|
|
| Dolby Digital
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 theatre with three channels of sound in the front
(left and right for music and effects and centre 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 theatre 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 theatre-type set-up in the home.
Today's Dolby Digital system takes the next step, providing six channels
of crystal clear digital surround sound. Left, centre 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 optimised 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 theatre 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. |
|
