How is surround sound achieved?

How is surround sound achieved?

what is surround sound

There are many ways to produce and present recordings. The simplest method, and the one used in the earliest sound films, is called mono. Mono means that all the sound is recorded onto a single track or channel (for example, a spiral groove on a record, or a track on a tape), which is usually played on a single speaker.

Binaural recording is often called stereophonic, in which the sound is played on speakers on either side of the listener. But that's not entirely accurate, as stereo actually refers to a wider range of multi-channel recordings. Binaural sound is the standard format for home stereo receivers, televisions, and FM radio broadcasts. The simplest two-channel recording, called binaural recording, involves setting up two microphones to replace a person's ears at a live event (such as a concert). When you listen to the two channels on two different speakers, it recreates the experience of being there.

Surround recording takes this idea a step further, adding more audio channels so that sound comes from three or more directions. While the term "surround sound" technically refers to a specific multi-channel system designed by Dolby Laboratories, it is more commonly used as a general term for theater and home theater multi-channel sound systems. In this article we will use it in this general way.

There are special microphones that can record surround sound (by picking up sound from three or more directions), but this is not the standard way to create a surround sound soundtrack. Almost all surround sound effects for movies are created in a mixing studio. Sound editors and mixers collect many different audio recordings, dialogue recorded on a movie set, sound effects recorded in a dubbing studio or produced on a computer, musical scores, and then decide which audio channel or channels to play.

early surround sound

Surround sound has been produced in many different ways over the years. Disney's "Fantasia" (1941) was one of the first surround sound films, immersing the audience in classical music. Disney sound engineer William Garrity separated each orchestra recording and mixed them to create four distinct soundtracks, which were recorded as optical soundtracks on a separate reel of film.

Four audio tracks drive different speakers around the theater. In a well-equipped theater, the music appears to move back and forth in the auditorium, an effect achieved through sound panning. Panning involves fading a sound (such as a violin melody) from one audio channel while building the sound on another channel.

To play "Fantasia" in surround sound, theaters would need an additional projector to play the original sound, as well as an expensive receiver and speakers.

This surround-sound system did not catch on (the necessary equipment was very expensive), but by the late 1950s, many Hollywood films were adopting the simpler multi-channel format. Several different theater settings emerged during this era, including the famous movie theater and movie projector, but most used the same basic sound technology. Taken as a whole, these systems are called stereo, or simply theater stereo.

stereo

Stereo uses four or more analog magnetic audio tracks around the edges of the film. Magnetic tracks don't produce as clear a sound as traditional optical soundtracks, and they fade away over time, but they take up much less space on the film. The standard film format doesn't have enough room for more than two tracks, but up to six tracks can be squeezed around the film frame.

In a stereo system, three to five channels drive the speakers behind the movie screen. Popular four-channel systems include one channel driving the left speaker, one channel driving the right speaker, one channel driving the center speaker, and one channel driving the surround speakers along the sides and rear of the theater. Some systems have five separate channels and a surround channel behind the screen.

In these movies, most of the sound is recorded on the front channel so that the text appears to be coming from the screen. When an actor speaks on the left side of the screen, the dialogue comes from the speakers on the left. When an actor speaks to the right, the sound comes from the speaker on the right. Most dialogue is also directed to the center speaker, which serves to anchor or focus the sound on the screen. Backtracks (or audio tracks) are often used for "effect sounds" such as ambient background noise or sounds coming from off-screen.

In the 1970s, Dolby Laboratories introduced a new sound format based on the same configuration.

Dynamic Dolby

Like stereo, the original Dolby Stereo® has three front channels and one surround channel. However, instead of using magnetic tracks, it reverts to superior optical track technology to allow for clearer sound playback. Dolby Stereo also uses advanced noise reduction technology to further improve sound quality.

The high-quality sound of Dolby Stereo enables filmmakers to make wider use of surround channels. George Lucas's Star Wars was one of the first films to be encoded in Dolby Stereo, using surround sound to enhance its epic space battle scenes. The sound engineer gradually moved the sound of the fighter jet from the front channel to the rear channel, making it appear as if it was flying across the screen.

Later films adopted the "Star Wars" formula, using surround tracks to create fantastic effects and filling in background noise to establish the setting of the scene. In newer versions of surround sound systems, theater owners can hook up a subwoofer to handle very low-frequency sounds (a crossover can separate these sounds from the two audio tracks). Many filmmakers use subwoofers to create powerful roars in theaters that shake the audience when an explosion or earthquake occurs on screen. The subwoofer channel in analog and digital surround sound systems is sometimes called the low frequency effect (LFE) channel.

In 1982, Dolby introduced Dolby Surround, a version of Dolby Stereo for home entertainment systems. und Dolby Surround recreates the effect of Dolby Stereo in a theater, but with a slightly different effect. Audio channels are encoded as tracks on magnetic tape or broadcast as television signals, rather than as optical tracks. The speakers are set up in the same basic way as in a theater, except that 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 extra channel for the front center speaker.

The real innovation of Dolby Stereo is that so much audio information is compressed into a small space in a movie. When Dolby engineers began working on the new format, they discovered they could only fit two light tracks in the available space. To allow four independent audio channels, they developed a special 4-2-4 processing system. This system was originally used for four-channel home stereo recording in the early 1970s, encoding four channels of audio information into two audio tracks.

4-2-4 handling system

The basic idea of the 4-2-4 processing system is to derive four information flows from two information flows. Basically, the four streams of information are: information in stream A, information in stream B, the same information in stream A and stream B, and the difference between the information in stream A and stream B.

