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MULTICHANNEL PERCEPTUAL CODING


INTRODUCTION

Dolby AC-3 is a perceptual digital audio coding technique of unprecedented efficiency, quality, and versatility. It has been providing multichannel digital sound in cinemas since 1992, and was launched in two-channel DBS applications in 1994. Demonstrations of its ability to deliver multichannel digital surround sound from both tape- and disc-based home video formats have already generated excitement within the consumer electronics industry. And it has been selected to provide digital surround sound with HDTV broadcasts in the US, which are expected to begin experimentally in 1997.

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.


Multichannel Sound: Past, Present, Future


Bell Laboratories' famous early experiments with stereo sound in the 1930s used three channels. The public heard stereo sound for the first time during the early 1950s in cinemas that used no less than four channels, and as many as seven. Yet when stereo finally made it into the home a few years later, it used only two channels, because that was all the phonograph record could accommodate. This technological limitation, and not listener preference, led to two-channel stereo becoming the standard for home sound reproduction.

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.


How Dolby AC-3 Came About


In 1987, the standardization process for a high-definition television (HDTV) system began formally in the US. For the system's audio, it was first proposed that four sound channels be matrix-encoded down to a digitally coded stereo pair for transmission. But by 1990, four or more discrete channels were considered preferable, to avoid the compromises of audio matrixing. With the technology then available, however, this would have required at leasttwice the bit-rate.

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.


Dolby Digital (AC-3 in the Cinema)


While conceived and later chosen for HDTV in the US, AC-3 was actually implemented for the cinema first, making it practical to provide multichannel digital sound with 35 mm prints. In order to retain an analog track so that these prints could play in any cinema, it was decided to place the new digital optical track between the sprocket holes, a key factor in defining its maximum practical bit-rate. It was also well documented that a 5.1-channel format would best satisfy the requirements of theatrical film presentation. Altogether, these needs dovetailed with the HDTV requirements that led to AC-3's conception.

Dolby 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 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.


Dolby Digital (Surround AC-3) in the Home


Dolby Digital for the home (aka Dolby Surround Digital or Dolby Surround AC-3), forms the final link from multichannel program producer to home listener. Like the film format, it provides separate channels for left, right, and center speakers at the front; two surround speakers at the sides; and a subwoofer at the listener's option.

Unlike analog Dolby Surround with its single band-limited surround channel (usually played over two speakers), Dolby 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 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 Digital can do. While Dolby 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 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:

Dolby Digital offers a dramatic step forward in listener involvement and excitement. It provides program producers, directors, recording engineers, and performers unprecedented creative opportunities. And it offers remarkable media adaptability within a single, far-reaching technological framework. .


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 deltamodulation (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 at the heart of the Dolby FAX ® system which 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 DSTL ® system used to link broadcasters' studios and transmitters.


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 isforced 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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