Continued from previous message:
AC-3 ICs
A major step in the progress of Dolby Surround Digital has been the
introduction of Zoran Corporation's ZR38000, the first single-chip DSP
capable of implementing AC-3 decoding. It is being used first in the
second-generation Dolby Stereo Digital theater decoder unit.
A lower cost, dedicated AC-3 decoder IC, particularly suited to consumer
products and also providing Pro Logic decoding, is slated for release by
Zoran in mid-1994, as is a two-channel chip for such applications as
cable TV set-top decoders. Other semiconductor manufacturers are
expected to begin supplying AC-3 decoder ICs in late 1994 or early 1995.
ABOUT DOLBY AND DIGITAL
AC-3 is the third perceptual coding system developed by Dolby
Laboratories. Each one combines a unique knowledge of psycho acoustics
with a progression of advanced digital signal processing techniques.
Dolby AC-l was first used in 1985 for DBS applications by the Australian
Broadcasting Corporation. Partly because of its very low decoder cost,
AC-l has since been adopted for other DBS services, satellite
communication networks, and digital "cable radio" systems; the data rate
is 220-325 kb/s per channel depending on application. A refined form of
adaptive delta modulation (ADM), the data stream contains information
not on the absolute value of the audio signal, but on the change in
value from sample to sample. Techniques adapted from Dolby noise
reduction, such as continually varying step-size and pre-emphasis,
greatly improve on basic ADM performance.
Dolby AC-2 uses advanced adaptive transform coding for professional
audio transmission and storage applications; its data rate is 128 or 192
kb/s per audio channel. Frequency-domain signal processing in a
multiplicity of narrow bands takes full advantage of noise masking,
resulting in effective data rate reduction combined with high signal
transparency.
Among other applications, AC-2 is used to link remotely located
recording studios and/or film post production facilities for
long-distance, real-time recording, mixing, and ADR sessions. It is also
at the heart of the Dolby DSTLr system used to link broadcasters'
studios and transmitters.
ABOUT DOLBY LABORATORIES
From analog noise reduction to digital coding, Dolby Laboratories
specializes in developing audio processes that are rooted in studies of
how the human ear perceives sound. During the development cycle, as much
emphasis is placed on critical listening as on making measurements. The
result is much the same, whether the process is analog or digital:
significant, cost-effective improvements of the media to which it is
applied. In some cases, the improvement is sufficient to create a new
product category, as with Dolby Surround and home theater.
Dolby Laboratories comprises engineering groups that invent and develop
the technologies, professional audio product design and manufacturing, a
unique licensing organization, and world-wide field support. Products
are built in modern plants at the company's main US and UK facilities.
Licensing involves every major consumer electronics manufacturer in the
word, and entails testing samples of each licensed product, inspecting
every facility worldwide that manufactures them, and co-operating
closely with IC manufacturers (more than 80 different ICs incorporating
Dolby technologies have been developed for use by licensees).
Dolby-trained consultants are available for every production center in
the world to assist in the preparation of multichannel soundtracks for
films, videos, broadcasts, and music media.
When it comes to multichannel digital sound, Dolby Laboratories is in an
unparalleled position not only to develop the technology, but to
introduce it effectively throughout the world.
HOW DOLBY AC-3 WORKS
The digital audio coding used on Compact Discs (16-bit PCM) yields a
total range of 96 dB from the loudest sound to the noise floor. This is
achieved by taking 16-bit samples 44,100 times per second for each
channel, 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 AC-3 is the first perceptual coding designed specifically to code
multichannel digital audio. It is also the only one to benefit from the
development of two other successful perceptual coding systems, Dolby
AC-1 and AC-2, and from the development of what are in essence analog
perceptual coding systems: the full gamut of Dolby professional and
consumer noise reduction systems. 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. 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
NR 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.
AC-3 has been designed to take maximum advantage of human auditory
masking. 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 perceptual coding system
like AC-3 is essentially a form of very selective and powerful noise
reduction.
In Dolby AC-3, bits are distributed among the filter bands as needed by
the particular frequency spectrum or dynamic nature of the program. A
built-in model of auditory masking allows the coder to alter its
frequency selectivity (as well as time resolution) to make sure that a
sufficient number of bits are used to describe the audio signal in each
band, thus ensuring noise is fully masked. AC-3 also decides how the
bits are distributed among the various channels from a common bit pool.
This technique allows channels with greater frequency content to demand
more data than sparsely occupied channels, for example, or strong sounds
in one channel to provide masking for noise in other channels.
Dolby AC-3's sophisticated masking model and shared bit pool arrangement
are key factors in its extraordinary spectrum efficiency. Furthermore,
where other coding systems have to use considerable (and precious) data
to carry instructions for their decoders, AC-3 can use proportionally
more of the transmitted data to represent audio, which means higher
sound quality.
Technically speaking, AC-3 can process at least 20-bit dynamic range
digital audio signals over a frequency range from 20 Hz to 20kHz x 0.5dB
(-3dB at 3Hz and 20.3 kHz). The bass effects channel covers 20 to 120 Hz
x0.5 dB (-3 dB at 3 and 121 Hz). Sampling rates of 32, 44.1, and 48
kHz are supported. Data rates range from as low as 32 kb/s for a single
mono channel to as high as 640 kb/s, thereby covering a wide range of
requirements. Typical applications include 384 kb/s for 5.1-channel
Dolby Surround Digital consumer formats, and 192 kb/s for two-channel
audio distribution.
Dolby is a registered trademark of Dolby Laboratories Licensing
Corporation.
Copyright © 1995 Lucasfilm, Ltd.
--- QScan/PCB v1.18b / 01-0249
---------------