reeve_mike wrote:
Sorry if it disappoints but I know well the work of Claude Shannon and
Harry Nyquist ...
Opps, sorry.
What I was trying to contribute was that the samples on the CD do not
faithfully represent the musical waveform,
they only represent a version of it filtered at 20.5KHz, which seemed
to be relevant at the time but now I can't remember why ...
I would not expect it to be 20.5kHz.
The sample rate is 44.1, so in theory, you could have signals as
high as 22.05kHz.
Realistically, there is nearly nothing above 20kHz to start.
Not only is 20-20kHz the standard "spec" but most microphones
have serious roll off above 17kHz or so. And all of the preamps
used for the microphones, especially the 'vintage' ones that people
swear sound best. Neve, SSL, etc.
And analog signals don't stop at clean numbers like 20.5, they just roll
off at X dB per octave.
As an aside, just because 20Hz-20KHz has been used for years doesn't
make it 'right', there is increasing psycho-acoustic experimental data
that suggests that even though pure tones above approx. 20KHz cannot be
heard directly their presence in music signals can be 'detected' in some way -
as
pointed out by Pat ...
Right is an interesting concept here. I believe that the idea of a brick
wall fall off at 20kHz is dumb, I believe that there are harmonics and
interactions. I don't know when the 20-20kHz idea became popular, but
by the early post-War days, when Hi-Fi was invented by signal corpmen
going to engineering school on the GI-bill (same guys who made ham radio
be real), it was established.
It is next to impossible to get a brick wall filter using coils and
capacitors. I think 24dB/octave is about it, but there might be more.
So you would expect that a good filter can only cut the signal by
24 dB going from 20kHz, up an octave to 40kHz.
But the mics, preamps, Neumann cutting lathes, and all the
anti-feedback controls all combine to each throw away a couple more
dB per octave. There just isn't much up there.
Arbitrary technical limits are often post-rationalizations of the
technology limits of the time (e.g. 44.1/16), and one might point to
the historic limits of valve-amp output transformers, speaker drive
unit technology etc. (and the immature state of psycho-acoustics) for
leading us to 20Hz-20KHz ...
All the early HiFi stuff was tubes and transformers. Getting even plus
or minus 3dB 20-20kHz was hard and expensive. And no speakers before the
early 80s tried for 20-20kHz.
One of the better speakers of the era was the Quad 63, which has a
writeup in Stereophile on their website
http://stereophile.com/floorloudspeakers/416/index11.html
The scale is not detailed enough to quote where the -3dB points are, but
my eyeballs estimate it as 50-15kHz.
Things like Bozak systems went lower, and getting into the 30 hz zone
was not all that hard (but took a lot of power/space).
All in all, I think that 44.1/16 was a good engineering choice at the
time. The key was that CDs were designed to replace casettes, and be
better quality, longer living, and harder to replicate. That is why the
labels wanted them.
They did replace cassettes. And in all but a few cases, replaced vinyl.
The current Stereophile (ro maybe it was TAS) has a quote from
Boothroyd/Stewart big wig who said that if they had chosen 20 bit and
50kHz (or maybe 55kHz) that we would have enough to have perfect sound
forever. :-)
--
Pat Farrell PRC recording studio
http://www.pfarrell.com/PRC
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