From: jj@alice.att.com (J. Johnston)
Newsgroups: comp.compression
Subject: Re: Too much lossy encoding?
Keywords: PASC, audio
Date: 13 Apr 92 14:32:47 GMT
Reply-To: jj@alice.UUCP (J. Johnston)
Organization: NJ State Home for Bewildered Terminals

In article <1992Apr10.225301.1659@cco.caltech.edu> madler@cco.caltech.edu (Mark Adler) writes:
>In article <22537@alice.att.com> jj@alice.UUCP (James D. Johnston) writes:

>I agree completely.  My question then is, what does 22 KHz mean in terms
>of the ear's frequency analysis? 
Just what it always means.
>The ear does not split things into
>impulse functions in the frequency domain, even though we can approximate
>that analysis (probably with a lot of brain involved) to about 1/1200
>of an octave.  (Well I can't, but piano tuners can.)
?eh?  The basic filter in the ear is about 1/3 octave wide at high
frequencies (that's a generalization, but mostly a true one).  The
detection mechanisms sharpen this up a lot for 1 tone presented
at a time.  For two beating tones, the waveshape of the beating is
also neurally detectable if they're close enough to
hit the same neurons.  (Yes, this is a mild oversimplification,
but mostly correct.)

In any case, it's easy to show, using fourier analysis, the
effect of windows, or, to put it another way, you don't
need delta functions in either frequency or time.  Since the
ear has a limited (but good) resolution, you clearly can
limit the time-interval to that resolution, if not more.

In fact, you quickly discover that you can limit the detection
interval to roughly the length of the impulse response of
the cochlear filters (with some smartness afterwards, of course).

Given these obvious methods, I don't understand why
you think you need infinite frequency resolution.  Face it,
there's something just like the Heisenburg limit for
frequency determination, i.e. your window limits your 
resolution.  That's simple, it's directly derivable from
basic Fourier mathmatics, and it's a good model of
how the ear works.  Now, you say "model" vs. "physical reality",
but the test of a model is how it mimics physical reality,
and current hearing models (at least a few of them) do
very, very well. (no, nothing is perfect. not even "nothing")

>My point was that that analysis is not a Fourier analysis, and dropping
>all Fourier components past 22 KHz probably has little validity with
>respect to the signal decomposition that the ear (and brain) does do.
Actually, it comes pretty close, for most people.  While it's true
that children can hear above 22 kHz, the cochlear damage due
to life in the mechanical world eliminates that by the age of 12
in most cases.  In any case, the filters at such frequencies
provide almost no frequency resolution and are very insensitive,
ergo hard to excite in any case, even when a person doesn't
have some damage.  Yes, for some people, for a while, they
are detectable, but the act of detection at those frequencies
is close to the levels that cause damage.  Basically,
the neurons that detect high frequencies are very close to the
part of the ear that gets the excitation (the farther away,
the lower-frequency the center frequency of the filter), and
take all the beating you hear...  This is also what limits the
frequency resolution at supersonic frequencies, because there
is very little detector space, and very little resolution as a
result.  (I.e. the 1/3 octave approximation goes bad at
frequencies above 16kHz or so...)

>As you have done.  It is also
>not necessary to precede every posting with "what follows is opinion and
>misconceptions based on hearsay and sometimes total fabrication", since
>everyone who reads newsgroups already knows that about every posting they
>see.

True, but I am quite dismayed to find some of the rampant
superstition usually put forth in wreck.audio working its way
into comp.compression (not necessarily YOUR article).

What I find dismaying about everyday life is that people are
looking for excuses to throw out technology, reject mathmatics
and science, and in general try to move back into a less
enlightened and less educated age, rather than trying to
extend the scope of knowledge and find out what to do when
knowledge proves insufficient.  You know that from your own
experiences in standards, I'm sure.
-- 
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