Can't get there from here

November 18, 2012
 by Paul McGowan

The day of the 16 bit ladder DAC is pretty much over. If you were particularly fond of the beast, despair not because we have something better called DSD.

The classic ladder DAC relies on a very accurate division of current (eventually voltage) between steps. If you think about it, this fact should now be an obvious one to you – if we’re building something out of steps, the accuracy of each step would be critical. Since each step represents a doubling of the last step, we get into finer and finer steps as we add bits. The problem with a ladder DAC is there’s a limit to how accurate we can make these steps using resistors, which is what a ladder DAC uses to divide the steps.

Once the accuracy of the steps gets into the parts per million figures we need to exceed 16 bits we run into trouble. Temperature changes and parts accuracy become the 300 pound gorilla in the equation. There’s only so much you can do to make a resistor accurate and stable in its value and so DAC designers starting running into major brick walls when they attempted to build DACS with enough accuracy to handle dynamic ranges necessary for 20, 24 and even 32 bits. Remember, 24 bits is a whopping 144dB of dynamic range which should accurately cover the range of a single molecule’s impact on your eardrum to that of the loudness of a jet blast – that’s a hell of a range. If you’re interested in more examples, go here to see.

Because PCM requires a ladder DAC to function, designers had to start figuring out a new scheme that could operate without need for the classic PCM conversion of the ladder DAC and they turned to a model called Delta Sigma. Delta Sigma modulation has other names as well, among those you may be familiar with are PDM (Pulse Density Modulation), 1 bit audio and, our old friend, DSD (Direct Stream Digital) which is a trade name by Sony for their version of PDM. DSD is also the methodology behind another Sony trade name, SACD (Super Audio Compact Disc).

So pretty much all modern 24 bit and beyond DACS accept PCM on their inputs but then convert that PCM data into a 1-bit stream that the output DSD process can recognize.

So for those of you who feel you’re not part of the DSD crowd because you have a high resolution PCM DAC, despair not, you’ve been listening to DSD all along.

Tomorrow I’ll begin to explain how this works.

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