As its many announced and potential applications attest, Dolby AC-3 is unusually versatile. Not a single rigid scheme, it is a flexible family of processes allowing such parameters as bit-rate and number of channels to be tailored to particular applications. All variations are based on the same operating principles, however, and have been engineered to ensure compatibility among formats and adaptability to future needs.
Most significantly, Dolby AC-3 combines high quality sound with remarkable data rate efficiency. Although, for example, it enables multichannel surround sound at a lower bit rate than is needed for just one channel on Compact Disc, the resulting sound quality remains fully consistent with listener expectations, thanks to Dolby Laboratories' more than 25 years of experience developing signal processing systems based upon how sound is perceived.
Filmmakers, however, continued to regard four channels left, center, right, and surround-as the minimum necessary to create a convincing, lifelike sound field. This was underscored by the widespread adoption by the motion picture industry in the late 1970s and early 1980s of the 35 mm Dolby Stereo format, which matrix-encodes four channels onto two optical soundtracks, then recovers them upon playback by means of a proprietary sound processor.
When Dolby Stereo films are transferred to two-channel video formats, their four-channel encoding survives intact. Since the early 1980s, Dolby Surround has made it possible to decode these multiple channels at home-the first real break with the consumer tradition of two channel stereo. Today millions of homes are equipped with multichannel Dolby Surround systems; in fact, more multichannel, surround sound audio receivers are now sold than two channel stereo receivers. Dolby Surround program material ranges from videos of theatrical films to TV shows and music-only CDs, helping "home theater" become the fastest-growing product category in the consumer electronics marketplace.
With the viability of multichannel sound for consumer formats so well established, the stage has been set for a more advanced technology that reproduces sound with unparalleled spatial realism, while providing exceptional adaptability to varying listening conditions and industry needs. That technology is Dolby AC-3, and, appropriately enough, its first application has been cinema sound.
It was at this point that Dolby AC-3 was conceived as a multichannel audio coder operating at little more than the bit-rate required by two independently coded channels. Previous adaptive-transform coding technologies such as Dolby AC-2 had already paved the way, but multichannel coding required further breakthroughs, including a new bit-allocation technique.
The 3/2 configuration and use of separate bass effects channels were both pioneered by Dolby Laboratories for the cinema. Two surround channels, called stereo or split surrounds by film sound mixers, were first heard in 1979 from 70 mm magnetic prints of Apocalypse Now, and have been offered as an option for 70 mm Dolby Stereo releases ever since. Supplemental bass tracks have been standard on Dolby Stereo 70 mm prints since the mid 1970s.
An important advantage of the 3/2 configuration is its compatibility with today's consumer surround sound systems. While Dolby Pro Logic Surround is a 3/1 system (i.e. just one surround channel), current playback systems all feature two surround speakers exactly as needed for 3/2 playback.
In consumer Dolby Surround Digital systems, reproducing the bass effects channel via a subwoofer is optional; the home listener can program the decoder to route the non-directional bass effects to any or all full-range speakers in the system. In cinemas, however, where sound effects often depend upon relatively high sound pressure levels at very low frequencies, use of subwoofers for Dolby Stereo Digital playback is a necessity.
Dolby Stereo Digital, the film sound format with AC-3 as its keystone, debuted in cinemas in June of 1992. Within less than two years, more than 50 feature films had been released in the new format, and nearly 600 cinemas in 27 countries had been equipped for playback of the digital track. This experience confirmed that prints with both digital and analog tracks could be manufactured economically, that such prints would play in any cinema, and that the AC-3 coded digital track provided high audio quality with extraordinary resistance to wear and tear.
Just as important, Dolby Stereo Digital provides a unique springboard for consumer formats based on AC-3, enabling the accumulation of invaluable experience in mixing, recording, and distributing multichannel digital audio. It is also fostering a library of program material immediately available for consumer release, and has facilitated the development of cost-efficient IC decoder technology. Dolby AC-3 is the only multichannel perceptual coding technology with this kind of real-life experience behind it.
AC-3 has made it possible to fit a 5.1 channel digital soundtrack between the sprocket holes on 35 mm prints. Among the benefits are not only high-quality sound, but also a track that is highly resistant to wear and tear thanks to a relatively large bit-size.
Unlike analog Dolby Surround with its single band-limited surround
channel (usually played over two speakers), Dolby Surround Digital features
two completely independent surround channels, each offering the same full
range fidelity as the three front channels. As a result, true stereo surround
effects can be achieved for an expanded sense of depth, localization, and
overall realism. And because Dolby Surround Digital maintains complete
separation of the audio channels, it is as suited to music-only recordings
and broadcasts as it is to video formats. Thus it has the potential to
open up new worlds of multichannel sound reproduction. That isn't all that
Dolby Surround Digital can do. While Dolby Stereo Digital is heard in cinemas
with a full complement of loudspeaker channels, a standardized playback
level, and full dynamic range capabilities, home listening circumstances
vary markedly. Therefore, for Dolby Surround Digital consumer formats,
AC-3 has been designed to satisfy many diverse requirements. At the outset,
at least, while some listeners will have multichannel systems, most will
be listening in mono or conventional stereo. Those with Dolby Surround
systems will want a two channel matrix encoded output from their decoders.
Many listeners may prefer a restricted dynamic range, but others will wish
to experience the full dynamic range of the original signal. Techniques
to satisfy these and other needs have been designed in from the beginning:
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.
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.
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.
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.
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