How Surround Sound Works

There are many ways to make and present a sound recording. The simplest method, and the one used in the earliest sound movies, is called monaural or simply mono. Mono means that all the sound is recorded onto one audio track or channel (a single spiraled groove in a record, for example, or a single magnetic track on tape), which is typically played on one speaker.

­ Two-channel recordings, in which sound is played on speakers on either side of the listener, are often referred to as stereo. This isn't entirely accurate, as stereo (or stereophonic) actual refers to a wider range of multi-channel recordings. Two-channel sound is the standard format for home stereo receivers, television and FM radio broadcasts. The simplest two-channel recordings, known as binaural recordings, are produced with two microphones set up at a live event (a concert for example) to take the place of a human's two ears. When you listen to these two channels on separate speakers, it recreates the experience of being present at the event.

Surround recordings take this idea a step further, adding more audio channels so sound comes from three or more directions. While the term "surround sound" technically refers to specific multi-channel systems designed by Dolby Laboratories, it is more commonly used as a generic term for theater and home theater multi-channel sound systems. In this article, we'll use it in this generic sense.

There are special microphones that will record surround sound (by picking up sound in three or more directions), but this is not the standard way to produce a surround soundtrack. Almost all movie surround soundtracks are created in a mixing studio. Sound editors and mixers take a number of different audio recordings -- dialogue recorded on the movie set, sound effects recorded in a dubbing studio or created on a computer, a musical score -- and decide which audio channel or channels to put them on.

In the next section, we'll learn a little bit about how surround sound was created and see how it was configured in older theaters.

Over the years, there have been many different approaches to surround sound. Walt Disney's "Fantasia" (1941), one of the earliest surround-sound movies, immersed the audiences in classical music. Disney sound engineer William Garity took separate recordings of each orchestra section and mixed them to produce four distinct audio tracks, which were recorded as optical tracks on a separate reel of film.

The four tracks drove different speakers positioned around the theater. In an equipped theater, the music seemed to move around the auditorium, an effect achieved by sound panning. Panning involves fading a sound (a violin melody, for example) from one audio channel while building it on another.

To show "Fantasia" in surround sound, a theater needed an additional projector to play just the soundtrack, as well as an expensive receiver and speaker assembly. (Check out Film Sound History for a thorough history of how "Fantasound" came about.)

This surround-sound system didn't catch on (the necessary equipment was prohibitively expensive), but by the late 1950s, many Hollywood movies were encoded with simpler multi-channel formats. Several different theater setups emerged in this era, including the famous Cinerama and Cinemascope, but most of them used the same basic sound technology. As a whole, these systems were referred to as stereophonic sound, or simply theater stereo.

Stereophonic sound used four or more analog magnetic audio tracks around the edges of the film. Magnetic tracks could not produce as clear a sound as the conventional optical audio tracks, and they tended to fade over time, but they took up a lot less space on the film. The standard film format did not have enough room for more than two optical tracks, but it was possible to squeeze as many as six magnetic tracks around the film frame. (See How Movie Sound Works to find out how optical and magnetic soundtracks work.)

In the stereophonic system, three to five channels drove speakers behind the movie screen. The popular four-channel system included one channel driving a speaker on the left, one channel driving a speaker on the right, one channel driving a center speaker and one channel driving surround speakers along the sides and back of the theater. Some systems boasted five separate channels behind the screen and one surround channel.

In these movies, most of the sound is recorded on the front channels so that the words seem to come from the screen. When an actor speaks on the left side of the screen, the dialogue sound comes from the left speakers. When an actor speaks on the right side, the sound comes from the right speakers. Most dialogue is also channeled to the center speakers, which serves to anchor, or focus, the sound on the screen. The rear track (or tracks) are typically reserved for "effect sounds," such as ambient background noise or a voice coming from off-screen.

In the 1970s, Dolby Laboratories introduced a new sound format based on this same configuration. In the next section, we'll see what made this system the new standard for theater sound.

Like stereophonic sound, the original Dolby Stereo^® had three front channels and a surround-sound channel. But instead of using magnetic tracks, it reverted back to the superior optical track technology to allow for clearer sound playback. Dolby stereo also used an advanced noise-reduction process, which improved sound quality further. (See How Movie Sound Works for details.) Today, Dolby Stereo is the analog sound standard, thanks to its superior sound quality and relatively simple installation.

The heightened sound quality of Dolby Stereo led moviemakers to make more extensive use of the surround channel. George Lucas' "Star Wars," one of the first films encoded for Dolby Stereo, used surround sound to heighten its epic space battle scenes. By gradually panning the sound of fighter ships from the front channels to the rear channel, sound engineers made it seem like the ships were flying off screen over the audience.

Later movies followed the "Star Wars" model, using the surround track to create fantastic effects, as well as fill in background noise to establish a scene's setting. In later versions of the surround-sound system, theater owners could hook up a subwoofer to handle extremely low-frequency sounds (a crossover unit can separate out these sounds from both audio tracks). Many moviemakers use the subwoofer to create a powerful rumbling in the theater, shaking the audience when there is an explosion or earthquake on-screen. The subwoofer channel in both analog and digital surround-sound systems is sometimes called the low frequency effects (LFE) channel.

