Yesterday I surprised a few of you when we learned that the output of just about every modern DAC with 24 bit resolution is actually DSD, or Pulse Density Modulation, also commonly known as 1-bit. We understand that classic ladder type DACS reach a brick wall at about 16 bits because the accuracy of parts needed to extended beyond that range simply do not exist.
I would ask if any of you remember back to the beginning of this series when I told you that PCM (which is what CD’s work with) is a code decipherable only to a machine that understands that code – and DSD is not a code at all and your analog stereo system can play it back directly. That’s a big difference.
We learned that PCM measures the musical signal with a series of snapshots called samples and memorializes each sample as a multi-bit word. We reconstruct the samples by reversing the process and creating a stepped output which is identical to the input once we smooth out the little steps.
1-bit audio is very, very different. Today we’re going to simply give you an overview of how it works, which is a simple process, and then we’ll get into the nitty gritty of the process which can be a little disturbing to say the least. But let’s save the disturbing stuff for later once we’re all on the same page.
1-bit audio is simple to understand in concept. There are no samples, there are no words, there is no code. Instead there is a continuous streaming “train” of single identical bits that are either on or off. The more bits that are on, the higher the eventual output voltage becomes. The more bits that are off, the lower the eventual output voltage. We refer to this type of scheme as Pulse Density Modulation because when you have a greater number of on bits it appears as more densely populated. Here’s a picture that will help you visualize a 1-bit system.
Pulse density modulation 2 periods How dense are you?
Note the blue areas are on and the white areas are off. Also note the periodicity between single bits is identical. The red sine wave overlaid on this image shows the results of more bits or fewer bits. Where there are no on bits (all white) the sine wave is at its lowest point – lots of on bits and it’s at its highest point.
The speed of the bits is 64 times the sample rate of a CD and some DSD schemes run at 128 times faster than a CD.
Here’s the interesting part of this: if you take a DSD stream and run it through a simple analog lowpass filter to smooth out the on/off transitions, you get music! This is amazing considering that if you do the same with PCM you get only noise.
DSD is a lot closer to analog than PCM ever thought to be.
Paul McGowan – PS Audio, Intl.