The Mechanics of Surround Sound

Experiencing a movie today is vastly different than it was 70 years ago—the image quality is sharper, movies are predominantly in color, and the ticket prices have risen. However, the most significant shift is likely in the sound. Back in the 1930s, movie theater audio was limited to a single speaker or a small cluster behind the screen. Now, audiences expect sound to come from every angle, and this technology, once exclusive to cinemas, is now a common feature in home entertainment systems.

This article will dive into the surround-sound systems that have become a standard in movie theaters, as well as explore home-theater setups. We'll guide you through the basics of creating your own system at home.

What is Surround Sound?

Sound recordings can be created and presented in numerous ways. The most basic method, and the one used in the earliest sound films, is known as monaural or simply mono. Mono means that all audio is recorded on a single track or channel (for instance, a single spiral groove on a record or a single magnetic track on tape), typically played through a single speaker.

Two-channel recordings, where audio is played through speakers positioned on either side of the listener, are commonly called stereo. This term is somewhat misleading, as stereo (or stereophonic) actually refers to a broader category of multi-channel recordings. Two-channel sound is the standard for home stereo systems, television broadcasts, and FM radio stations. The simplest form of two-channel sound, known as binaural recordings, is made with two microphones placed at a live event (such as a concert) to replicate a human's two ears. Listening to this sound on separate speakers recreates the sensation of being at the event in person.

Surround recordings expand on this concept by adding multiple audio channels, so the sound emanates from three or more directions. While "surround sound" specifically refers to multi-channel systems designed by Dolby Laboratories, it is often used more broadly to describe theater and home theater sound systems. In this article, we will use the term in this general context.

There are specialized microphones designed to capture surround sound by recording audio from three or more directions. However, this is not the typical method for creating a surround soundtrack. Most movie surround soundtracks are crafted in a mixing studio. Sound editors and mixers combine various audio recordings, such as dialogue from the set, sound effects from a dubbing studio or computer-generated sounds, and the musical score, then decide which audio channel or channels to assign them to.

In the following section, we'll explore how surround sound was created and examine how it was set up in older movie theaters.

Early Surround

Throughout the years, there have been various methods for creating surround sound. One of the earliest examples is Walt Disney's "Fantasia" (1941), which used surround sound to immerse the audience in classical music. Disney's sound engineer William Garity took separate recordings from each section of the orchestra and mixed them to create four distinct audio tracks, which were recorded as optical tracks on a separate reel of film.

These four audio tracks powered different speakers placed around the theater. In a properly equipped theater, the music appeared to move throughout the auditorium, an effect achieved by sound panning. Panning involves gradually shifting a sound (such as a violin melody) from one audio channel to another while building it on the new channel.

In order to showcase "Fantasia" in surround sound, theaters needed an extra projector solely for the soundtrack, along with a costly receiver and speaker system. (For an in-depth history of the development of "Fantasound," check out Film Sound History.)

This surround-sound system didn't gain widespread use due to the high cost of the required equipment. However, by the late 1950s, many Hollywood films began incorporating simpler multi-channel formats. During this period, several theater systems emerged, including the renowned Cinerama and Cinemascope, although most of them relied on the same core sound technology. Collectively, these systems were known as stereophonic sound or simply theater stereo.

Stereophonic Sound

Stereophonic sound utilized four or more analog magnetic audio tracks placed along the edges of the film. Although magnetic tracks didn't produce as clear a sound as conventional optical audio tracks and tended to degrade over time, they occupied much less space on the film. The standard film format couldn't accommodate more than two optical tracks, but it was possible to fit as many as six magnetic tracks around the film's perimeter. (For more on how optical and magnetic soundtracks function, see How Movie Sound Works.)

In the stereophonic system, three to five channels were used to power speakers placed behind the movie screen. The widely-used four-channel system included one channel for the left speaker, one for the right speaker, one for a center speaker, and one for surround speakers along the sides and rear of the theater. Some setups featured five distinct channels behind the screen with an additional surround channel.

