Special thanks to Bill Peebles, owner of
Rialto, Colony and Studio theaters in Raleigh, NC, for
the screen and theater photos and his general
you ever watched a movie at home with the sound muted? It is
amazing what a difference the sound makes in a movie
experience. Sound, especially dialogue, makes it easier to
understand what is happening. But it also provides texture and
emotion to each scene. Most movies would not be interesting at
all if you took away the sound. And when we go to the
theaters, we expect the sound to be as exciting and
encompassing as the images on the screen.
In this edition of HowStuffWorks,
you will learn how analog and digital sound
systems work. You will also learn about the three major
Digital Theater Systems (DTS)
Sony Dynamic Digital Sound (SDDS)
Sound in movies has come a long way. As early as 1889,
Thomas Edison and his associates were experimenting with
synchronizing sound to moving pictures. In 1926, Warner
Brothers released "Don
Juan," the first commercial film to have accompanying
recorded sound. "Don Juan" had a musical score but no
dialogue. The next year, Warner Brothers released "The
Jazz Singer" with music, sound effects and a few lines of
dialogue. Sound had finally arrived in the movies.
Analog Sound The mechanism for delivering
sound in the early days of cinema was incredibly simple.
Vitaphone, used in "The Jazz Singer," consisted of a
record player playing a wax record. This was known as
sound-on-disc. The sound recording was usually done
after the movie was filmed. The record was played on a
turntable that synchronized sound to the film by controlling
the speed of the projector.
It was a simple but very effective way to add audio to a
the early 1930s, sound-on-film began to supplant
sound-on-disc as the technology of choice for adding a
soundtrack to a movie. An interesting thing about
sound-on-film is that the sound is several frames away from
the corresponding images. This is because the audio
pickup, or reader, is set either above or below
assembly of the projector. Most analog
pickups are in the basement (below the lens), while digital
pickups are normally in the penthouse (fastened to the
top of the projector). A test film is run to calibrate the
sound to the picture. Once this calibration is done,
projectionists can splice film together knowing that
the sound will synchronize properly.
Sound-on-film uses one of two technologies:
The most common method is an optical process whereby
a transparent line is recorded along one side of the film.
This strip varies in width according to the frequency of the
sound. For this reason, it is known as a variable-area
soundtrack. As the film passes the audio pickup, an
exciter lamp provides a bright source of light,
focused by a lens through the transparent line. The light that
passes through the film shines on a photocell.
The light is changed to electrical current by the
photocell. The amount of current is determined by the amount
of light received by the photocell. The wider parts of the
strip allow more light, which causes the photocell to produce
more current. Since the width of the transparent strip changes
the amount of light, this results in a variable electric
current that can be sent to a pre-amplifier. The
pre-amplifier boosts the signal and sends it to the amplifier,
which distributes the signal to the speakers.
A variation of this method is known as variable-density
soundtrack. It uses a strip that varies in transparency
instead of width. The more transparent the strip is, the more
light shines through. The biggest problem with this method is
that the natural graininess of the film can create a lot of
In the 1950s, magnetic
recording became popular. Magnetic sound-on-film had a couple
of advantages over optical at the time:
Magnetic was stereo, while optical was mono.
Magnetic had better sound quality.
were disadvantages, too:
Magnetic had to be added to the movie after it was
Magnetic was more expensive.
Magnetic didn't last as long as optical.
Magnetic was more easily damaged.
Even though magnetic recording provided as many as six
discrete tracks of sound on a film, the expense was simply too
much. There had been experiments with stereo optical tracks,
but there was too much noise to make that sound system
worthwhile. But when Dolby
Laboratories introduced Dolby A in 1965, a noise
reduction method originally developed for professional
recording studios, the movie industry saw an opportunity to
reinvent the optical track.
A breaks the incoming audio signal into four discrete bands. A
technique called pre-emphasis boosts the signal of each
band above 10 decibels,
the level of ambient noise. Each signal then travels through a
compander, where the signal is compressed to further
eliminate low-level noise and is then expanded again. The
signals are combined, and the result is much cleaner sound.
