Jagan Seshadri

DVD-Audio and Super Audio CD: Next-generation digital audio

Digital Audio: Then and Now
When people think of digital audio they think "Compact Disc". Though the CD revolutionized consumer audio by being compact, durable, and great-sounding, it was implemented in the early 1980s and designers have learned a lot more about digital audio since then. Nipping at the heels of the CD are two new audio formats: SACD and DVD-A.

What could be better than CD?
For most people, the biggest difference between CD and the new DVD-A and SACD formats is that the new formats are capable of multichannel sound. CD has two channels (stereo: left and right) whereas the new formats can playback up to six channels to completely envelope the listener with sound.

And you thought that CDs were great!
When Compact Discs came onto the market, they were marketed as having "perfect sound forever". While CDs certainly sounded better than audio cassettes and dusty, worn-out LPs, CDs were great, but not perfect. Quite simply, the numbers (i.e. bits per sample and sampling rate) were not high enough to precisely capture all of the sound from studio master tapes.

Performance Descriptions of CD/DVD-A/SACD
Digital audio is recorded by taking 'snapshots' of sound waves. Since sound waves constantly vary in volume, each snapshot measures the volume at a specific point in time. Furthermore, each of these volume measurements is measured with a certain precision (like measuring a distance to the nearest inch). When a digital recording is played back, these volumes are played back snapshot-by-snapshot so quickly that it sounds continuous (much like watching a movie looks continuous rather than a bunch of individual snapshots).

Compact Discs record each channel of sound by taking 44100 samples each second, and measuring each sample "from the ground-up" using 16-bits (like being able to measure up to a mile within an accuracy of an inch). This is called 16/44.1 PCM (Pulse Code Modulation). In other words, the CD *at best* can accurately record sounds from 0 to 22.05kHz with about 96 decibels of difference between the faintest and loudest sounds (i.e. dynamic range). That's pretty impressive, but if the sound you're recording exceeds those numbers, you get ugly distortions upon playback.

DVD-Audio is similar to CD in the way it is recorded, except that its numbers are better. Each channel of sound can take up to 192000 samples each second, and measure each sample "from the ground up" using 24-bits (like being able to measure up to a mile within four thousandths of an inch!). This is called 24/192 PCM. In other words, DVD-Audio *at best* can accurately record sounds from 0 to 96kHz with about 144 decibels (theoretically) between the faintest and loudest sounds (i.e. dynamic range). It's actually overkill, but we are basically assured that the sound from a studio master tape will be accurately recorded without distortion. With numbers like these, recordings can sound more like live music and less like recordings.

Super Audio CD (developed by the makers of the original CD: Sony and Philips) is the other high-resolution digital audio format available. Each channel of sound is recorded by taking millions of samples every second (2822400 samples per second, to be exact) BUT the samples are not measured "from the ground up". Instead, each sample is measured relative to the previous sample, and is measured using 1-bit. Playback is done by recreating the ups and downs of the recorded waveform: If the sample is '1' then increase the volume, if the sample is '0' then decrease the volume. Silence is achieved by playing back '1 0 1 0 1 0 1 0...' etc. This method is called DSD (Direct Stream Digital) by Sony, and it related to pulse density modulation. SACD is said to have a dynamic range of over 120 decibels (although this figure would be highest for low frequencies and lowest for high frequencies, instead of being a fixed value as in PCM), and a frequency range between 0 and 100kHz.

Oh no, another format war?
Just like the oft-mentioned VHS/Beta format war, SACD and DVD-A are vying for the same spot in your sound system, but you might not have to choose between one or the other. It helps that all of these digital audio formats are on a CD-shaped disc. Just as DVD video players can playback CD audio, it is possible that "universal" players will emerge that can play CD/DVD-Audio/SACD as well as DVD-video.

Which should I choose? DVD-A or SACD?
This choice is actually not a matter of who's technology is superior - both systems are outstanding and both marketing campaigns have stretched the truth a bit (i.e. DVD-Audio for stating 144dB of dynamic range when electronic noise limits you to around 130dB, and SACD for stating that its recording rechnology is superior to PCM when they often use PCM for mixing, mastering, and editing). Instead, buy the technology for the music. Sony controls a lot of entertainment media companies, so some artists will only be released on SACD. DVD-Audio has a number of record companies on-board with it, such as Warner Music, and those artists would be issued on DVD-A.

Technically, SACDs cannot be played on conventional CD players unless it is a hybrid dual-layer disc (one layer for SACD and one layer for regular CD). DVD-Audio discs cannot be played on conventional CD players at all, but can be played on DVD-players with DVD-A capability. If the DVD player does not have DVD-A playback capability, you often can still play DVD-A discs but you'll hear a lossy-compression version of the recording (Dolby Digital or DTS).

DVD-A and SACD outputs (or, "Once Upon a Copyright")
These high-resolution digital audio formats were conceived partly to migrate masses of consumers to 'digitally secure' media, and to eventually stop producing the easily ripped and copied CD (and thus slow the MP3 piracy phenomenon). With this mindset, DVD-A and SACD players are equipped with analog outputs to pass along the high-quality sound to your sound system, but to prevent perfect digital copies from being made. Although DVD-A players still output a digital stream as well, it is intentionally downgraded for copyright protection reasons. These moves result in the user having to string six analog cables from their player to their receiver (or preamp/processor) to get the best-quality sound.

Another copyright protection mechanism that DVD-A used is called watermarking, where a special authenticity code is embedded in the digital recording. While it is said to be inaudible (which is indeed possible), some people suspect it is in fact audible, and therefore inferior to SACD. SACD's authenticity code is done using a manufacturing technique involving Pit Signal Processing (PSP) which does not alter the audio data at all.

Should I buy it now?
As of the time of writing (April 2002), neither format has very many recordings to be enjoyed. Of the recordings that have been released, DVD-Audio tends to issue more classic-rock and modern-rock albums (from the 1970s to today) whereas SACD tends to issue more classical music and jazz. Those who favor SACD tend to say that it sounds more natural, which may actually be due to better-sounding source tapes. DVD-Audio should be capable of equaling or SACD performance, and those who engineer 5.1 channel surround recordings will soon provide better mixes for both formats.

The fact that 24/96 PCM recordings (i.e. Digital Audio Disc (DAD) recordings...basically a regular DVD with less video and more sound) sound remarkable, bodes well for the future of DVD-Audio. However, everyone can win if more universal players are manufactured. Hopefully by 2003 or 2004 the outlook for these formats will be more clear.

-JNS