Thank you! However, my question was specifically with regards to DSD, which I would understand as Sony proprietary. And it was my understanding in the early days anyway, not so sure now, that it was also not public in any way. In order to get the story on DSD, what specific technique(s) must be used or not, one had to be a Sony licensee, and many early technical commenters, in early AES papers, could not comment accurately because they did not understand the required noise shaping, which itself might be "adaptive" in some way, truley clever perhaps and inimitable wihout similarly intense effort.
My feeling is nobody can really comment well without really knowing what is going on. And for that matter, I would recommend that nobody use DSD, as I an philosophically opposed to closed things that I am not allowed to understand (let alone play with, manipulate, etc., but I'll have more to say about that, I hope). At this time PCM systems are entirely open, very well understood, and easily manipulated by end users.
But I won't let lack of knowledge inhibit me in making some interesting speculations, which you have prompted me to describe. And I'll concede I don't understand even the general principles of SDM itself well enough, let alone the secret mysteries of DSD.
But firstly I would prefer for starters not to think of the final conversion to analog as being what distinguishes what I consider the true PCM from not PCM. PCM, as DSD, is fundamentally a coding format, a way of transmitting information, or storing it, etc. Thinking about the conversions are the next step. The realization to analog could take many different means, SDM, or whatever. I prefer real PCM end-to-end with successive approximation converters (such as the PMI units, still among my favorites) and flash converters, my favorite being PCM 1704. So I reeled in horror seeing Sony's first marketing abolishing both my favorite kind of ADC and my favorite kind of DAC in one blow, and establishing a trend that non-DSD capable DAC's could not be sold.
Please bear with me, and I'll explain why I think as I do.
First start with a naive view of DSM and DSD in particular I've seen on this blog and confess I still harbored until digesting this blog a little. In this naive view DSD can't respond to steep transients because it can only go up or down one LSB in each time interval.
The reality is quite different, we are shown without explanation DSD64 even producing picture perfect impulses, no visible slewing at all.
Aha, this is the magic of "noise shaping." And also DSD isn't a set of bricks that can be added to or subtracted from, as in the naive view. Instead, it is a series of deltas feed to the algorithmic feedback loop known as the noise shaper.
So it isn't like we're dealing with fixed bricks anymore. A straight sequence of 1's isn't going to slew gradually. The noise shaping feedback loop is going to reinforce the upward movement so we get a straight up transient. Miraculous!
But when you see such miracles it is often a good idea to look under the hood or in this case think (or speculate, as I have no real information about DSD noise shaping).
Now that we have applied our deltas to create the straight up transient, we have a little problem. We have used up the deltas that might have covered a period of time just to better describe one instant of time. So now we have fewer deltas to describe the time interval.
Here we have a decision to make. Either we can reduce the information content of the post-transient interval, sort of like lossy compression. Or we can warp the time a little. An adaptive algorithm could account for both cases when they seemed to be less audibly dangerous.
Now PCM isn't natively inclined to such time warping or information level discontinuity. PCM realized end-to-end without DSM is I still believe the best way, because it uniquely represents no warping of time or information density. DSM is dishonest in it's use of time.
But you could argue, DSM did not take over simply because of Sony marketing and industrial muscle. DSM took over because it was cheap, and because it was and apparently (but I believe dishonestly) remains the best way to get to perfect amplitude linearity, or as close as possible.
But the piper to be paid for the perfected linearity is the messed up time.
To my ears, PCM end-to-end has unmatched pace and rhythm. The linearity, done well, is good enough.