In these films, most of the audio is routed through the front channels to ensure that dialogue appears to come from the screen. When a character speaks from the left side of the screen, the sound originates from the left speakers. Likewise, speech from the right side is projected from the right speakers. The majority of dialogue is also sent to the center speakers, which help anchor the sound and focus it on the screen. The rear channels are generally reserved for background effects, such as ambient noise or voices from off-screen.

During the 1970s, Dolby Laboratories introduced a new sound format based on this same layout. In the next section, we’ll explore what made this system the new standard for cinematic sound.

Dynamic Dolby

Similar to stereophonic sound, the original Dolby Stereo featured three front channels and a surround-sound channel. However, instead of relying on magnetic tracks, it reverted to superior optical track technology, offering clearer sound playback. Dolby Stereo also introduced an advanced noise-reduction process, which further enhanced sound quality. (For more details, see How Movie Sound Works.) Today, Dolby Stereo stands as the analog sound standard due to its exceptional sound quality and relatively straightforward installation.

The improved sound quality provided by Dolby Stereo encouraged filmmakers to utilize the surround channel more effectively. One of the earliest films encoded in Dolby Stereo, George Lucas' "Star Wars", made remarkable use of surround sound to intensify the space battle scenes. By gradually panning the sound of the fighter ships from the front channels to the rear, the sound engineers gave the sensation that the ships were flying off the screen, sweeping over the audience.

Subsequent films followed the example set by "Star Wars", using the surround track to create captivating effects and fill the background with sounds that helped establish the scene’s atmosphere. In later versions of the surround-sound system, theater owners could connect a subwoofer to handle extremely low-frequency sounds, with a crossover unit separating these sounds from the main audio tracks. Many filmmakers use the subwoofer to produce powerful rumbles in the theater, such as shaking the audience during an explosion or earthquake on-screen. In both analog and digital surround-sound systems, the subwoofer channel is sometimes called the low frequency effects (LFE) channel.

In 1982, Dolby introduced Dolby Surround, a version of Dolby Stereo designed for home entertainment systems. Dolby Surround recreates the theater experience in the home, but with some differences. The audio channels are encoded as magnetic tracks on videotape or broadcast as a television signal, rather than being recorded as optical tracks. The speaker setup is similar to a theater's, though the original home Dolby system only had three channels — left, right, and rear speakers. In 1987, Dolby released Dolby Pro Logic®, which added an additional channel for a front central speaker. (For further details, refer to How Home Theater Works.)

The true breakthrough of Dolby Stereo lies in how it manages to fit so much audio data into a compact space on the film. When Dolby engineers began developing the new format, they realized they could only fit two optical tracks in the available space. To accommodate four distinct audio channels, they devised a unique 4-2-4 processing system. This system, initially used in the quadraphonic home stereo recordings of the early 1970s, encodes four audio channels into just two tracks. In the following section, we’ll explore the clever technique that made this possible.

Four From Two

At its core, the 4-2-4 processing system works by deriving four distinct streams of data from just two. These four streams of information are as follows:

The first two channels are quite simple to understand. The A stream sends sound to the left speaker, and the B stream sends sound to the right speaker. However, the 'same' and 'difference' channels are a bit more intricate. To grasp how this works, it's helpful to have some knowledge of how speakers generate sound.

A basic speaker consists of an electromagnet, which is a metal cylinder wrapped with wire. This electromagnet is positioned within a permanent natural magnet. When an electrical current flows through the electromagnet, it becomes magnetized, acting like a natural magnet with a distinct north pole and south pole. The coiled wire at each end of the electromagnet is connected to the (+) and (-) speaker wires. The audio amplifier continually changes the direction of the current, causing the poles to alternate.

By reversing the polarity of the poles, the attraction between the electromagnet and the surrounding natural magnet changes. This causes the electromagnet to move back and forth, which in turn pushes and pulls a speaker cone. This motion moves air in and out rapidly, creating the sound we hear. (For more details, check out How Speakers Work.)

