The text on this page is reprinted with permission from the Dolby Labs web site. This document is simply intended as an opener to understanding Dolby surround technology. For all the details on Dolby Digital, Dolby Surround and Dolby Multimedia, please refer to http://www.dolby.com/.

You're probably beginning to hear a great deal about Dolby Digital - the next and perhaps last evolutionary step toward realistic sound in the home.What is this new technology? How is it related to Dolby Surround Pro Logic and what kind of investment in new equipment will it require? These and other questions will be answered on this page.

But first, a point regarding nomenclature. Over the past couple of years Dolby Laboratories has used the term Dolby Digital to refer to this new digital system in the film and theater industries, and has used the term Dolby Surround AC-3 to refer to the system in the home theater market. In practice both the cinema and home theater applications are variations of the same basic technology (slightly different data rates). So to minimize the possibility of confusion by consumers the Company has decided that Dolby's consumer multichannel digital format will be known by the same name used in the film and theater industries -- Dolby Digital.

This is expected to help consumers more readily identify products that feature it, and to help distinguish it from Dolby Surround and Dolby Pro Logic formats which are based on analog matrix technology. For the new generation of laserdiscs, and new formats such as the use of Dolby Digital on digital video/versatile discs (DVD) and high definition television (HDTV), the Company will refer to Dolby Digital, just as it does for the professional film application. The Dolby Digital nomenclature will also help to clear up growing consumer confusion over the term "AC-3" (audio code number 3), the technical designation for the digital audio coding technique Dolby developed for multichannel applications.

I've heard of Dolby Surround Pro Logic, but what's Dolby Digital (AC-3)? What's Dolby "5.1"?

Dolby Digital delivers six totally separate (discrete) channels of sound. Like Dolby Surround Pro Logic, it includes Left, Center and Right channels across the front of the room. Dolby Surround Pro Logic provides a single limited-bandwidth (100 Hz to 7,000 Hz) surround channel which is typically played back in the home through two channels of amplification and two speakers. In comparison, Dolby Digital provides separate (discrete) left surround and right surround channels, for more precise localization of sounds and a more convincing, realistic ambience. And, with Dolby Digital, all five main channels are full range (3 Hz to 20,000 Hz). A subwoofer could be added to each channel, if desired.

The sixth channel, the Low Frequency Effects Channel, will, at times, contain additional bass information to maximize the impact of scenes such as explosions, crashes, etc. Because this channel has only a limited frequency response (3 Hz to 120 Hz), it is sometimes referred to as the ".1" channel. When added to the 5 full range channels, the Dolby Digital system is sometimes referred to as having "5.1" channels.

All six channels in a Dolby Digital system are digital, which means that they can be transferred without loss from the producer's mixing console to your home playback system. But Dolby Digital packs all six channels into less space than a single channel on a Compact Disc requires. This advanced approach to handling the digital audio data is what makes it possible for Dolby Digital soundtracks to be added to conventional laser discs, and to a whole host of other sources. Dolby Digital has been used to bring you Dolby Digital movies for several years, and now, due to the advent of Dolby Digital laser discs, you are able to enjoy this unique performance in your own home. And because it was developed by Dolby Laboratories, you know it will sound great!

Does Dolby Digital require a lot of new equipment?

If you are starting a new system, you can buy laser disc players, amplifiers, and speakers that are perfect for Dolby Pro Logic now and ready for Dolby Digital in the future. You will need one of the new laser disc players which includes an Dolby Digital RF signal output (providing the Dolby AC-3 data stream) already available. The main component that is new is the Dolby Digital demodulator/decoder (for decoding that AC-3 data stream for playback). If you have a Dolby Surround Pro Logic system, you probably already have the required five channels of amplification and five speakers (Figure 1). Adding a subwoofer is recommended, and more than one subwoofer is your option.

Where did they put the Dolby Digital soundtrack on the laser disc? Do I have to give up any important audio information on the laser disc? Will I have to replace my existing laser disc library?

There are more similarities than there are differences between "conventional" laser discs and the "new" discs which include Dolby Digital (Figures 2 & 3). The new discs are completely compatible with your existing laser disc player. The two PCM digital stereo tracks will remain intact, so you can continue to enjoy the quality and excitement of Dolby Surround Pro Logic from these "new" compatible discs. The Dolby Digital track is encoded on what used to be the right FM analog track of the disc. The left analog track which remains may contain a mono version of the soundtrack, commentary, or other audio material.

The last laser disc player to rely on the FM analog tracks as its only audio source was produced over 10 years ago. Since then, almost everyone has opted to use the superior PCM digital tracks as the audio source for their stereo and Dolby Surround listening. Of course, the new AC-3-ready laser disc players will continue to play all of your existing laser discs, including both PCM tracks and both analog tracks.

So Dolby Digital doesn't make Pro Logic obsolete?

Not at all. Dolby Surround Pro Logic will be with us for a long time to come. The Dolby Surround Pro Logic matrix system "folds" four channels (Left, Center, Right and a limited bandwidth Surround channel) into two channels. These two channels are added together ("summed") for playback in monophonic systems. And, of course, all the information is available for playback in stereo systems. But when the two channels are fed into a Dolby Pro Logic Decoder, the matrix is "unfolded," and four channels are again available (the limited bandwidth Surround channel is reproduced through the Left Surround and Right Surround speakers).