In 1982, Dolby launched Dolby Surround^®, a version of Dolby Stereo for home entertainment systems. Dolby Surround reproduces the effect of Dolby Stereo in the theater, but it works a little bit differently. The audio channels are encoded as magnetic tracks on video tape or broadcast as a television signal, rather than put down as optical tracks. The speakers are set up in the same basic way as in a theater, except the original home Dolby system only had three channels --- left speaker, right speaker and rear speaker. In 1987, Dolby introduced Dolby Pro Logic®, which had an additional channel for a front central speaker. (See How Home Theater Works for more information.)

The real innovation of Dolby Stereo is how so much audio information is squeezed into a small space on the film. When Dolby engineers started working on the new format, they figured out they would only be able to fit two optical tracks in the available space. In order to allow for four separate audio channels, they developed a special 4-2-4 processing system. In this system, originally used in the quadraphonic home stereo recordings of the early 1970s, four channels of audio information are encoded into two tracks. In the next section, we'll find out about the clever trick that makes this possible.

The basic idea of a 4-2-4 processing system is to derive four streams of information from two streams of information. Essentially, the four streams of information are:

The first two channels are fairly straightforward. The A stream feeds the left speaker, and the B stream feeds the right speaker. But the "same" and "difference" channels are a little more complex. To understand how this works, you need to know a little bit about how speakers produce sound.

A basic speaker is built around an electromagnet, a metal cylinder with a wire coiled around it. The electromagnet is surrounded by a permanent natural magnet. When you send an electrical current through the electromagnet, it becomes magnetized and acts like a natural magnet, with a north pole and a south pole. The coiled wire in the electromagnet is connected to the (+) speaker wire on one end and the (-) speaker wire on the other end. The audio amplifier is constantly changing the direction of the current so that the orientation of the poles keeps switching.

Changing the orientation of the poles shifts the attraction between the electromagnet and the surrounding natural magnet. This causes the electromagnet to move back and forth. As the electromagnet moves, it pushes and pulls a speaker cone, which rapidly pushes out air and then pulls back in. This movement of air particles produces the sounds we hear. (For more information, see How Speakers Work.)

An audio signal, then, is just a fluctuating electrical current. When the current fluctuates one way, the speaker cone moves in; when it fluctuates the other way, the cone moves out. This signal can be represented as an oscillating wave. The particular sound produced depends on how rapidly and how far the cone moves, which is dictated by the fluctuation pattern in the electrical current.

In a surround-sound setup, the signal for the center channel is recorded on both the A stream and the B stream. The center signals on both streams are identical in amplitude and frequency, and they are synchronized exactly.

A surround-sound decoder that supports a central channel will pick out the identical signals in the A stream and B stream based on their pattern and amplitude. In a surround setup with no center speaker, the perfectly balanced center signals will create a "phantom speaker" (the illusion of a speaker) directly in between the left and right speakers.

The sound signal for the surround channel is also recorded on stream A and stream B, but the identical signals in each stream are out of phase with each other. Instead of playing in synchrony, they are shifted in time in both audio streams. The result is that the two signals work opposite one another: When the surround signal in stream A tells the left speaker cone to move out, the signal in stream B tells the right speaker cone to move in. Because of this, the surround signal information coming from the front left and front right speakers largely cancels itself out, and you don't hear it.

A surround-sound decoder receives both stream A and stream B and shifts them relative to one another so the surround signals are in phase again. With this shift, the right, left and center signals are all out of phase, and so tend to cancel each other out.

In addition to separating the different signals, proper surround decoders pass the audio information through different filters and noise-reduction elements to balance sound levels and reduce noise. Pro Logic decoders use active "steering" elements to control the process more precisely. Check out Dolby Surround Pro Logic Decoder: Principles Of Operation (PDF) for more information.

Lots of home audio hobbyists have figured out a way to partially unlock the surround channel using only a two-channel home stereo and an extra set of speakers. In the next section, we'll see how this bare-bones surround-sound setup works.

The easiest way to access the surround-sound channel is to get a receiver with a surround-sound decoder. The decoder recognizes the out-of-phase information and extracts it into a third channel. To balance the sound, the receiver also boosts the channel to an appropriate level, and adds a slight time delay. It is possible to access surround sound with a standard stereo receiver, however, since all the information is actually included in the left and right channels.

To do this, get a pair of rear speakers and position them to the left and right of the listener. Connect the (+) amplifier terminal for the right channel to the (+) speaker terminal on the right rear speaker and the (+) amplifier terminal for the left channel to the (+) speaker terminal for the left rear speaker. Then you connect the two (-) terminals on the rear speakers.

The stereo signals that are in phase in the front channels cancel each other out in the rear speakers: The (+) currents for left and right will arrive at the (+) and (-) terminals of each speaker at the same time, so the current won't change the electromagnet at all.

