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Music servers and other computer based digital audio technologies.

RE: I stand slightly corrected..HDTracks does offer SOME process info-

That's pretty much the only way it can be done.

The so-called "DSD" (a marketing term, not an engineering term, meaning anything that Sony wants it to mean) signal has a sharp rise in noise level starting at 20 kHz (not the 25 kHz stated in the HDTracks paper).

There is almost no instance where the energy of the music signal above 20 kHz exceeds the noise floor of the "DSD" process. This was just one of the duplicitous things about SACD. On the one hand they claimed frequency response "up to 100 kHz" and at the same time they said that the out of band noise was inaudible.

You can't have it both ways! If the noise above 20 kHz was inaudible, then why not just use 44/24 PCM? Or if the high-frequency content of the music was important, why bury it under massive amounts of noise? The real answer is simply that they wanted a system that they could license and make royalties from, and that was just about all.

The fact that SACD ended up sounding fairly good in most instances is a happy accident of the fact that there was virtually no filtering on the recording side. Then on the playback side, the Scarlet Book specified:

To protect analog amplifiers and loudspeakers, it is recommended that a Super Audio CD player contain at its output an analog low pass filter with a cut-off frequency of maximum 50 kHz and a slope of minimum 30 dB/Oct. For use with wide-band audio equipment, filters with a cut-off frequency of over 50 kHz can be used.

Thirty dB/octave is a steep enough filter to have a negative sonic impact. This could be dispensed with for "wide-band" audio equipment, whatever that means...

Some players had a switch on the rear panel (really only a handful) to select between the filter and no filter. But when converting to PCM for download, HDTracks (and anybody else) needs to filter out at least some of the noise to prevent problems with systems that are sensitive to that high-frequency noise.

An 88.2 kHz transfer will need a brickwall filter by 40 or 44 kHz, so most of the noise is filtered out anyway. A 176.4 transfer has more flexibility. They have to decide how much noise they want to let through, but they have the advantage of not requiring such a sharp filter. This results in better sound quality.

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