First introduced to moviegoers in 1992 with Batman Returns, and heard on almost a thousand films
around the world since then, Dolby Digital is the latest sound innovation from Dolby Laboratories.
Dolby revolutionized tape recording in the late 1960s and early 1970s with Dolby A-type (for
professional applications) and Dolby B-type (for consumer applications) noise reduction. Later in the
1970s Dolby revolutionized film sound with the Dolby Stereo analog sound system. Dolby Stereo
brought 4 channel sound to the movie theater with three channels of sound in the front (left and right
for music and effects and center for dialog) and a surround channel for effects and atmospheres. Then
in the 1980s both tape recording and film sound saw significant improvements through the use of
Dolby SR ("Spectral Recording").
Dolby also revolutionized home entertainment in the late 1980s and early 1990s with the Dolby
Surround and later Dolby Pro Logic home theater systems (basically using the Dolby Stereo
technology in the home environment for video tapes and laser discs). This allowed home viewers to
create the same 4 channel theater-type setup in the home.
Today's Dolby Digital system takes the next step, providing six channels of crystal clear digital
surround sound. Left, center and right channels in front of you provide precise, clear positioning of
dialogue. Separate or "discrete" left and right surround channels on the side and in the rear immerse
you in the film with atmospherics and ambient sounds. And a subwoofer/bass effects channel
contributes extra punch to action and special effects sequences.
The principles of Dolby Digital follow from the analog noise reduction work that Dolby has been
engaged in for more than 30 years. Dolby noise reduction works by lowering the noise when no audio
signal is present, while allowing strong audio signals to cover or mask the noise at other times. Thus it
takes advantage of the psycho acoustic phenomenon known as auditory masking. Even when audio
signals are present in some parts of the spectrum, Dolby noise reduction reduces the noise in the other
parts so the noise remains imperceptible. This is because audio signals can only mask noise that
occurs at nearby frequencies.
When moving from analog recording to a digital recording medium like the compact disc, one finds
that the digital audio coding used on CDs yields an amount of data often too immense to store or
transmit economically, especially when multiple channels are required. As a result, new forms of
digital audio coding - often known as "perceptual coding" - have been developed to allow the use of
lower data rates with a minimum of perceived degradation of sound quality. Dolby's third generation
audio coding algorithm (AC-3) is such a coder.
This coder has been designed to take maximum advantage of human auditory masking in that it
divides the audio spectrum of each channel into narrow frequency bands of different sizes optimized
with respect to the frequency selectivity of human hearing. This makes it possible to sharply filter
coding noise so that it is forced to stay very close in frequency to the frequency components of the
audio signal being coded. By reducing or eliminating coding noise wherever there are no audio signals
to mask it, the sound quality of the original signal can be subjectively preserved. In this key respect, a
coding system like AC-3 is essentially a form of very selective and powerful noise reduction.
Indeed, Dolby Laboratories' unique experience with audio noise reduction is essential to AC-3's
effective data rate reduction: the fewer the bits used to describe an audio signal, the greater the noise.
In the film industry, the Dolby Digital soundtrack is optically encoded right on the filmstrip, in the
space between the sprocket holes. Having the soundtrack directly on the film allows it to coexist with
the analog track without involving any other media like a CD - this not only makes it simple for film
distributors and theater owners to handle, but also allows Dolby Digital prints to be prepared at
virtually no additional cost. The sprocket hole area has also proven highly resistant to wear and tear,
so that a Dolby Digital soundtrack will remain free of pops and hiss for the useful life of the print.
In the consumer electronics industry the Dolby Digital soundtrack can be found on the latest
generation of laser discs (in space where one of the analog tracks used to be), can be found as the
standard audio track on Digital Versatile Discs (DVD), is the standard audio format for High
Definition Television (HDTV), and is being used for digital cable and satellite transmissions.
All
Dolby Digital related content is Copyright © 2002 Dolby Laboratories, Inc. All
rights reserved.
Dolby and the double-D symbol are registered trademarks of Dolby Laboratories.
All other trademarks are the property of their respective owners.
|
