The News about DSD
For many, the current hot topic in the world of high-end audio is Direct-Stream Digital (DSD), a method, developed by Sony and Philips, of digitally encoding an analog signal. The irony is that DSD is nothing new. The basis of the technology dates to 1946. Stereophile described it in “Industry Update,” as early as Vol.19 Nos.1 and 5, and again in Vol.20 No.9. And, almost exactly 14 years ago, in November 1999, John Atkinson went into greater detail, contrasting DSD with the more common Pulse-Code-Modulated (PCM) encoding used on CD:
The most straightforward way of encoding an analog signal as a Pulse-Code-Modulated (PCM) digital datastream is to use an A/D converter operating at the sampling frequency that puts out digital words of the desired length. If you are talking about the CD's 16-bit/44.1kHz data, an ADC samples the analog signal 44,100 times every second, each time describing the instantaneous signal amplitude to the nearest one of 65,536 voltage levels (2 to the 16th power). This is, in fact, how digital audio recordings were made up to the mid-'80s. But the complexity of the ADC increases almost exponentially with the number of bits required, and the critical demands made on the analog antialiasing filter needed to eliminate every trace of signal above half the sample rate are extreme. A different A/D paradigm was required to achieve resolution greater than 16 bits and to achieve more accurate 16-bit resolution at lower cost and circuit complexity. . .
Instead of trying to attain higher resolution by increasing the number of bits, it was thought: why not increase the sample rate instead? In the limit, if you increase the sample rate to a sufficiently high frequency, you can use a 1-bit quantizer: a simple voltage comparator that outputs a "1" if the analog signal level is higher than it was at the previous sampling moment, or a "0" if it is lower. Because this "delta modulation" technique uses a sample rate very much higher than the baseband audio signal, the requirements for a "brickwall" analog antialiasing filter on the ADC's input can be relaxed. You can then either feed the high-rate pulse stream to a simple low-pass filter to reconstruct the analog original, or you can use a low-pass digital filter to "decimate" the low-resolution, high-sample-rate data to derive the desired multi-bit, low-sample-rate data. . .
The elegance of the idea behind DSD is that this decimation filter can be eliminated. Why not, Sony's engineers thought, just store the output of a 7th-order noise-shaped delta-sigma modulator running at a very high frequency (in DSD's case, 2.8224MHz, or 64 x 44.1kHz) on an appropriate medium. For playback, this datastream could be fed, in theory at least, to a D/A converter consisting of just a simple low-pass filter.
The use of such a high sampling frequency would mean the ADC's analog antialiasing filter needn't be a brickwall type but could instead be a sonically benign low-order type; linearity would inherently be excellent; there would be no digital decimation filter, with its necessary mathematical approximations on either the A/D or D/A conversions reintroducing PCM quantization noise or time-domain dispersion problems; there would be no multi-bit DAC, with its possible performance compromisesthis would be the closest thing to a digital topology with analog-like properties.
DSD encoding became commercially available with the SACD format, which, for various reasons best discussed over Buffalo wings and beers, never reached its full potential. At least for hi-fi enthusiasts, then, the news is that DSD has finally become practical: Mastering engineers have the necessary tools, record labels are beginning to release the music, and hardware manufacturers are responding with DSD-capable digital-to-analog converters that most enthusiasts can actually afford.
Elsewhere, in that same November 1999 issue, JA wrote: "I can't help wondering if sounding better is, on its own, sufficient reason for SACD to become established. To become dominant, a new medium needs to be different in kind, not just offer more of the sameeven if that more is much better."
JA told it like it was then, while, at the same time, telling it like it is now.
As I see it, the problem with audiophiles is that they care too much about sound. Good sound is not enough; has never been, and will never be, enough.
CD succeeded, JA argues (and I agree), not only because it offered digital encoding, but because it provided random access, portability, greater longevity, and lack of surface noiseconveniences, mostly, and things largely unrelated to good sound.
DSD's day has finally come. The encoding is freed from the disc; things are now as they should have always been.
Why such fine news should be at all controversial is beyond me, but, hey, I suppose there is something in human nature that enjoys a good waste of time. And who am I to argue with human nature?
Interestingly, back in 1999, JA concluded his DSD discussion with these words:
The proof of any audio pudding is in the hearing, and in that respect DSD-encoding would seem to be beyond reproach. Every Stereophile writer who has auditioned DSD under critical conditions has found it both very much better than 16/44.1k CD and much closer to the analog experience.
Again, nothing much has changed.
In case you missed it, last week, AudioStream.com’s Michael Lavorgna interviewed Andreas Koch of Playback Designs, a proponent of DSD encoding and one of the early developers of SACD. ML and Andreas Koch aim for the heart of the matter. It’s an excellent read: informative, entertaining, and clear. Check it out.