Digital-Audio Myths
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Even worse is the phrase “near CD quality.” For those unfamiliar with marketing doublespeak, “near” is the same as “virtually.” In plain English, both words translate to “not.” So what was once a technical term is now simply advertising gibberish.

Finally, I have to ask: is CD quality still supposed to be a good thing? At one time, 16-bit, 44.1 kHz audio was synonymous with state-of-the-art digital technology. But that was then. In today’s 24-bit world (with 96 kHz sampling rates gaining in popularity), those CD specs are looking a bit long in the tooth. Maybe instead of CD quality, the industry can agree on a more appropriate term like “real old-fashioned CD goodness.” It’s just a thought.

Myth No. 4: 24-bit is 24-bit is 24-bit. Resolution is an easy way to specify a digital device’s quality. Unfortunately, it is not a reliable benchmark. I remember a meeting with a representative from a major digital-audio–chip manufacturer in which two of the manufacturer’s models of 20-bit D/A chips were evaluated. When asked why one of the chips was abnormally noisy and performing more like a 14-bit D/A than its 20-bit spec suggested, the representative responded that it was “20 bits—with 6 bits of marketing.”

FIG. 3: The clock stability in Apogee’s AD-8000 is an important selling point that the company touts in its ads
(click image for larger view).

So what’s the moral of the story? Just because two devices are both “24-bit” does not mean they exhibit the same audio quality. In fact, fidelity can vary so widely that a well-designed 16-bit device may sound better than a poorly designed 24-bit instrument.

One variable is the quality of the D/A or A/D chip. The major manufacturers of these chips may have a line of parts with the same general specifications (such as 24-bit, 44.1 to 48 kHz) but with widely diverging noise amounts and differing prices. The clock-circuit quality is also important for minimizing jitter. (For more on jitter, see Myth No. 5.) In fact, several high-end A/D/A manufacturers specifically cite the their clocks’ stability as an important selling point (see Fig. 3).

Finally, remember that the A in D/A stands for “analog.” You know that there are good and bad sounding analog mixers, preamps, and other gear, so it should come as no surprise that a “digital” device’s analog parts can make a real difference in its overall sound quality. High-quality analog parts and clever analog design are absolutely essential for a digital device to realize its true potential.

FIG. 4: During a jittery recording, the sampling points are not spaced evenly in time, though they follow the curve (top). During playback from a device with low jitter, the points are spaced much more evenly, but they retain the amplitudes of the jittery recording (bottom). After the timing is corrected, the amplitudes no longer create the original shape.

Myth No. 5: Jitter is recorded during digital dubs. In a perfect world, each digital-audio sample is recorded and played back at exact, even intervals derived precisely from each tick of the digital-audio word clock. For instance, a 44.1 kHz system should sample the incoming audio exactly 44,100 times per second. Real-world clocks aren’t quite perfect, however, and each tick of the clock may be slightly behind or slightly ahead of where it’s supposed to be. That difference between the ideal timing and the actual timing is called jitter.

Jitter causes distortion in digital audio, but it’s different from what you generally think of as distortion. Instead of distortion in amplitude, such as overdrive in a guitar amp, jitter is distortion in time that causes slight variations in the audio waveform’s shape. In a sine wave, for example, varying each sample’s timing causes the waveform to bulge out and cave in at different points, as opposed to following the ideal smooth curve (see Fig. 4).

Every digital-audio device produces some amount of jitter, but some devices exhibit much more than others do. Jitter can also be cumulative: as a signal passes through multiple signal processors, mixers, and so on, the jitter may get progressively worse. Jitter becomes “frozen” when you record an analog source with a digital system. In other words, every time you play back the audio, you hear the effect of the jitter that was present during the recording. You also hear the jitter produced by the digital-to-analog converter.

Reprinted with permission from

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