Paul McGowan writes:
I’ve been thinking hard about how to simply and easily explain DSD, 1-bit, Pulse Density (all pretty much the same thing) and decided it’ll be easier to break it up into a few small posts. Let me first explain why we’re talking about it, what it is and why it matters.
PCM is on its way out. The very language that started the digital audio resolution is going the way of Esperanto. Once a valiant attempt at a universal numeric representation for all things audio, PCM is used to store the music, but it isn’t what we listen to in a growing number of modern DACS. What’s replacing it is a form of what’s known as PDM, Pulse Density Modulation (DSD). Now, having read that statement your first reaction may well be “Hey, wait a minute, DSD is what I’d like to listen to but my DAC is PCM as is most of my library”. You’re right, but probably wrong as well.
Most modern DACS convert the classic PCM signals into a type of DSD for you to listen to. Whether the internal DAC chip is based on Wolfson (like ours), ESS Sabre, Analog Devices, TI, Burr Brown etc., just about every new and advanced DAC chipset coming out today are all based on Delta Sigma technology (1-bit, DSD). So what’s the first thing that happens in a Delta-Sigma converter found in the vast majority of all high-end DACS? The digital audio (PCM) is sent through a converter and changed over to DSD. They all do pretty much the same thing. Here, take a look at the block diagram of the Wolfson (click it to open a bigger version):
Note the very first block in this picture is the audio interface. This is where the outdated and soon to be replaced PCM data enters into the system and is soon to be converted to DSD. In fact, you can see that most of what’s going on in this DAC chip is handling the PCM, running it through filters, volume controls etc. and finally, near the end of the chain, it is sent to the actual D to A Converter – a digital version of a Delta Sigma modulator. (You may also note that DSD entering this chip is converted to PCM so it can be processed, but that’s for another discussion).
Why do I make such a big deal out of this and why does it matter at all? Because the two schemes are so fundamentally different that I believe it’s important to have a basic understanding of how they work and why they are different.
So, to get ready for tomorrow’s post I have something for you to think about. I’ve brought this up before. It bears repeating.
One of the most fundamental of differences between PCM and PDM is how far away one is from analog and, of course, how close is the other.
PCM is about as far away from analog as you could get. PCM is a computer code. If you place the digital output of your CD player into your preamplifier and give it a listen, you get nothing but noise.
PDM (DSD) is about as close to analog as you can get without being analog. If you place the digital output of of a DSD player (were there such a beast in the first place) into your preamplifier and give it a listen, you get music.
That’s a big difference and one we should explore a bit more.