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Original Message

RE: To be precise...

Posted by Charles Hansen on May 31, 2017 at 11:53:13:
>> I have been getting some anomalous results ripping HDCD discs
with dbpoweramp, in that the true bit depth of the resultant file
varies. I did wonder if this was due to interpolated disc errors
during the rip. <<

I seriously doubt that any possible errors in HDCD decoding would lead to this result. Following is the curve for the Peak Extend (PE) function given by Keith Johnson in the AES preprint:

The vast majority of the possible values map in a 1:1 relationship, which results in the 45 degree line at the left part of the graph. The end point is also trivially calculated, as it simply compresses the input signal by 1 bit (= 6.02dB). The errors I've noted in all of the available versions of software HDCD decoders are simply in the shape of the curve. This would translate as a slightly non-linear transfer function and therefore introduce a small amount of harmonic distortion - but *not* a meaningful change in the amplitude of that signal.

I would guess what you are seeing is simply variations in the amount of "headroom" the mastering engineer left in the recording. Modern "loudness war" pop material would always have full-scale audio data. With Peak Extend, this would expand the 16 bits available from the Redbook format to 17 bits. But dBpoweramp puts the decoded data into a 24-bit container (for compatibility reasons), and pads the 7 LSBs with zeroes.

On the other hand I have seen some recordings, especially those of classical music and/or audiophile labels that leave more headroom. The signal rarely (if ever) even gets within a few dB of 0dBFS. Depending on the tools used to measure the bit depth, it seems possible that they might simply report the number of *active* bits and ignore the static zero bits used for padding. In that case a recording that never used the very top bits might be reported as having a lower bit depth. However, this is all just guesswork on my part as to what you are seeing.

As always my posts reflect my own opinions, and not necessarily that of my employer or cable installer.