DSD and Class D

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A common engineering joke around PS Audio is the TLA, a self-referencing acronym of the finest order. TLA, DSD, PCM, oh my, oh my. Sometimes I get a little crazy with all these initials representing technologies. One of the most common misconceptions is that Class D and DSD are the same. Nothing could be further from the truth. DSD is PDM (crap - there those TLAs go again). Pulse Density Modulation—sometimes known as 1-bit audio—is about as close to analog as digital audio gets. Instead of PCM (there we go again...) which involves ones and zeros in undecipherable code, DSD bunches greater or fewer numbers of "on" bits together to form music. Take a look at this picture: The white 1-bit spaces are zeros while the blue 1-bit spaces are ones. Notice when the ones outnumber the zeros the red sine wave goes up in level. White spaces have less energy and the sine wave goes down. This is very different than Class D. Class D is another clump of letters, PWM (… sigh). Pulse Width Modulation has only one "bit" as well, but instead of many 1-bits of greater or lesser density, as in DSD, Class D keeps that bit on or off for shorter or longer periods within a given frame of time. Here's an example. Note how the red shapes get wider as the sine wave rises, narrower as it gets smaller. This is what Class D is. If you'd like to get a personalized view of this, I produced a video called The Difference between DSD and Class D. Wrapping our arms around all these terms so we have an inkling of what they mean helps navigate the complex world of high-end audio.
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Paul McGowan

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