Re: What's the use of a 192 kHz sample rate?

"robert bristow-johnson" <rbj@[EMAIL PROTECTED]
> wrote
in message
news:4f0de839-fa35-4c51-93c8-751c12118f79@[EMAIL PROTECTED]
> now, long ago, when DSD was just coming out, i
> participated in something like this that an audio guy
> (now an author) named Bob Katz did.  this was before Pro
> Tools HD and they were using some expensive system from a
> company called Sonic Solutions that could do 192 kHz.
> they recorded at 192 kHz some test sounds (including some
> high frequency percussive sounds like castenets, cabasas,
> cymbals) along with synthesized bandlimited (to 96 kHz)
> tones of all sorts of waveshapes at a variety of
> frequencies from below 10 kHz up to 90 kHz.  i don't
> remember all of the test sounds, but they made sure that
> most, if not all, had content well above 30 kHz.  the
> subject listened to those sounds in two different forms,
> but both with a 192 kHz playback rate.  one form was the
> raw recorded sound, the other was processed through a
> phase-linear FIR filter with a lot of taps (i thought it
> was around 300 something taps, it was not a real-time FIR
> filter but proceessed one sound file into another) that
> was flat to within 0.01 dB up to 20 kHz, had a smooth
> transition band from 20 to 22 kHz, and then was down by
> more than 130 dB for 22 kHz to 96 kHz. we padded the
> beginning an end of the original sound file with zeros,
> of half the length of the FIR on both ends and the
> non-real-time FIR was not "causal" and had a delay of 0
> (so it started responding before the first non-zero
> samples) and the filtered sound file was lined up in time
> with the original.

> now this isn't what Bob Katz did, but it is what i wished
> he did: called the original sound "A", and the LPFed
> sound "B".  so sometimes the subject hears AA, sometimes
> AB, sometimes BA, and sometimes BB (all four permutations
> exist in equal quantity) and for each pair, the subject
> simply has to say if they think the sounds are the same
> or different.  every time a subject says that AA or BB
> sound different, we count that as a false positive and
> subtract that count from the number of times the subject
> says that AB or BA sound different. likewise for false
> negatives.

That is called same/difference testing.  It is an older form of testing, 
valid enough as far as it goes. The most im****tant question in a test is
not 
the order of presentation of variables (within reason) but rather whether 
the listener can determine the identity of the so-called unknowns by any 
means other than listening. Obviously, if one can determine the the
identity 
of the unknows by any means other than listening, the unknowns are not 
unknown but known, and there is no test.

> now Bob didn't do that, but he did something like it (i
> think it was ABX) and there was no statistically measured
> difference.

At this point 100's if not 1,000s or perhaps even 10,000s of people have 
done this kind of test, and they obtained the same results as Katz. Please

see www.pcabx.com.

>  people could simply not reliably tell if the
> stuff above 20 kHz was removed or not.  they could not
> tell at all.  no one could.

Welcome to the well-known limitations of the human ear.

> now, if they cannot tell if the content existed above 22
> kHz or not, is there a need to have it there in storage
> or in transmission?

No.

>   if there is no need to have it
> there, and it is removed, what does the sampling theorem
> tell us regarding sufficient sampling rate?

It tells us that a well-implemented 44 KHz sample rate might even be 
overkill.

>> I'm not trying to "prove" anything. I am presenting
>> information which you can consider and believe or can
>> ignore. But you can't say my statements are false unless
>> you have some information to "prove" they are.

Requiring absolute proof can be a sort of excluded-middle argument. IOW 
there are a great many things that have not been absolutely proven false, 
that are generally and correctly treated as being false.  In fact, many
many 
negative hypotheses, such as the idea that anything in particular is
false, 
are difficult or impossible to prove.

Just because I can't absolutely prove that pigs can't fly is not a good 
reason to start an airline based on flying pigs. ;-)

>> Human hearing is not a microphone connected to an amplifier.

Often human hearing is far less sensitive than a microphone connected to
an 
amplifier. So what?

>> It is a very complex process which even includes the
>> brain and we certainly don't understand it completely.

We don't have to understand everything completely in order to come up with

effective working hypothesis about them.

> but we can't hear anything above 20 kHz.

Actually, often many of us can hear something above 20 KHz if it is an 
isolated tone and also loud enough. The question at hand is about
something 
very, very  different - whether or not we can hear the elimination of 
information above 20 KHz from music.

>  even with
> percussive sounds with sharp attacks.  and if we cannot
> hear anything above 20 kHz, then
> 40.0001 kHz sampling rate can store all of the
> information we need. for practical reconstruction
> purposes, 44.1 and 48 kHz are sufficient.

That turns out to be a hypothesis whose contradiction is difficult or 
impossible to prove.

> now, in *processing* sounds with some nasty
> non-linearities in the process, it very well may be
> necessary to upsample to 192 kHz or higher to do that
> non-linear processing, and when it is done, LPF to 20 kHz
> and downsample back to 48 kHz.  but, except for
> experimental purposes, 192 kHz storage or transimssion is
> not necessary.

Agreed.