An audio signal is essentially a varying electrical current. As the current fluctuates in one direction, the speaker cone moves inward; when the current fluctuates in the opposite direction, the cone moves outward. This signal can be visualized as an oscillating wave. The sound produced depends on how fast and how far the cone moves, which is determined by the fluctuations in the electrical current.

In a surround-sound system, the center channel's signal is recorded on both the A stream and the B stream. The center signals on these two streams have the same amplitude and frequency and are perfectly synchronized with each other.

Phase Shift

A surround-sound decoder capable of handling a center channel will extract the matching signals in both the A and B streams by recognizing their pattern and amplitude. In systems without a center speaker, these perfectly matched center signals will create a "phantom speaker", giving the illusion of a speaker positioned exactly between the left and right speakers.

The sound information for the surround channel is recorded in both streams A and B, but in this case, the signals are out of phase with each other. Instead of playing synchronously, the signals in both streams are slightly shifted in time. This causes them to cancel each other out: As the signal in stream A pushes the left speaker cone outward, the signal in stream B pulls the right speaker cone inward. Consequently, the surround signal from the front left and right speakers largely cancels out, and you don't perceive it.

A surround-sound decoder takes both streams A and B and adjusts them in relation to each other, ensuring the surround signals are once again in phase. This adjustment causes the right, left, and center signals to become out of phase, leading them to cancel each other out.

To ensure optimal sound quality, surround decoders not only separate the different signals but also pass them through various filters and noise-reduction components to balance levels and minimize unwanted noise. Pro Logic decoders go a step further by utilizing active "steering" mechanisms to fine-tune the process. For more details, refer to Dolby Surround Pro Logic Decoder: Principles Of Operation (PDF).

Many home audio enthusiasts have discovered how to unlock some of the surround channel's potential using just a basic two-channel stereo system and an additional pair of speakers. The next section will explain how this minimalist surround-sound setup operates.

Accessing the Surround Channel

The most straightforward way to tap into the surround-sound channel is by using a receiver equipped with a surround-sound decoder. This decoder identifies out-of-phase information and separates it into a third channel. To maintain a balanced sound, the receiver also adjusts the channel volume to a suitable level and introduces a slight delay. While it's possible to access surround sound with a regular stereo receiver, all the necessary information is already embedded in the left and right channels.

To implement this, you need to get a pair of rear speakers and position them on either side of the listener. Attach the (+) terminal of the amplifier for the right channel to the (+) terminal on the right rear speaker, and do the same for the left channel, connecting it to the left rear speaker. Lastly, connect the (-) terminals of both rear speakers.

Stereo signals that are in phase in the front channels will cancel each other out when they reach the rear speakers. The (+) currents from both left and right channels will arrive at the (+) and (-) terminals of each speaker simultaneously, meaning the electromagnet won't be activated.

However, signals that are out of phase in the stereo channels will generate an alternating current. In this scenario, the current will flow from the (+) terminal of the left amplifier while the (+) terminal of the right speaker draws current from the (+) terminal of the right amplifier. This causes the rear speaker's electromagnet to move, controlling the rear sound output.

To set up a basic central speaker—one that serves as the foundation between the left and right stereo speakers—simply power on your television. If your TV is mono, it will combine both stereo channels. Even stereo TVs work fairly well for this purpose, as both channels emanate from the vicinity of the television.

Another essential component for this setup is a potentiometer, a device that can adjust the resistance in a circuit, reducing the voltage. In this surround-sound arrangement, the potentiometer functions as a volume control for the rear speakers. You can place it anywhere along the circuit leading to the rear speakers. For more detailed guidance on building this type of custom system, refer to Chris Kantack's Surround Sound Information Source.

This configuration won't provide the same high-quality surround sound as a dedicated surround-sound receiver. However, constructing a homemade system is an excellent exercise to understand the principles behind analog surround sound.

During the 1990s, a new type of surround sound emerged in movie theaters, gradually replacing the traditional 4-2-4 format. In the next section, we’ll explore these new digital theater sound systems.

