OTHER TECH

Inside Super Audio CD

As names go, Super Audio CD (SACD) is remarkably descriptive. It is almost literally a compact disc with super audio: wider bandwidth, greater dynamic range, and, perhaps most important, more channels - all on a 4 3/4-inch plastic disc. In addition, an SACD can carry text (disc title, artist names, track titles, lyrics, liner notes, and so forth), graphics, and even video to spice the mix up a bit.

Delivering so much information without compromise requires a lot more data capacity than the old CD-type 650-megabyte disc has to offer, however. So the foundation of SACD is a DVD-style high-density disc that can hold up to 4.7 gigabytes - about six times as much as a regular CD.

Arguably the most interesting technical characteristic of SACD is what actually goes on that high-density disc. An ordinary music CD normally carries two channels of pulse-code-modulated (PCM) digital audio using 16-bit data words and a 44.1-kHz sampling rate. That's good for a dynamic range of about 96 dB and a frequency bandwidth of a little over 20 kHz. An SACD, on the other hand, can carry as many as six channels of audio in a relatively new digital format that Sony and Philips call Direct Stream Digital, or DSD. As recorded on SACDs, it uses 1-bit data words at a ferocious sampling rate of a little over 2.8 MHz. The result is a dynamic range of about 120 dB in the audible range below 20 kHz (approximately equivalent to 20-bit PCM) and an upper frequency limit of about 100 kHz. The numbers tell only part of the story, however. DSD's primary virtue is simplicity. Direct Stream Digital is an example of what is known generically as delta-sigma or, sometimes, bitstream audio coding. In SACD's DSD, a single bit is toggled on and off very quickly (at that 2.8-MHz rate mentioned earlier). As the amplitude of the signal goes up, the proportion of the time that the bit is toggled on (set to one) also goes up. As the signal goes down, the trend reverses, with off settings (zeros) gaining dominance. The result is a pulse train whose density varies according to signal level - a system known as pulse-density modulation, or PDM. At full positive level, the DSD bitstream is all ones; at full negative level, it is all zeros; and at zero signal level, ones and zeros precisely alternate.

The amount of noise in a 1-bit signal is inherently rather large, but the high sampling rate distributes it over a very wide frequency range, which improves the signal-to-noise ratio down in the audio band. Performance is further enhanced by aggressive noise shaping that squeezes still more of the noise out of the audible range up into the ultrasonic region (above 20 kHz), where it won't bother us.

Delta-sigma coding is actually used in the front ends of most modern high-quality PCM analog-to-digital (A/D) converters and at the back ends of many PCM digital-to-analog (D/A) converters. In between, however, the bitstream signal must be converted to PCM (decimated) and back (interpolated). These processes are accomplished by digital filters.

DSD effectively cuts out the middleman. Instead of converting the bitstream signal to PCM, DSD simply records it. Then, on playback, all that's required is what would be the very last stage of a conventional PCM D/A converter: an analog low-pass filter to remove components above the system's 100-kHz upper frequency limit. What emerges is an extremely close replica of the original analog input signal.

DSD pays for its simplicity with relative inefficiency. Compared with PCM, it tends to hog both storage capacity and transmission bandwidth. Sony and Philips have developed a lossless compression system for DSD that makes it possible to fit both six-channel and two-channel versions of a CD-length program onto one SACD, but bit for bit and after compression, PCM is still more efficient. And since all current digital signal-processing (DSP) chips operate on PCM signals, any DSP in the playback chain will require conversion from DSD to PCM, at least in the near term. Besides, none of the players so far supply a digital DSD bitstream for an output. SACD's last big trick is one of its most appealing. The standard allows for dual-layer discs with a regulation CD recording on the top (nearest the label) and an SACD beneath it. Most ordinary CD players will see only the top layer of a dual-layer SACD release and play it just like an ordinary CD. Put the disc in an SACD player, however, and you'll get the high-resolution DSD version. Given all the car and portable CD players people already have, that's a wonderfully practical transition feature. -Michael Riggs