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DSD vs PCM Interview with Ed Meitner

DSD vs PCM Interview with Ed Meitner

First presented at Positive Feedback, Issue #11, Mike Pappas interviews master engineer Ed Meitner about the originations of DSD audio.  We have highlighted one section below that describes the differences between DSD and PCM digital audio.  You can read the entire article at Positive Feedback.

Meitner: See, I always have to come back to the same thing, it’s that your A to D converter starts off its life — I should say the audio through an A to D converter starts off its life — as a one-bit, or similar to a one-bit signal, and then you muck it up.

Pappas: And as you refer to that process, PCM is alien.

Meitner: Yes, it has really nothing to do with audio. You have minimal resolution at zero crossing, whereby with DSD you have maximum resolution and on and on.

Pappas: So in other words, DSD is pretty much the —

Meitner: It’s the least conversion of a conversion.

Pappas: And with PCM, you were mentioning that you know your biggest problems are when you get around the least significant bit, which is at zero crossing; with DSD, that’s where you have your maximum resolution.

Meitner: That’s right.

Pappas: That’s very interesting.

Meitner: You have, in fact, a situation that is very akin to what we hear. We mostly hear velocity changes. Now velocity changes are at maximum at zero crossing of the sine wave. So this is where you have to be so careful. And if you look all through the high-end audio industry, it’s class A, class A, class A. And you know the old solid state zero crossing distortion amplifiers and stuff like that never worked.

Pappas: Because the ear is more sensitive at that point.

Meitner: Yes, "maximum velocity" means "maximum intensity," which means the point where we hear the most. So now look at a PCM signal at zero crossing, and all you’ve got at that moment where it crosses zero is you have zero-bit resolution. The only resolution you’ve got is dither.

Say you have a 16-bit system where one bit is dithered. You now have your noise floor quantized at one bit, and that’s why, when you go to higher resolution tracks or higher resolution PCM converters, people think they are better — and they might be — but the really fundamental reason in my mind why they are better is because you quantize your noise with more bits.

Pappas: And that exists through the zero crossing issue.

Meitner: Well, anywhere near zero crossing. Because we know that as your signal level goes down, with every 6dB decrease you lose one bit.

Pappas: Right.

Meitner: That’s why when you do digital recordings, you nail this thing to zero dB as much as you can. So let’s look at a typical mike pre-amp that might have, say, an equivalent input noise of -131 or -130 dB, right?

Pappas: Right.

Meitner: So then we put, say 40 dB of gain, which is a typical gain, I guess, and we end up now with a noise at -90. So now we have a 16-bit converter. That converter is going to be now dithered from that noise with one or two bits. Right?

Pappas: Right.

Meitner: So the resolution of that noise at the bottom is one or two bits. Now, we take a 20-bit converter. All of a sudden we have the same situation, the same noise floor still, except that that noise floor is now quantized with five or six bits, and so on and so on. So I call that "the bits that are dancing."

Pappas: Dancing bits.

Meitner: Which is what gets you through the parts where you need resolution and you just don’t have the bits to do it.

Pappas: And DSD gets around these problems.

Meitner: Yes. And that’s what I call a friendlier conversion or a lesser conversion or minimal conversion, I should almost say.