In article , Johnny B Good
wrote:


The point he was making was that once the final mix was down mixed and
normalised to fit within the dynamic range of the CD format (with shaped
dithering this is a massive 120dB,

Erm. It is misleading to call that the 'dynamic range'.

The basic dynamic range is more like 90 to 96 dB depending on the choice of
dither, etc. But because of the nature of human hearing the perceived range
can be larger, depending on a lot of details.

What is correct is that - correctly dithered/noise shaped - the system
allows the recording and detection on replay of signal patterns well below
-96dBFS. However that comes with a lot of "provided that..." qualifiers.
Simply stating the above value without explaining that can be very
misleading and confuse people.

The "120dB" figure tends to stem from people quoting against
power-frequency spectra that divide the entire range into enough bins that
each individual frequency bin only has an amount of noise/dither that is at
-120dBFS. But the human isn't only hearing a single bin. What the human
*is* doing is employing complex signal processing in their brain/ear to
'pattern recognise' so the 'recognised' waveforms sound clearer above
the noise.


comfortably matching the widest limits between the sensitivity and pain
thresholds, marked in red, of human hearing demonstrated by the
Fletcher-Munson equal loudness curve plots) you had every aspect of the
performance you could usefully present (and then some!) nicely
encapsulated by the CD standard.

Again, the FM curves only show the perceived loudness *for isolated single
sustained tones*. Human hearing reacts differently when that isn't the
*total* input. The curves are also averages, so your hearing or mine might
be markedly different.



The only remaining issue, pointed out by Jim, is the question of the
quality of the DACs used to effect a flawless replay of the audio so
carefully encoded into a Music CD. On a technical level at least, this
is a problem that was solved over two decades ago using oversampling
techniques to neatly sidestep both the demands for a 'brickwall'
analogue filter and to push the aliasing/digital artefact noise an
octave or more beyond the 22.05KHz region.

In principle, yes. In practice people still use flawed ADCs, DACs, and
signal processing. This sometimes shows up when the results are analysed.
e..g the recent Verdi Requiem cover CD from BBC Music Magazine. You might
assume these problems ceased decades ago, but alas, real life isn't so
simple. :-/

[snip other examples of how our understanding has developed, but alas not
everyone may have kept up.]


One would hope by now that the more reputable manufacturers of CD
players who subscribe to the best practices of "High Fidelity" have long
since 'put this one to bed', removing any final (misplaced) criticisms
of the, now venerable, CDDA format.

In practice some makers of replay equipment and CDs have dealt with these
issues. But not every one has, in every case.

Jim

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