Pace, Rhythm, & Dynamics Page 7

Such design practice goes far beyond simple engineering considerations. Where high-quality audio is concerned, the dollars/watt ratio is no longer the prime consideration.

Other established design factors also make sense. For example, operating an output stage in class-A largely stabilizes its operating characteristics. One may also include full regulation of the power supply, which removes the uncertainties and randomnesses of power-supply behavior. Deleting various low-frequency time constants in the circuit also appears to be helpful, hence the trend to DC coupling, with servo circuits used to keep the amplifier's output terminal at mean ground potential.

The primary target
I have not yet touched on the primary target: digital audio. This is because the digital operating system is so complex. We have the interfacing of the digitized data, the digital signal-processing operations, and finally the variety of conversion techniques used to transform the data back to the analog domain. This system is not open to the kind of simple analysis so far seen for loudspeakers or even power amplifiers.

It's disturbing to realize that most digital audio systems are not designed by seasoned audio engineers but by digital theoreticians at chip level. Without a strong and critical audio influence, the ultimate sound quality of digital audio will itself have a random quality, lacking heart and direction. So far, the results obtained suggest that all a high-end designer can do is pick his way through the available technologies and try to devise applications which best express his audio goals. This is most definitely not the same as designing your own speaker or amplifier: within digital technology, the number of variables available to the designer has been severely reduced. Those vital system decisions have already been made at the keyboard of a large digital CAD system fundamentally tailored for the design of digital logic circuitry. (Footnote 6)

Considering the two primary DAC technologies, broad generalizations are now possible, notwithstanding individual designer variations. Low- or 1-bit, high-oversampling decode systems have a pleasant "undigitized" sound, "analog-like" in tonal quality and often capable of fine transparency and very good low-level resolution. In addition, good results are often obtainable at low cost. The weaknesses of such systems are in the areas of transient life, dynamic expression, and rhythm. In classical subjective terms, the sound quality of MASH or Bitstream may appear very plausible, like a very-low-coloration monitor speaker, but may ultimately fail to satisfy. A secure feeling of rhythmic, dance-like coherence is often absent, careful analysis of the reproduction frequently revealing bland politeness. High-profile proponents of low-bit D/A technology include Meridian, Deltec, Altis, and California Audio Labs, as well as Pioneer, Philips/Marantz, Sony, and Matsushita (Technics, Panasonic).

While much mud has been slung at the alternative and older, high-bit, low-oversampling technique, or multi-bit, mainly by avid proponents of low-bit systems, the fact remains that multi-bit is still the system of choice for high-end CD replay. Handled well, it is capable of good timbre and good transparency, while it also has the potential for a good dynamic and rhythmic performance, even if this is not yet up to the best analog standards. High-profile adherents of multi-bit technology in the High End include Accuphase, Mark Levinson, Stax, Linn, Naim, Theta, Audio Research, Krell, PS Audio, Audio Synthesis (UK), Mission-Cyrus, and Wadia, to name but a few.

Now that digital audio has matured sufficiently to overcome its initial harshness and mechanistic quality---"digititis," as it has often been called---it is time for us to advance its performance in the areas of pace, rhythm, timing, and dynamics.

The future
Rhythmic and dynamic qualities are fragile and easily diluted. They are precious yet vital aspects of the musical experience, and demand wider recognition and greater understanding. A stronger commitment is required from the audio industry as a whole to address their manifest deficiencies in this area.

Footnote 6: The conflict between circuit theory (and simulation) and the practicalities of design is expanded upon in an excellent collection of essays in the EDN Series for Design Engineers, Analog Circuit Design: Art, Science, and Personalities, edited by Jim Williams (Butterworth-Heinemann, 1992).---JA