The main compromise in Dolby A is a narrower frequency
response, resulting in a smaller dynamic range. Dolby
noise reduction has evolved from Dolby A to Dolby Spectral
Recording, an enhanced process that reduces noise twice as
much as Dolby A.
In 1971, "A
Clockwork Orange" used Dolby A on magnetic sound-on-film
with great success. Eastman
Kodak worked with RCA and Dolby in the early 1970s to
develop stereo variable area (SVA), an optical method
that offered stereo sound by using two variable width lines in
the space that was originally allocated for one.
Surround Sound Surround sound first
showed up with Walt Disney's "Fantasia"
in 1941. To show the movie with surround sound, a movie
theater had to spend $85,000 for a special setup that included
custom loudspeakers and required two projectors, one running
the film and one track of audio plus a second one dedicated to
four special audio tracks.
Because of the expense, the full surround-sound system was
only installed in two theaters: one in Los Angeles and the
other in New York. Many theaters offered surround sound as
magnetic-based sound became popular, allowing four or even six
channels of sound. Dolby A noise reduction allowed films to
have stereo optical tracks, but even Dolby A couldn't
compensate for the level of noise if more than two optical
tracks were put on the film. A major breakthrough in surround
sound came when Dolby Stereo was created.
Film with Dolby Stereo optical
Using an amazing process called matrixing, Dolby
devised a way to use the two optical lines on the film to
create four distinct channels of sound:
Matrixing works like Boolean
logic by comparing the information on the left and right
optical tracks to determine which speaker to send the signal
to. For example, if a signal on the left track AND the right
track is encoded completely out of phase, it is considered
surround sound. When the pickup in the projector
reads the optical tracks, it decodes this signal as surround
sound and sends it to the rear and side speakers in the
theater. If the in-phase signals from the left track AND the
right track are identical, it sends the signal to the center
channel. Otherwise, it sends the left track signal to the left
front speaker and the right track signal to the right front
It is interesting to note that Dolby Surround and
Dolby ProLogic are the home versions of Dolby Stereo.
The same principle applies in these home systems. Four tracks
of audio information are condensed into the space of two
tracks. If the system does not have a surround-sound decoder,
the tracks are treated as normal stereo (right and left)
tracks. The key difference in Surround and ProLogic is the
center channel. A Dolby Surround system uses the right and
left speakers to create a phantom center speaker. This works
fine if you are sitting exactly halfway between the two
speakers. ProLogic sends the center channel sound to an actual
With the advent of digital
sound, the capability to offer discrete channels of
sound has grown tremendously. "Discrete" means that each
channel of sound is encoded separately from every other
channel, instead of the averaging process used in matrixing.
Digital Theater Systems The first commercial
use of digital sound on a large scale debuted with the release
Park." It is called DTS, an acronym for Digital
Theater Systems, the name of the company that patented
the process. At its essence, DTS is an updated version of the
classic sound-on-disc technology used in the early days of
cinema. DTS employs a special optical time code that is
part of the film. The time code is a series of dots and dashes
along the side of each frame between the image and the analog
optical sound tracks.
A special optical reader is mounted on the
projector. The film is threaded through the reader before it
enters the projector. Similar to the analog audio pickup, the
DTS reader uses a light-emitting diode (LED) to focus
light on a lens, through the film and onto a photocell. This
creates pulses of current that the reader decodes as the time
code. It sends this information via a serial cable to a
computer. The computer controls an audio system with three CD players. The
movie soundtrack consists of six tracks (right, left, center,
left-surround, right-surround and subwoofer) compressed on one
or two CDs, depending on the length of the movie. One CD holds
about two hours of audio in the special compressed format used
by DTS. The third CD player is used for a CD that contains
current DTS movie previews.
Both the film and the soundtrack CDs have an identifying
code. The computer checks these codes to make sure that the
correct soundtrack is played for the movie being shown. To
make sure that the audio does not lag due to accessing the CD,
the system buffers the audio in memory using
the FIFO (first in, first out) method. Because the
computer is constantly analyzing the timecode and matching the
audio from the CD to it, the sound is seldom out of sync with
the picture. And, since the sound is not actually encoded on
the film, movie-goers don't hear that annoying "pop" that
sometimes occurs when the audio pickup encounters a splice.