Digital Domain: DTS

Today, many movie theaters are equipped with digital surround-sound systems. Digital audio operates on a fundamentally different principle compared to analog systems.

In analog recordings, sound is represented as a continuous, fluctuating stream of data. In digital recordings, sound is represented as a series of 1s and 0s, similar to a computer program. This method allows for encoding much more data within a limited space, resulting in clearer, more accurate audio. (Refer to How Analog and Digital Recording Works for further details.)

Digital theater sound was first introduced to the masses with the release of "Jurassic Park" in 1993. The film used a technology known as DTS Digital Sound®, developed by Digital Theater Systems, the company behind the patent for this process.

In this system, six distinct audio channels are encoded onto one or two CDs. The theater is outfitted with a CD player and a decoder that separates these channels and sends them to various speakers placed around the theater. Like Dolby Stereo, DTS includes three front channels and a subwoofer. However, instead of a single surround channel, it features individual channels for speakers located on the left and right sides of the theater.

The CD is synchronized with the film through a special time code embedded on the movie. This code, consisting of a series of dots and dashes along the edge of each frame, is detected by an optical reader attached to the projector. The projector shines light through the film using a light-emitting diode (LED), and the light that passes through the film is captured by a small photocell. The photocell sends pulses corresponding to these flashes of light to the DTS processor. The dash pattern in the code matches a similar pattern on the CD, and the processor ensures both codes are in sync, aligning sound and image. (For more details, see How Movie Sound Works.)

Digital Domain: Dolby Digital

Dolby followed with its own digital format, known as Dolby Digital®. This system is also referred to as Dolby Digital 5.1® (referring to five audio channels plus a subwoofer channel), Dolby AC-3® (for Dolby’s third audio-coding method), or Dolby SR-D® (for Spectral Recording Digital). Although the basic speaker setup in Dolby Digital mirrors that of DTS and produces similar sound, it operates on a distinct system. Rather than storing audio on CDs, digital data is encoded directly into patterns on the film itself, placed between the sprocket holes.

As the film moves through the projector, the Dolby Digital reader shines an LED through the encoded pattern. On the opposite side, the light hits a charge-coupled device (CCD), which is a light sensor also used in digital cameras (for more on this, see How Movie Sound Works: Dolby Digital). The CCD captures a pattern made up of hundreds of tiny spots, representing 1s, with gaps between them representing 0s. The Dolby Digital Processor decodes this pattern into an audio signal.

Dolby Digital Surround EX® operates similarly to Dolby Digital, with the addition of an extra surround channel. This extra channel powers speakers placed along the rear wall of a theater. Much like the central front speaker, it serves to anchor sounds from both the left and right surround channels.

Digital Domain: SDDS

The latest innovation in digital cinema sound is Sony Dynamic Digital Sound® (SDDS), which provides five distinct audio channels at the front of the theater, along with left and right surround channels, totaling eight channels, including the subwoofer.

Similar to Dolby Digital, SDDS encodes digital data through a unique pattern of light and dark areas on the film. However, the reader for SDDS uses a laser on one side of the film and an array of photocells on the opposite side. The laser shines light through transparent sections of the film, while the opaque sections block the light. The photocells that receive no light send a small current to the processor, while those that do not are inactive. This allows the processor to interpret the pattern as a sound signal. SDDS distinguishes itself by utilizing two identical digital tracks for enhanced error correction.

Both Dolby and DTS have introduced home theater versions of their respective formats, and there is also a SDDS Surround 7.1® system (seven audio channels plus a subwoofer channel) available for home use. While digital sound cannot be recorded on video tapes or transmitted through traditional cable broadcasts, it is the exclusive format for encoding information on DVD. Digital sound can also be broadcast via satellite and digital cable. To learn more about these home systems, see How Home Theater Works.

For movie enthusiasts around the world, surround sound has become an essential part of the cinematic experience. For filmmakers, creating the surround sound mix is now a pivotal stage in the production process. This immersive sound technology has transformed films into a three-dimensional experience, placing audiences right in the heart of the action like never before.