I'm not sure exactly why you would only want to use half the signal to measure SNR or dynamic range or whatnot. Your discussion/perspective applies only to symmetrical signals centered around the "center" axis. In that case, then yeah, I guess you could use only half of the wave to qualify the "dynamic range" of "16-bit signals" (in actuality, you would be using only 15 bits, hence your 90dB figure.)
In practice, the "point of silence" really doesn't have to be at "-inf" or right in the middle. If you feed a straight DC offset into a speaker, it won't make any noise, the cone will remain at a stable position. Silence isn't categorized/qualified by a position on the axis, but by a presence or absence of movement on the medium/mechanism (aka, signal.)
That said, in practice, if you feed a straight dc offset to a speaker, it will most likely get damaged, explode, and/or catch fire, which might be a bit noisy
.
Still in practice, your half-wave dynamic range measure would only be usefule for balanced signals, and not the whole of the signals that can be represented in 16 bit.
About Nyquist, you are right that you would have a problem sampling a pure 22050hz tone with a 44100hz sampling rate, since it's the "limit" value. The theorem says that all signals *below* half-the-sampling-frequency can be reconstructed almost perfectly. With a signal of exactly half-the-sampling-rate, it depends on where you will sample the signal. With all other frequencies below 22050hz, you won't have that kind of syncing effect and you will get much better reconstruction.
If the signal exceeds the sample rate, it will slowly get mirrored back into the sampled spectrum. For example, a 22051hz frequency will appear at around 22049hz (not that you can make the difference with your current auditive apparatus
), while a say 30000hz signal will get aliased at around 14khz.
So yeah, 44.1khz is a bit of a small value, I guess for the time it was considered "good enough", but you'll definitely still hear some artifacts from it. I'd still rate it at fairly below "annoyance" level, just slight coloring, which can be altered a bit given the converters and signal path you use at reconstruction and things.
All that human-ears-max-out-at-20khz stuff is, as far as I know, based on studies made in the 20s involving "pure signals", like say I feed you a straight 30khz signal by itself, you won't hear it. Great. But what if that 30khz signal is a higher harmonic of a much lower signal that you are actively hearing and listening? I suspect then that it might play a much bigger role than a 30khz signal by itself, and that's partly why vinyl sounds so good even the dynamic range is much smaller than CD and so on.
So yeah, for the average untrained hear, CD sampling rate is alright, with a bit of training know, it's like everything else, you can push it a bit further.
Mmm rambling, need coffee
