Introduction
It is difficult to overstate the significance of the past five years in terms of their effect on how we listen to our music and movies—today and in the future. Naturally, perspective always seems to be easier to grasp after the fact, and it may take years before the full impact of this period hits the entire population of home theater and music buffs. But make no mistake about it: Multichannel audio is here to stay—and our listening rooms and home theaters will never be the same.

Naturally, the sound of multichannel audio isn't new. Dolby has been giving us a taste of what life beyond stereo could sound like with movie soundtracks since the early '70s, and at roughly the same time, the short-lived quadraphonic experiment proved that even then, people were wondering how far musical recordings could go. Quad, of course, never made it—it may have simply been too far ahead of its time. While Dolby Surround did make a considerable impact, it was still in essence a stereo signal being run through a matrix decoder—and those in the know were well aware that this was only the beginning.

Fast forward to 1997 and the birth of the DVD. After it stalled initially, and expectedly, in the purgatory that all new formats face, it has quickly become the fastest selling consumer electronics format of all time. While DVD's video aspects can hardly be ignored, it is DVD's audio, and its role in launching discrete multichannel audio, that will secure it in the annals of consumer electronics history. Dolby Digital and DTS 5.1 were the first major steps in what has become a revolution in the way we think about sound.

Not to be outdone by movie sound, the music world did some rethinking of its own. The result was the debut of two new music formats, DVD-Audio and SACD, which not only institutionalize the idea of discrete multichannel playback for music, but also further the progress of high-resolution musical recordings that had begun with formats like HDCD. Both formats are currently experiencing growing pains similar to those that DVD-video faced and haven't yet captured the hearts of the masses. The unfortuitous timing of these formats' launches cannot be ignored either: They come at a time when people appear to be far more preoccupied with the distribution of music than with its quality. But if the DVD-video precedent has taught us anything, it may only be a matter of time before SACD and/or DVD-A takes control.

Naturally, the ripple effects of both DVD formats and SACD have been significant on the hardware side. DVD-video, having already hit its stride, has had the most impact thus far, including major changes in processor and source design, and the recent surge in the multichannel-amplifier segment. Speakers, too, have seen considerable change. Subwoofers have never been more involved in the audio mix, and both surround and center-channel speakers have taken on far-more-significant role--center channels are arguably the most important speaker element in a discrete, 5.1-channel array for soundtrack reproduction. High-resolution, multichannel music will have a similar impact on processor, source, and amplifier design and precipitate its own form of speaker specialization. We're already seeing the emergence of switchable monopole/dipole surrounds to offer a more specialized approach to movie and music playback. With SACD and DVD-A's potential for frequency responses to 100 kilohertz and dynamic-range capabilities around 120 decibels, speaker manufacturers must rethink some of the most fundamental principles of speaker design for the first time in many years.

It's anybody's guess what the listening environment of the future will sound like. It will likely involve more channels, more exploitation of perspective and ultimately, even more advanced attempts at breaking down the barrier between fantasy and reality—building on the foundation that has been set in our current home theaters, and more listening rooms now too. There's little doubt our audio world has gotten a lot rounder in the past few years, but there's also little doubt that the best is yet to come.

DVD-Audio
The more direct successor to the venerable but outdated CD in the high-resolution camp is DVD-Audio, based on the fact that its foundation is the Pulse Code Modulation system (PCM) that drove its predecessor's success. The primary differences between CD and DVD-Audio center around word lengths and sampling rates more than fundamental differences in execution. The CD was based on the Red Book specification, which, among other things, dictated word lengths of 16 bits and a sampling rate of 44.1 kHz for two-channel audio. The DVD-Audio specification, however, allows for word lengths anywhere between 16 and 24 bits, sampling rates from 44.1 kHz to 192 kHz, and a variety of channel-number options (although 24-bit/96kHz, 5.1-channel mixes will be the most common here in the early stages of DVD-A). DVD-Audio discs are similar to DVD-Video discs, and although a DVD-A's high-resolution tracks are not compatible with standard DVD-Video players, many DVD-As also offer a Dolby Digital mix that is backwards-compatible with DVD-Video players.

