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RE: Tom - I appreciate your indulgence in responding so thoughtfully to my posts

Chris, you're asking a great question that has been discussed with many thousands of words. I'll give you the short answer, and you can research the underlying discussions. I'd suggest if you're interested, the many postings at Computer Audiophile. Do a search on Miska, the provider of HQ Player, particularly on the DAC Forum.

The short answer to your concluding question is yes, the decimation filter has to eliminate the out of band noise, and in doing so, injects phase artifacts (unfortunitly required by the steepness of the filter) that render the resulting PCM words not convertible back to the original PDM/DSD bitstream(s). Therefore, it's a lossy process.

It's the effects of the necessary decimation filter required in the DSD > PCM conversion to preclude folding back into the audio band any energy above half the sampling rate of the converted PCM result. Since the vast majority of energy in any PDM (DSD) bitstream lies above the audio band, being almost all the energy approaching the bit rate frequency, the decimation filter has to be very steep approaching the upper frequency of the audio band of interest. The decimation filter shaping, with its subsequent phase distortions and nonlinearities (necessary to achieve the required filtering shape), make the reconstruction process impossible back to the original bitstream. Therefore, it's lossy, and in my experience quite perceivable.

A PDM (DSD if 1-bit) bitstream is almost entirely uncorrelated white noise. It's a useable A/D converting format through the process of noise shifting (an activity of the Delta-Sigma Modulator), where the uncorrelated noise is shifted to above the audio band, then filtered out when the audio content is retrieved in playing. Since PDM/DSD is already an analog bitstream, it simply requires low pass filtering to retrieve the audio content (there's many ways to do this, which I won't go into here, but the filter has to be either very steep (not good) if the audio is to be retrieved at the original bit rate, or the bit rate upconverted to a much higher bit rate, and a gentler shaped filter employed).

But for digital post production processing, since PDM/DSD is an analog representation of changing input analog signal levels, it contains no digital values ala PCM. It only contains analog signal level changes represented by the density of bits in the bit stream. Since no values are represented (actually, the density of bits is directly proportional to percent of modulation, which in turn CAN be translated to absolute signal level values in the conversion to PCM), the bit stream must be converted to a value based format (PCM) to be further digitally processed.

So, back to my original point; if as a recording producer you can archive the A/D converters original front end Delta-Sigma modulation bitstream(s), in the form of a 1-bit DSD bit stream(s) at the original recorded bit rate, why not do that? Then you can convert to whatever required/desired 2nd generation format meeting the needs/desires of the producing engineer/label, and have as backup the original PDM/DSD content for future possibilities.

2L's Morten Lindberg, as well as Bert van der Wolf of Northstar/Turtle/Challenge Records (among others) say they prefer the sound quality of DXD, and so record in that format (they say). That's their prerogative and choice, but it is a 2nd generation conversion.


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