The first two channels are fairly simple. Stream A is provided to the left speaker, and stream B is provided to the right speaker. But "same" and "different" channels are a little more complicated. 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, which is a metal cylinder with a ring of wire surrounding it. The electromagnet is surrounded by permanent natural magnets. When you pass an electric current through an electromagnet, it becomes magnetized, just like a natural magnet, which has a north and south pole. The coil in the electromagnet is connected to the (+) speaker wire on one end and the (-) speaker wire on the other end. The audio amplifier constantly changes the direction of the current, keeping the direction of the magnetic poles switching.

Changing the orientation of the magnetic poles changes the attraction between the electromagnet and the surrounding natural magnets. This causes the electromagnet to move back and forth. As the electromagnet moves, it pushes and pulls on the speaker cone, which quickly pushes the air out and then pulls it back in. The movement of air particles creates the sounds we hear.

Then, the audio signal is a fluctuating current. When the current fluctuates one way, the speaker cone moves inward; when the current fluctuates another way, the speaker cone moves outward. This signal can be represented as an oscillating wave. The special sound produced depends on how fast and how far the cone moves, which depends on the wave pattern of the current.

In a surround setup, the center channel's signal is recorded on both the A and B streams. The center signals on both streams are identical in amplitude and frequency, and they are completely synchronized.

phase shift

A surround sound decoder that supports the center channel will pick up the same signal based on the pattern and amplitude of the A and B streams. In a surround sound setup without a center speaker, a perfectly balanced center signal will create a "ghost speaker" (a ghost of a speaker) directly between the left and right speakers.

The sound signals of the surround channels are also recorded on stream A and stream B, but the same signals in each stream are not synchronized with each other. Instead of playing synchronously, they move over time in both audio streams. The result is that these two signals interact: while the surround signal in stream A tells the left speaker cone to move outward, the signal in stream B tells the right speaker cone to move inward. Therefore, the surround signal information from the front left and right speakers is largely canceled automatically.

The surround decoder takes stream A and stream B and moves them relative to each other so that the surround signal is in phase again. With this offset, the right, left, and center signals are all out of sync and therefore tend to cancel each other out.

In addition to separating different signals, a proper surround decoder also passes audio information through different filters and noise reduction components to balance sound levels and reduce noise. Professional logic decoders use active "steering" elements to control the process more precisely.

Many home audio enthusiasts have found a way to partially unlock the surround sound channels using just a two-channel home stereo and an extra set of speakers.

get surround channel

The easiest way to access surround channels is to get a receiver with a surround decoder. The decoder identifies the out-of-phase information and extracts it into the third channel. To balance the sound, the receiver also boosts the channel to the appropriate level and adds a slight time delay. However, surround sound is accessible using a standard stereo receiver because all information is actually contained in the left and right channels.

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

Stereo signals in phase with the front channels cancel each other out in the rear speakers: the (+) currents from the left and right reach the (+) and (-) terminals of each speaker at the same time, so the currents don't change the electromagnet poles at all.

But out-of-phase signals in the stereo channels will form alternating currents, and the current from these signals will flow out of the (+) amplifier terminal of the left channel, while the current from the (+) speaker terminal flows into the (+) amplifier terminal of the right channel. The effect is that these out-of-phase signals move the rear speakers' electromagnets, thereby controlling the sound from the rear speakers.

To set up a simple center speaker (a speaker that holds the left and right stereo speakers), just turn on the TV. If it is a mono TV, it will play two stereo channels simultaneously. Stereo TVs also work well for anchoring purposes since both channels are beamed from the TV area.

Another component you'll need in this setup is a potentiometer, which can apply varying degrees of resistance to the flow of electricity, thus reducing the voltage in the circuit. In this surround setup, the potentiometer simply acts as a volume control for the rear speakers. You can hang it anywhere along the circuit leading to the speakers at the back.

Of course, this setup won't provide the same quality surround sound as an actual surround sound receiver.

In the 1990s, a new type of surround sound began to appear in theaters, and since then it has gradually surpassed the standard 4-2-4 method.

Digital domain: DTS

Today, many theaters have digital surround sound systems. Digital sound works very differently than analog sound systems.

In analog recording, sound is encoded as a long, fluctuating stream of information. In digital recording, sound is encoded as a series of ones and zeros, much like a computer program. This way you can encode more information in a limited space, resulting in clearer and more precise audio tracks.

In 1993, with the release of "Jurassic Park", digital cinema sound effects became available to the public. "Jurassic Park" uses a technology called DTS Digital Sound®, named after Digital Theater Systems, the company that patented the technology.

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

Digital Domain: Dolby Digital

Dolby subsequently launched its own digital format, Dolby Digital®. Dolby Digital is also known as Dolby Digital 5.1® (for 5 audio channels and a subwoofer channel), Dolby AC-3® (for Dolby’s third audio encoding design), or Dolby SR-D® (for spectrum digital recording). Dolby Digital has the same basic speaker configuration as DTS and sounds similar, but it works on a very different system. Instead of recording audio on a CD, the digital information is encoded in tiny patterns on the film between the holes in the sprocket.

As the movie passes through the projector, the Dolby Digital reader passes through this pattern of LEDs. On the other side of the film, light strikes a charge-coupled device (CCD), the same type of light sensor used in digital cameras. The CCD records an image consisting of hundreds of small spots representing ones, with hundreds of spaces between the spots representing zeros. The Dolby Digital processor interprets the digital information in this image as an audio signal.

Dolby Digital Surround Ex® works the same as Dolby Digital, but it includes an additional surround channel. Additional channel-driven speakers line the back wall of the theater. Like the front and center speakers, it can be used to anchor the sound for the left and right surround channels.