Reference

If there is one thing I've learned in almost 28 years (ouch) of audio writing, it's that audience reaction is fickle. Sometimes readers will swallow the most contentious pronouncements without indigestion, only to choke on throwaway lines you've invested with little importance. It just goes to confirm that human communication involves senders and receivers, and they aren't always in synchrony.

Pick an expletive—one you would normally use to express deep intellectual frustration—but don't vocalize it. Hold it in reserve for a few minutes, letting it simmer to concentrate its intensity. I'll tell you when to let rip.

Someday we may speak wistfully to our grandchildren about the "golden age" of digital audio when consumer formats (CD and DAT) contained a bitstream that was an exact bit-for-bit duplicate of the original studio master recording—not a digitally compressed, filtered, copy-resistant version whose sound is "close enough" to the original. Digitally compressed formats such as DCC and MiniDisc represent, in effect, a return to the pre-CD era when consumer-release formats were always understood to be imperfect copies of the studio original. Even the most ardent audiophile accepted the fact that LPs and mass-produced tapes did not, and could not, sound as good as the master tapes they were derived from.

Until recently, all problems in digital audio systems were blamed on either the analog/digital converters (ADCs) used in mastering or the digital/analog converters (DACs) needed for playback (footnote 1). As the performance of both ADCs and DACs improved, however, a previously unrecognized mechanism for distortion was unmasked: jitter. As we shall see, jitter—or, more correctly, word-clock jitter—can be a significant limitation in the technical and sonic performance of digital audio systems (footnote 2).

"How sour sweet music is
When time is broke and no proportion kept!"

The promise of "perfect sound forever," successfully foisted on an unwitting public by the Compact Disc's promoters, at first seemed to put an end to the audiophile's inexorable need to tweak a playback system's front end at the point of information retrieval. Several factors contributed to the demise of tweaking during the period when CD players began replacing turntables as the primary front-end signal source. First, the binary nature (ones and zeros) of digital audio would apparently preclude variations in playback sound quality due to imperfections in the recording medium. Second, if CD's sound was indeed "perfect," how could digital tweaking improve on perfection? Finally, CD players and discs presented an enigma to audiophiles accustomed to the more easily understood concept of a stylus wiggling in a phonograph groove. These conditions created a climate in which it was assumed that nothing in the optical and mechanical systems of a CD player could affect digital playback's musicality.

Jitter is not what digital sound quality induces in the listener; rather it is the instability in the clock signal that controls exactly when the analog waveform is sampled in the original A/D conversion, or when the digital word input into a DAC results in an analog voltage being produced at the chip's output. "So what?" is the response of digital advocates, "As long as a digital one is recognized as a one and a digital zero as a zero, then how can there be any difference in sound?" goes their argument, normally culminating in a fervently expressed "Bits is bits!"