The amazing part of the Dolby Surround Pro Logic system is that the matrixed ("folded") signal is carried in the stereo signal, whatever form it takes: stereo TV broadcasts, stereo satellite transmission (C-band, DSS or PrimeStar), stereo cable transmission, stereo FM radio, stereo laser disc, stereo video tape, and even some stereo video games. Today your Dolby Surround listening choices include dozens of regularly scheduled broadcast television programs, a growing list of CDs, and of course VHS and laser disc recordings of the thousands of movies encoded in Dolby Stereo (Dolby Stereo is the movie name for what you know as Dolby Surround at home). For as long as stereo is with us, your Dolby Surround Pro Logic decoder will be needed...and that's why all Dolby Digital decoders will also have digital Dolby Pro Logic decoders built in.

See this chart for a point-by-point comparison between Dolby Surround Pro Logic and Dolby Digital, as well as a diagram of how each works at the mixing, mastering and playback stages.

OK, there are plenty of sources for Dolby Pro Logic programming. What about Dolby Digital? Will there be sources for Dolby Digital besides laser disc?

The beauty of Dolby Digital is that it is a very flexible audio coding process. A wide variety of Dolby Digital formats are technically possible, and several are expected to appear before the end of the decade:

High Definition Television (HDTV) is one of the first new entertainment formats to select Dolby Digital as its primary audio system, after extensive listening tests by the "Grand Alliance" who are setting standards for the HDTV system to be used in the United States.

Direct Broadcast Satellite (DBS) systems are already taking advantage of Dolby Digital 's high quality and ease of transmission: the DMX for Business service, for instance, is using Dolby Digital to transmit 120 stereo music channels to business and commercial establishments, all via a single transponder. PrimeStar is adding Dolby Digital to their DBS consumer satellite TV service in the near future.

Digital Cable TV systems are adopting Dolby Digital, both for performance considerations and also to be prepared for standardization with future HDTV systems.

The Digital Video Disc (DVD) format includes Dolby Digital.

Other formats like Digital Video Cassette (DVC), and Digital Audio Broadcast (DAB) are prime candidates for Dolby Digital 's unique combination of sound quality, spectrum efficiency, and multichannel capability.

What is it about Dolby Digital that makes it so flexible -- or, for that matter, so good?

The "AC" of Dolby AC-3, stands for Audio Coding. Digital Audio Coding is most often referred to as "perceptual coding." Simply put, perceptual coding seeks to eliminate the data we can't hear, while maintaining all the data we can hear. The purpose of perceptual audio coding is to get more information into the available spectrum. A car pooling analogy:

Suppose you needed to get 4,000 people (essential information) from one place to another within one hour. The highway (available spectrum) can only carry 1,000 cars in one hour. By getting the 4,000 people (the essential information) to ride in only 1,000 cars, the unneeded additional information (the 3,000 cars left at home) is eliminated. That's high efficiency transportation, and that's what Dolby Dolby Digital is about.

One reason compact discs yield such high quality audio is that they use an immense volume of data: 16 bit samples are taken 44,100 times per second, for each channel. That's 1,411,200 bits of data per second. The Compact Disc is such a vast storage medium that it can store up to 74 minutes of two channel music on a single disc. But what if one wanted to store 2 hours of 20 bit signals and there had to be six channels? Today such huge bit rates are impractical for storage or transmission.

Dolby Digital is the first perceptual coder designed specifically to handle multichannel audio. Dolby Laboratories' unique experience with audio noise reduction is essential to Dolby Digital 's effective data rate reduction because 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, but allowing strong audio signals to cover or "mask" the noise at other times. Those signals, however, can only mask noise that occurs at nearby frequencies. Dolby Digital therefore divides the audio spectrum of each channel into narrow frequency bands which correlate closely to the frequency selectivity of human hearing. This allows coding noise to be very sharply filtered so that it stays close in frequency to the audio signal being coded. The audio signal effectively masks the noise, making it imperceptible. Where there are no audio signals present to mask the noise,Dolby Digital concentrates on reducing or eliminating the coding noise. You could say that Dolby Digital is a form of very selective and very powerful noise reduction...and the result is sound quality which is subjectively extremely close to the original signal.

Dolby Digital uses a "shared bitpool" arrangement, as well as a model of human auditory masking, to make its use of the transmitted data as efficient as possible. Bits are distributed among the different narrow frequency bands described above -the distribution is different in each case, to meet the needs of the frequency spectrum or dynamic nature of the program being coded. Then, applying a model of auditory masking ensures that a sufficient number of bits are used to describe the audio signal in each band. Bits are also distributed among the various channels in accordance with this model, so that channels with greater frequency content can demand more data than sparsely occupied channels, or strong sounds in one channel can provide masking for noise occurring in other channels. The result is that Dolby Digital can use proportionally more of the transmitted data to represent audio, yielding higher sound quality and allowing multichannel surround sound to be encoded at a lower bit rate than required by just one channel on a CD.

TECHNICALLY SPEAKING

Dolby Digital can process at least 20-bit dynamic range digital audio signals over a frequency range from 20 Hz to 20,000 Hz �0.5 dB (-3 dB at 3 Hz and 20,300 Hz). The bass effects channel covers 20 to 120 Hz �0.5 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 Digital consumer formats, and 192 kb/s for two-channel audio distribution.

For a more complete technical explanation of Dolby Digital browse http://www.dolby.com/. The text on this page is reprinted with permission from the Dolby Labs web site.