Going to the movies today is a very different experience
from going to the movies 70 years ago -- the picture is
clearer, most of the movies are in color, and the admission
price is a lot higher. But the biggest change is probably the
sound experience. In movie theaters of the 1930s, the entire
soundtrack was played on a single speaker or collection of
speakers positioned behind the movie screen. Today, theater
audiences expect to hear sound coming from every direction.
In this edition of HowStuffWorks,
we'll take a look at the surround-sound systems that
have become standard movie theater equipment. We'll also look
at home theater surround-sound setups and get you started
building your own.
What is Surround Sound? 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
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
Early Surround Over the years, there have
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
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
To show "Fantasia" in surround sound, a theater needed an
to play just the soundtrack, as well as an expensive receiver
and speaker assembly. (Check out this
site for a thorough history of how "Fantasound" came
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
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
Typical layout for stereophonic
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
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
Dynamic Dolby 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.
Dolby Stereo stores sound information on two
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.
The Dolby Stereo theater
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.
Four From Two 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 information in stream A
The information in stream B
The information that is the same in stream A and stream
The difference between the information in stream A and
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
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
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
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.
The surround-sound encoder takes four
channels and combines them all onto two
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
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.
The surround-sound encoder splits the
surround channel in two and shifts them in time so they
are "out of
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.
The surround-sound decoder picks out the
information in the right and left channel that is out of
phase, shifts it so it is in phase again and directs it
to the surround-sound
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 this
page 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.
Accessing the Surround Channel 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 "poor man's surround sound"
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
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 this
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
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.
Digital Domain 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-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 theater setup used in DTS and Dolby
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 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
here 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.
A Dolby Digital
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.
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
Dolby and DTS have both released home theater versions of
these popular formats. 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.