|
Although DVD-Audio builds on the familiar high-density optical-disc technology of DVD-Video, it's probably easier to understand if we compare it with the CD, which it's designed to succeed. CD is a relatively simple "dumb" format. Every CD contains two channels of 16-bit pulse-code-modulated (PCM) digital audio at a sampling rate of 44.1 kHz. Additional information, computed from the audio data, is included for error correction, along with elementary control data (timing, tracks, index points, and so forth). That's it, except for basic title/artist information on CD Text discs. DVD-Audio, on the other hand, is a much more sophisticated format that gives both producer and listener much greater flexibility. Like CD, DVD-Audio uses the PCM digital audio format. But the data rate isn't fixed. Digital "word" length (also called bit depth) can range from 16 to 24 bits, allowing greater dynamic range, and the sampling rate (or frequency) can be 44.1, 48, 88.2, 96, 176.4, or 192 kHz, allowing high-frequency response that extends well into the ultrasonic region. The only restrictions are that the 176.4- and 192-kHz rates are available only in two-channel mode because the system can't get enough data off the disc fast enough for more channels at those sampling rates. DVD-Audio is so flexible that it's not even necessary for all the channels on a disc to have the same sampling rate. A producer could, for example, choose to have three channels at 96 kHz/24 bits and two others with less critical information, such as surround-channel ambience, at 48 kHz/ 20 bits. The DVD-Audio standard provides for a lossless data-compression method called Meridian Lossless Packing (MLP), which allows a substantial reduction both in the amount of data that must be stored on a disc and in the rate that data has to stream off the disc, which can't exceed 9.6 megabits per second (Mbps). Yet unlike lossy compression systems, such as Dolby Digital and DTS, which actually discard some of the original audio data, MLP allows exact, bit-for-bit reconstruction of the original, uncompressed signal on playback. With MLP, a DVD-Audio disc can hold CD-length (74-minute) programs containing six channels, all at 96 kHz and 24 bits. The DVD-Audio standard does allow for other audio coding systems, such as Dolby Digital, DTS, and even the Direct Stream Digital (DSD) system that Sony and Philips use in their rival Super Audio CD (SACD) format. But they must be used in addition to PCM, not instead of it. It might seem that this would open the door to easy compatibility with existing DVD-Video players, all of which will recognize at least Dolby Digital, but by itself it doesn't: The basic DVD-Audio file format — the way data is arranged on the disc — is different from that used for DVD-Video. This means an ordinary DVD-Video player simply can't find the information on a DVD-Audio disc. The only way to achieve this sort of backward compatibility is by making a section of the disc in the DVD-Video format or by adding a second layer in the CD format. Every disc released as of this writing includes a Dolby Digital version of the DVD-Audio mix, but none of them have a CD-compatible layer. DVD-Audio programs can have as many as six channels, all of them full-bandwidth if desired. Producers don't have to stick with 5.1-channel mixes, familiar from DVD-Video discs, since there's no reason to have an additional low-frequency-effects (LFE) channel for music except for such exotica as the 1812 Overture performed with real cannons. It's also possible to have multiple versions of the same program on the same disc. All of the early Warner Music Group releases, for example, include both a six-channel and a two-channel version. This is important because many people do a great deal of their music listening on portable players with headphones. It's not essential for producers to include a separate two-channel mix to get regular stereo, however. They can embed instructions in a multichannel data stream that tell players how to create "fold-down" two-channel mixes on the fly. When MLP is used, these instructions can be extremely detailed, to the point where there may be little reason for producers to consider including a separate stereo mix. The DVD-Audio format is elaborate in part because it uses a more complex and robust encryption method to deter piracy. It also allows for functions that don't exist in DVD-Video, such as cueing directly to a specific lyric chosen from an onscreen display. A DVD-Audio disc can provide all manner of information in addition to the audio, including text, still pictures, Internet links, and even some video. None of the discs released so far have Web links, but Emerson, Lake & Palmer's Brain Salad Surgery, for example, includes concert footage from the time of the album's making, and Aaron Neville's Devotion has an autobiographical documentary. Most of the discs also include lyrics, and some of them contain slideshows that change while the music plays. This is truly a brave new whirl for music in the home. —Michael Riggs
|