The downside to DTS is:
It requires additional steps in the production process
to create the CDs.
DTS relies on additional equipment to operate.
The soundtrack CDs occasionally do not arrive at a
theater with the film reels.
In the event of a failure of the DTS computer or CD
players, the film still has the analog tracks. DTS
Stereo, which is compatible with Dolby Stereo audio
pickups, is the process used to create the analog tracks. As
with all digital formats, the optical analog tracks are only
used under certain conditions:
when there is no digital information (such as the local
theater information or some previews)
when the digital format is incompatible with the local
equipment (for example, a DTS film in a Dolby Digital setup)
when the digital equipment fails
when the digital information on the film is unreadable
DTS has lasted much longer than anyone expected. The
original concept was viewed as a temporary solution while
theaters made the transition to digital. But the ease of use,
relatively low cost and the simple fact that many theaters
have already made the investment in the format have combined
to keep DTS a viable alternative to the sound-on-film digital
Dolby Digital Quite possibly the most
popular of the digital formats is Dolby Digital, which
is also known by several other names:
Dolby Digital 5.1 (more on 5.1 below)
Dolby AC-3 (Dolby's third audio-coding design)
Dolby SR-D (Spectral Recording Digital)
Dolby Digital uses the space between the sprocket holes to
encode information. Look at the photo below and notice the
gray dots between the holes. If you look closely, you can even
make out the Dolby Digital logo in the center of each segment!
Film with 5.1 channels encoded
The Dolby Digital reader mounts on top of the projector
(some projectors now have the reader built right in) and scans
the film as it passes through. Light from an LED shines
through the film onto a CCD.
The image, containing little specks that represent 1s and
spaces that represent 0s, is sent by the reader to a DSP-based Dolby
Digital Processor unit that turns the binary data
back into sound.
Just like DTS, Dolby Digital uses six tracks:
LFE (low-frequency effects)
This configuration is commonly referred to as 5.1,
for five main channels plus an effects channel. The effects
channel uses a subwoofer and is often called the boom
channel because its main use is for explosions and other
powerful, teeth-rattling sounds.
In the event of failure of the Dolby Digital reader or
problems reading the digital information, the film has Dolby
Stereo analog tracks.
Sony Dynamic Digital Sound The latest entry
in cinema digital sound comes from an entertainment industry
Dynamic Digital Sound (SDDS) uses the outside edge of the
film to stripe digital audio information. Unlike any of the
other formats, analog or digital, SDDS provides error
correction through the use of an identical redundant
stripe on the other edge of the film. SDDS supports increased
surround-sound options by offering eight channels of sound:
The SDDS reader uses a laser to
direct focused beams of light. The light passes through the
film, through a lens that magnifies the light and finally is
received by an array of photocells. Wherever there are dark
areas on the film, the photocells in that part of the array do
not receive any light. Any photocell that does receive light
emits a small amount of current. SDDS reads each cell as a 1
or 0 based on whether or not it is generating current. As the
film streams by, this creates a constant stream of binary
information that the reader sends to the digital processor.
Film with Sony Dynamic Digital Sound
Because it requires additional digital sound equipment,
SDDS is more expensive to implement than DTS or Dolby Digital.
Both formats convert their digital signals to analog after
decoding, but SDDS uses a digital connection to send the
decoded signal to a proprietary sound processor. Even so, the
addition of two more channels of sound make it a very
As you may have noticed in the images of film in this
article, more than one sound format is usually recorded on the
film. Since each format uses a different portion of the film,
it is very economical for the distributor
to include at least two of the digital formats on the same
film. Virtually all commercial films today have Dolby Stereo
as the analog format, and some films actually have all three
digital formats as well!
You may be wondering why THX is not
listed. Although commonly confused with a movie-theater sound
system, THX is not a sound format -- it's something else
entirely. See How THX Works
to learn about it.
For more information on movie sound and related topics,
check out the links on the next page.