Currently, the main limiting factors that DVD-A engineers/producers face are disc storage capacity, which is 4.7 gigabytes for the most commonly used discs, and player-output capability, which is 9.6 Mbps for all DVD players—whether they're video, audio, or both. Both of these numbers will increase with advances in disc and laser pickup technology, but for now, compression is the key.

One can't understand DVD-A without understanding its accompanying acronym, MLP. Short for Meridian Lossless Packing, MLP is a compression system that allows DVD-A engineers to work around the 4.7GB and 9.6Mbps barriers with virtually no loss in audio quality—a statement that can be backed up on the measurement bench as well as in the listening room. Unlike lossy coding schemes--like Dolby Digital and DTS, which remove information that the system deems unnecessary--MLP simply codes information more efficiently. For a simple analogy, think of MLP operating like a computer compression scheme such as WinZip. The result is a 2:1 compression ratio and smaller data rates with measurable bit-for-bit accuracy to the original source.

High-resolution sound, with silky-smooth highs, deep, well-defined soundstages, and many other sonic benefits, isn't DVD-Audio's only advantage. The format's increased flexibility and disc capacity give producers a variety of audio options, such as whether to include both standard- and high-resolution tracks, two-channel and/or multichannel tracks, and even what word lengths and sampling rates to use on each particular channel. Several video options are available as well, including interactive menu systems, liner notes, music videos, concert footage, or just about anything else producers may want to include. Just about anything that can be put on a DVD-V can be put on a DVD-A. Naturally, all of these options will continue to expand with advances in disc and pickup technology.

SACD
There is no small amount of irony in the fact that Sony and Philips—the companies that are directly or indirectly responsible for some 500 million CD players and 10 billion CDs in circulation today—chose to move away from their digital cash cow, PCM, and completely rethink consumer audio coding for their version of high-resolution audio. The result is SACD, and with it, a simpler approach to digital audio from start to finish.

The foundation for SACD is Sony's Direct Stream Digital (DSD) coding system, a process that has been established on the professional side but now makes its way to the consumer world. Rather than increasing both word lengths and sampling frequencies over CD's 16-bit/44.1kHz values, as the PCM-based DVD-Audio format does, DSD reduces word lengths to a single bit and drastically increases sampling frequency up to rates of 2.8224 megahertz. The idea behind the 1-bit word length is to simplify the process that all digital signals go through from recording to playback in order to minimize anomalies that materialize along the way. All digital signals begin as 1-bit representations of the analog waveform after they're converted by 64x-oversampling Delta-Sigma modulation. Whereas the PCM system then sends the signal through a series of filters, which can cause audible problems during recording and playback, DSD records the 1-bit signal directly and eliminates several steps of the record/playback process.

Like other forms of digital audio, SACD requires compression to fit all of its information onto the disc, which it gets in the form of a Philips technology called Direct Stream Transfer (DST). Like DVD-A's MLP system, DST is a lossless scheme that compresses at a rate of 2:1 and offers bit-for-bit accuracy to the original source.

Even with DST compression, Sony and Philips knew that more-advanced disc technologies would be needed to take full advantage of SACD's potential and included three disc varieties in the Scarlet Book specification (which covers all aspects of SACD, just as the Red Book covered CD). The most common SACD is a dual-layer hybrid disc with a semitransmissive layer (4.7 GB) and a reflective outer layer (780 megabytes). The semitransmissive layer holds the high-resolution tracks (six-channel, two-channel, or both) and also has an outer layer that can house video extras like menus, liner notes, etc. The reflective layer holds a downmixed DSD-to-16/44.1-PCM track that boasts compatibility with all existing CD players—which may give SACD a major advantage in the high-resolution format war, if this track is included on all SACD releases. Whereas DVD-As are at best compatible with existing DVD-video players, SACDs are theoretically compatible with anything that can play a CD, including car systems, portable players, and so on. The SACD camp is also trying to convince retailers that this compatibility means that they only need to carry the SACD version of an album, rather than both CD and SACD versions, which will allow them to avoid double stocking.

If DVD-Audio is an evolutionary process, then SACD is revolutionary in its approach to consumer audio recording and playback. The question over which format is superior is a lively debate—both have their strengths and weaknesses. The more important question is which, if either, will survive the rigors of format infancy. That, of course, remains to be seen.