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1-bit, 2-bit, oh my!

— By Paul McGowan —
A quick note correcting what I wrote yesterday about editing DSD. While it is true you cannot directly edit DSD, I reported that Cookie Marenco of Blue Coast Records recorded in DSD then converted to PCM, made her edits and then went back again to DSD. Not! While this is what some DSD editing systems do, Blue Coast converts to analog, makes any adjustments required then back again to DSD. Duh. That's a much better solution and an obvious one as well. I am sure thankful there's plenty of people smarter than me. One more note. We are embarking on a quick journey to try and understand Sigma Delta converters - the heart of DSD and the core of most modern A/D's and D/A's. I will refer to these as 1-bit converters. In practice they range from 1-bit to as many as 6-bits. I bring this up to make sure the overly attentive readers don't get bogged down in what is, for explanatory purposes, minutia. 1-bit, 2-bit, 4-bit, doesn't matter. The idea I will try and explain is the same. :) So we understand that a traditional PCM A/D Converter takes a snapshot (voltage sample) of the analog musical signal, converts that single voltage level to a number and that becomes a "word". The PCM stream is a series of these words, each a numeric representation of a voltage level. Because each word is in a type of code, you can't listen to the data and make sense out of it. Listening to a PCM stream would sound like nothing more than noise (perhaps a swarm of nasty insects). To get music from this code you need a converter that understands the numbering system used to encode the music, converts it back to voltage, and cleans up the output through a filter. 1-bit converters, also known as Delta Sigma, DSD or PDM are a completely different format altogether. Instead of breaking up the incoming analog into discrete samples, converting those samples into numbers, the 1-bit system is much simpler (on the surface). Take a look at what a PCM digital signal looks like in this illustration from Wikipedia: PCM Doesn't look like anything recognizable does it? Nope, just a series of rectangles and, coincidently, if you try and listen to this stream directly you get noise (as I mentioned). Now, take a look at what a PDM (DSD) data stream looks like. Pulse-density_modulation_1_period The red line showing the analog output when you play this stream back. Note how the top of the sine wave has all the rectangles filled in with blue, the bottom of the sine wave empty and the middle of the sine wave half and half? That's the secret to how you can playback DSD stream directly. In fact, all you need to make a simple DSD DAC is a resistor and capacitor. Together they form what we call a low pass filter, and that simply smooths out the choppy lines. DSD is really simple on the surface. How do we get these 1-bit pulses to "do their thing"? Let's dive deeper tomorrow.

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