Movie Sound

The following section contains excerpts from "How Movie Sound Works" by Tom Harris.
Full article is available at

Have 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.

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.

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 movie.

In 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 the lens
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.

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.

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 speaker.

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
center speaker.

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.
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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.
Just like DTS, Dolby Digital uses
six tracks:

Left surround
Right surround

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.
The latest entry in cinema digital sound
comes from an entertainment industry
giant. Sony 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:

Left center
Right center
Left surround
Right surround
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