Digital Audio

Mike Bollinger

Digital Audio

DVD-Audio and Super Audio CD's will eventually replace current CD technology and will be the formats of the future for audio replication. DVD-Audio and SACD formats are vast improvements over current CD quality. It will not happen over night, but eventually everyone will be buying either SACD's or DVD-Audio Discs instead of CD's.

There is somewhat of a format war brewing right now amongst new digital audio formats. The two formats in the running are DVD-Audio, which boasts higher sampling rates than that of the Compact disc, as well as higher quantization. Current CD technology is limited in its musical performace by its sampling rate of 44.1kHz and 16-bit word length. This means that when an audio signal is converted to digital for a CD, the analog-to-digital converter samples the audio waveform 44,100 times each second. The converter also assigns a number to each sample; this number represents the waveform's amplitude at the sample point, this process is called quantization. In other words, the CD uses 16-bit quantization. Therefore each sample's amplitude is represented by a 16-bit binary number. The new DVD-Audio format has not only higher sampling rates, but also higher bit rates. Sampling rates of DVD-Audio discs can be 48kHz, 96kHz, or 192Khz. Quantization can be either 16, 20 or 24 bit. DVD-Audio discs can have two formats on the disc; a two-channel mix recorded on the disc at 192kHz at 24-bit resolution, as well as a multi-channel mix with the main (left and right) channels encoded at 96kHz at 24-bit resolution, while the rear channels may be sampled at 48kHz to save data. The drawback to DVD-Audio is that they require special players to play them back. However, many of the current DVD-Audio discs also have a version on the disc that can be played in most DVD players.

The other competing audio format on the market is the SACD or Super Audio CD, this format was created by the joint efforts of both Sony and Philips, and is also capable of higher resolution audio than that of the Compact Disc. The SACD has a sampling rate of 2.8224 MHz, or 2,822,4000 samples per second. Because of the higher sampling rate, the SACD samples music 64 times the rate of Compact Discs.

Sampling preserves the time information from an audio signal while quantization preserves the amplitude information. Wider range of audio frequency can be preserved by faster sampling rates. "More bits in the quantization 'word', the greater dynamic range, the lower the noise, and the greater the resolution (Robert Harley, The Complete Guide to High-End Audio, pp.282). When SACD's were first marketed, one of the features that they were trying to highlight was the fact that SACD's would be a hybrid disc, meaning that they would be dual