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RE: Thanks...

Posted by Charles Hansen (M) on May 28, 2017 at 12:17:48

>> is there perhaps an analogy to vinyl/RIAA here? <<

I don't see how there could be. Pre-emphasis and subsequent de-emphasis is virtually mandatory for phonograph because of the underlying physics of the transducers. The Redbook CD specification also allows for pre-emphasis/de-emphasis, and it provides a slight advantage (about 1.5 bits extra resolution in the top octave) due to the typical spectral content of music. It seems that it's more trouble than it's worth, as only a handful of very early CDs employed pre-emphasis.

MQA is claiming to do something completely different - specifically, "correct" for "timing errors" created during the original A/D conversion process (so-called "de-blurring"). These "timing errors" are actually artifacts of the steep anti-aliasing filters with sharp "knees" at the corner frequency. The only thing that can be done to "correct" these "timing errors" ("de-blur") is to filter out the "ringing" created by the filter and *hope* that the new filter doesn't create worse artifacts. There are *many, many* digital filters that reduce the artifacts created on the A/D side, the first being from Wadia in the late 1980s. MQA is far from breaking any new ground in this area.

A fundamentally better approach would be to use an A/D converter that does *not* create artifacts during conversion. All DSD A/D converters are free from this problem, as they do not require any anti-aliasing filters at all. However DSD creates a new problem in that it is impossible to process the signal (change volume levels, mix, EQ, and so forth) without first converting to PCM. Conversion to PCM is done with anti-aliasing filters, so the problem springs back to life (think Whack-a-Mole here). Plus each conversion back to DSD adds additional noise.

I believe the best solution is to use true high-res PCM (trivially easy to post-process), but use digital filters on both ends (anti-aliasing for A/D and reconstruction for D/A) that don't introduce sonic degradation. The Ayre QA-9 ADC does exactly that (for dual- and quad-rates only - the single rate minimizes the artifacts but cannot eliminate them). Many companies have created DACs with special filters designed to minimize playback artifacts and also reduce the artifacts from the A/D converter (beginning with Wadia in the late 1980s). There are many companies that have followed the path that Wadia created, or in a few cases pushed that envelope even further.

As always, my postings reflect only my personal opinions and not necessarily those of my employer or son's pet snake.


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