although the previously outlined Dolby surround sound formats are designed to decode sound already encoded on DVDs or other material, there are thousands of CDs, VHS tapes, laserdiscs, and television broadcasts that contain simple analog two-channel stereo or Dolby Surround encoding.

Also, with surround schemes such as Dolby Digital and Dolby Digital-EX primarily designed for movie viewing, there is a lack of an effective surround process for music listening. Many audiophiles reject much of the surround sound schemes, including the new SACD (Super Audio CD) and DVD-Audio multi-channel audio formats, in favor of traditional two-channel stereo playback.

Some manufacturers, such as Yamaha, have developed sound enhancement technologies (referred to as DSP—Digital Soundfield Processing) that can place the source material in a virtual sound environment, such as a jazz club, concert hall, or stadium, but cannot "convert" two or four-channel material into a 5.1 format.

The Benefits of Dolby Pro Logic II Audio Processing

With this in mind, Dolby Labs has come to the rescue with an enhancement to its original Dolby Pro-Logic technology that can create a "simulated" 5.1 channel surround environment from a 4-channel Dolby Surround signal (dubbed Pro Logic II). Although not a discrete format, such as Dolby Digital 5.1 or DTS, in which each channel goes through its own encoding/decoding process, Pro Logic II effectively uses matrixing to deliver an adequate 5.1 representation of a film or music soundtrack.

With advancements in technology since the original Pro-Logic scheme was developed over 10 years ago, channel separation is more distinct, giving Pro Logic II the character of a discrete 5.1 channel scheme.

Extracting Surround Sound From Stereo Sources

Another benefit of Dolby Pro Logic II is adequately creating a surround sound listening experience from two-channel stereo music recordings. We have been less than satisfied trying to listen to two-channel music recordings in surround sound, using standard Pro Logic. Vocal balance, instrument placement, and transient sounds always seem to be somewhat unbalanced.

Still, there are many CDs that are Dolby Surround or DTS-encoded. Some are mixed for surround listening, but the majority are not and could benefit from the application of the Dolby Pro Logic II enhancement.

Dolby Pro Logic II also has several settings that allow the listener to adjust the soundstage to suit specific tastes. These settings are:

  • Dimension control: Allows users to adjust the soundstage either towards the front or towards the rear.
  • Center Width Control: Allows variable adjustment of the center image so it may be heard only from the center speaker, only from the left/right speakers, or through combinations of all three front speakers.
  • Panorama Mode: Extends the front stereo image to include the surround speakers for a wraparound effect.

A final advantage of a Pro Logic II decoder is that it can also perform as a "regular" 4-channel Pro-Logic decoder. In essence, that means receivers with Pro Logic decoders can also include Pro Logic II decoders, giving the consumer more flexibility without the expense of two different Pro Logic decoders in the same unit.

Dolby Pro Logic IIx

A more recent variant of Dolby Pro Logic II is Dolby Pro Logic IIx, which expands the extracting capabilities of Dolby Pro Logic II, including its preference settings, to 6.1 or 7.1 channels of Dolby Pro Logic IIx-equipped receivers and preamps.

Dolby Pro Logic IIx serves to deliver the listening experience to a greater number of channels without having to remix or reissue the original source material. This makes your record and CD collection easily adaptable to the latest listening environments.

Dolby Prologic IIz

Dolby Prologic IIz processing is an enhancement that extends surround sound vertically. Dolby Prologic IIz offers the option of adding two more front speakers above the left and right main speakers. This feature adds a "vertical" or overhead component to the surround sound field, which is great for rain, helicopter, or plane flyover effects. Dolby Prologic IIz can be added to either a 5.1 or 7.1 channel setup.

Yamaha offers a similar technology on some of its home theater receivers called Presence.

Dolby Virtual Speaker

Although the trend toward surround sound relies on adding additional channels and speakers, the requirement of multiple speakers around an entire room is not always practical. With that in mind, Dolby Labs has developed a way to create a reasonably accurate surround experience that gives the illusion that you are listening to a complete surround speaker system while using only two speakers and a subwoofer.

When used with standard stereo sources, Dolby Virtual Speaker creates a broader sound stage. However, when stereo sources are combined with Dolby Prologic II or Dolby Digital-encoded DVDs are played, Dolby Virtual speaker creates a 5.1 channel image using technology that takes into account sound reflection and how humans hear sound in a natural environment, enabling the signal to be reproduced without needing five or six speakers.

Audyssey DSX (or DSX 2)

Audyssey, a company that develops and markets automatic speaker room equalization and correction software, has developed its own immersive surround sound technology: DSX (Dynamic Surround Expansion).

DSX adds front vertical-height speakers, similar to Prologic IIz, but also incorporates the addition of left/right wide speakers positioned between the front left and right and surround left and right speakers. For a more detailed explanation and speaker setup illustrations, check out our Audyssey DSX page.


DTS is another well-known player in surround sound and has adapted its surround sound process for home use. Basic DTS is a 5.1 system that's like Dolby Digital 5.1, but since DTS uses less compression in the encoding process, many feel that DTS has a better result on the listening end. While Dolby Digital is mainly intended for the movie experience, DTS is also used to mix and reproduce music recordings.


DTS has come up with its own 6.1 channel systems, in competition with Dolby Digital EX, referred to as DTS-ES Matrix and DTS-ES 6.1 Discrete. DTS-ES Matrix can create a center rear channel from existing DTS 5.1 encoded material, while DTS-ES Discrete requires that the software have a DTS-ES Discrete soundtrack. As with Dolby Digital EX, DTS-ES and DTS-ES 6.1 Discrete formats are backward compatible with 5.1 channel DTS Receivers and DTS encoded DVDs.

