CD Player/Transport Reviews

Sidebar 1: Is a Bit Bits?

If you haven't seen reams of copy on the latest 1-bit D/A conversion schemes, you haven't been paying attention. Suffice it to say that all of these techniques abandon the common multi-bit, resistive ladder D/A converter topologies. Instead, the 16-bit signal is processed in the digital domain to convert it (requantize it, to use the technical term) to a series of identical amplitude pulses which vary in either their width (so-called Pulse Width Modulation, or PWM) or their density (Pulse Density Modulation, or PDM).

The result of this is that the signal variations are then represented by either the density or width (and in some designs, the polarity) of these single-bit pulses, instead of by the original 16 bits. Relatively simple output low-pass filtration is then used to reconvert these pulses into the analog signal. Oversampling is part and parcel of all of the 1-bit variations, and a technique called noise shaping is also necessary both to minimize requantization noise in the audio band and to preserve the digital data's original 16-bit resolution. The main differences between all of the 1-bit configurations lie in whether they use PDM or PWM (and in the latter by the number of pulse widths used to represent the signal), and in their type of noise shaper, degree of oversampling, and clock rate (footnote 1).

It goes without saying that each manufacturer has a name for its implementation that boasts (it hopes) the appropriate sizzle. Each claims that its technique is the best, and can show you the technical measurements and explanations to prove it. The upshot of all this is that 1-bit systems claim to improve a number of "flaws" in multi-bit systems—most significant of which is low-level linearity. It's not exactly a secret that the linearity of multi-bit converters is difficult to maintain in mass-production players—especially of the low-priced variety. There's no reason that a 1-bit player has to be inherently superior to a carefully designed and aligned multi-bit design; some of the best converters around are of the latter variety. It's just more expensive to make them, and to do so consistently. And there is a certain appealing elegance to the 1-bit variations, even if the actual implementations end up being rather complex.—Thomas J. Norton

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Footnote 1: All 1-bit systems use a very high clock rate.