But the signals that are out of phase in the stereo channels will form an alternating current -- the current for these signals will flow out of the (+) amplifier terminal for the left channel while the (+) speaker terminal for current is flowing into the (+) amplifier terminal for the right channel. The effect is that these out-of-phase signals move the electromagnet for the rear speaker, and so control the rear sound.

To set up a simple central speaker -- one that anchors the left and right stereo speakers -- just turn on your television. If it's a mono-speaker television, it will play both stereo channels mixed together. Stereo televisions will also work decently for anchoring purposes, because both channels emanate from the area of the television.

The other piece you need in this setup is a potentiometer, a device that can apply different degrees of resistance to a current, thereby reducing the voltage in a circuit. In this surround-sound setup, the potentiometer simply acts as a volume control for the rear speakers. You can hook it up anywhere along the circuit leading to the rear speakers. For detailed instructions on setting up this sort of homemade system, check out Chris Kantack's Surround Sound Information Source.

This setup won't give you the same quality surround sound as an actual surround-sound receiver, of course. But assembling a homemade system is a great exercise for understanding how analog surround sound works in the first place.

In the 1990s, a new kind of surround sound started popping up in theaters, and since then it has been gradually eclipsing the standard 4-2-4 approach. In the next section, we'll take a look at these new digital theater sound systems.

Today, many theaters boast digital surround-sound systems. Digital sound works on a very different principle from analog sound systems.

In analog recordings, sound is encoded as a long, fluctuating stream of information. In digital recordings, sound is encoded as a series of 1s and 0s, just like a computer program. With this approach, you can encode a lot more information in a limited space, making for crisper, more precise audio tracks. (See How Analog and Digital Recording Works for details.)

Digital theater sound was introduced to the public with the release of "Jurassic Park" in 1993. "Jurassic Park" used a technology called DTS Digital Sound®, named for Digital Theater Systems, the company that patented the process.

In this sound system, six separate audio channels are encoded onto one or two CDs. The theater is equipped with a CD player and a decoder that splits these channels up and plays them on different speakers arranged throughout the theater. As in Dolby Stereo, DTS has three front sound channels and a subwoofer. But instead of a single surround channel, it has separate channels for speakers on the left side of the theater and speakers on right side of the theater.

The CD is synchronized with the picture by a special time code on the film. The code, a series of dots and dashes along the side of each frame, is read by a special optical reader mounted on the projector. The reader shines light on the film with a light-emitting diode (LED). The light that passes through the film hits a small photocell. The photocell sends pulses of current representing these flashes of light to the DTS processor. The dash pattern corresponds to a pattern encoded onto the CD. The processor makes sure the two codes are synchronized so that the sound and picture fit together. (See How Movie Sound Works for more information.)

Dolby followed suit with its own digital format, Dolby Digital®. Dolby Digital is also known as Dolby Digital 5.1® (for five audio channels and a subwoofer channel), Dolby AC-3® (for Dolby's third audio-coding design) or Dolby SR-D® (for Spectral Recording Digital). Dolby Digital has the same basic speaker arrangement as DTS, and it sounds similar, but it works on a very different system. Instead of recording audio on CDs, digital information is encoded as tiny patterns on the film in the space between the sprocket holes.

The Dolby Digital reader shines an LED through this pattern as the film passes through the projector. On the other side of the film, the light hits a charge coupled device (CCD), the same sort of light sensor used in a digital camera (see How Movie Sound Works: Dolby Digital to learn more). The CCD registers an image made of hundreds of little specks that represent 1s and hundreds of spaces between the specs that represent 0s. The Dolby Digital Processor unit interprets the digital information in this image as an audio signal.

Dolby Digital Surround EX® works the same way as Dolby Digital, but it includes an extra surround channel. The extra channel drives speakers along the rear wall of a theater. Like the front center speaker, it can be used to anchor sounds from the left and right surround channels.

The latest entry in cinema digital sound, Sony Dynamic Digital Sound® (SDDS), boasts five separate channels at the front of the theater as well as left and right surround channels, for a total of eight available channels including the subwoofer.

Like Dolby Digital, SDDS encodes digital information with a distinct pattern of light and dark areas on film. In this case, the reader includes a laser on one side of the film and an array of photocells on the other side. The laser passes light through transparent areas of the film, but not through opaque areas. The photocells that are not exposed to light pass a small current on to the processor, but the exposed photocells do not. In this way, the processor receives the digital pattern, which it interprets as a sound signal. Unlike the other digital formats, SDDS uses two identical digital tracks to allow for better error correction.

Dolby and DTS have both released home theater versions of these popular formats, and there is a SDDS Surround 7.1® system (seven audio channels and a subwoofer channel) available for consumers. While digital sound cannot be recorded on video tape or broadcast over conventional cable, it is the only way to encode information on DVD. Digital sound is also broadcast on satellite systems, as well as digital cable. Check out How Home Theater Works to learn all about these home systems.

For movie fans everywhere, surround sound has become an integral part of the theater experience. And for moviemakers, putting together the surround mix has become a crucial step in the production process. Surround sound has effectively expanded movies into three dimensions, putting the audience in the middle of the action like nothing else can.

To learn more about surround sound, including its long history and technical details about particular systems, check out the links on the next page.