DTS Neo:6

In addition to DTS 5.1 and DTS-ES Matrix and Discrete 6.1 channel formats, DTS also offers DTS Neo:6. DTS Neo:6 functions similarly to Dolby Prologic II and IIx. With receivers and preamps with DTS Neo:6 decoders, it will extract a 6.1 channel surround field from existing analog two-channel material.


The next step that DTS has taken is to introduce its 11.1 channel Neo:X format. DTS Neo:X takes cues present in either 5.1 or 7.1 channel soundtracks and creates height and wide channels, enabling a more enveloping "3D" sound. To experience the maximum benefit of DTS Neo:X processing, it is best to have 11 speakers, with 11 channels of amplification, and a subwoofer. However, DTS Neo:X can be modified to work with a 9.1 or 9.2 channel configuration.

DTS Surround Sensation

Surround Sensation creates phantom center, left, right, and surround channels within a two-speaker or stereo headphone setup. It can take any 5.1 channel input source and recreate a surround sound experience with two speakers. In addition, surround sensation can also expand two-channel compressed audio signals (such as MP3) for a more surround-like listening experience.

SRS/DTS Tru-Surround and Tru-Surround XT

SRS Labs is another company that offers innovative technologies to enhance the home theater experience. (SRS is now a part of DTS.)

Tru-Surround can take multi-channel encoded sources, such as Dolby Digital, and reproduce the surround effect by using two speakers. The result is not as impressive as true Dolby Digital 5.1. The front and side surround effects are impressive, but the rear surround effects fall a little short, with the sense they are coming from just to the rear of your head rather than from the back of the room.

However, with many consumers reluctant to fill their room with six or seven loudspeakers, Tru-Surround and Tru-SurroundXT offer the ability to enjoy 5.1 channel sound within a normally limited two-channel listening environment.

SRS/DTS Circle Surround and Circle Surround II

Circle Surround approaches surround sound in a unique way. While Dolby Digital and DTS aim for a precise directional experience with specific sounds emanating from specific speakers, Circle Surround emphasizes sound immersion. To accomplish this, a normal 5.1 audio source is encoded down to two channels, then re-decoded back into 5.1 channels and redistributed to the five speakers (plus the subwoofer). It is done in such a way as to create a more immersible sound without losing the directionality of the original 5.1 channel source material.

The results are more impressive than those of Tru-Surround or Tru-Surround XT.

First, panning sounds such as flying planes, speeding cars, and trains sound even as they cross the sound stage. Often in DD and DTS, panning sounds will "dip" in intensity as they move from one speaker to the next.

Rear-to-front and front-to-rear sounds flow smoother. Environmental sounds, such as thunder, rain, wind, and waves fill the sound field much better than DD or DTS. For example, instead of hearing the rain coming from several directions, the points in the sound field between those directions are filled out, thus placing you within the rainstorm, not just listening to it.

Circle Surround enhances Dolby Digital and similar surround sound source material without degrading the original intent of the surround sound mix.

Circle Surround II takes this concept further by adding an additional rear center channel, thus providing an anchor for sounds emanating from directly behind the listener.

Headphone Surround Sound

Surround sound is not limited to large multi-channel systems. It can also be applied to headphone listening. SRS Labs, Dolby Labs, and Yamaha have incorporated surround sound technology with the headphone listening environment.

Headphone surround options include Dolby Headphone, CS Headphone, Yamaha Silent Cinema, Smyth Research, and DTS Headphone:X.

Typically, when listening to audio (either music or movies), the sound seems to originate from within your head, which is unnatural. Dolby Headphone, Yamaha Silent Cinema, and Smyth Research employ technology that gives the listener an enveloping sound and removes it from the listener's headspace and places it in the front and side spaces around the head, which is more like listening to a regular speaker-based surround sound system.

In another development, DTS has developed DTS Headphone:X that can provide up to an 11.1 channel surround sound listening experience using any pair of headphones plugged into a listening device, such as a smartphone, portable media player, or home theater receiver equipped with DTS Headphone:X processing.

High Definition Surround Sound

With the introduction of Blu-ray Disc and HDMI, the development of high definition (HD) surround sound formats in both DTS (in the form of both DTS-HD and DTS-HD Master Audio) and Dolby Digital (in the form of Dolby Digital Plus and Dolby TrueHD) provides extended accuracy and realism.

High-definition surround sound technologies include Dolby Digital Plus, Dolby TrueHD, and DTS-HD Master Audio.

The increased storage capacity of Blu-ray and wider bandwidth transfer capabilities of HDMI (which is required for accessing Dolby Digital Plus, Dolby TrueHD, and DTS-HD) have allowed for true, discreet audio reproduction for up to 7.1 channels of surround sound. And it is still backward compatible with older 5.1 channel surround sound formats and audio/video components.

Dolby Atmos and More

Although building on the foundation established by previous Dolby Surround Sound formats, Dolby Atmos frees sound mixers and listeners from the limitations of speakers and channels by emphasizing where sound needs to be placed within a three-dimensional environment. For more details on Dolby Atmos technology, applications, and products, refer to Dolby Atmos: From the Cinema to Your Home Theater

Additional advanced surround sound formats include:

Conclusion—For Now...

Today's surround sound experience is the result of decades of evolution. The surround sound experience is now easily accessible, practical, and affordable for home theater enthusiasts, with more to come in the future.

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