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Topic: Nyquist was wrong?! (Read 55650 times) previous topic - next topic
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Nyquist was wrong?!

Reply #75
Something I just found about audibility of ultra-high frequencies:

From http://groups.google.com/groups?hl=es&lr=&...a%2Ben%2BGoogle

From an AES lecture from David Griesinger:

"Adding ultrasonics to a recording technique does NOT improve time
resolution of typical signals â?? either for imaging or precision of
tempo.  The presumption that it does is based on a misunderstanding of
both information theory and human physiology.
Karou and Shogo have shown that ultrasonic harmonics of a 2kHz signal
are NOT audible in the absence of external (non-human) intermodulation
distortion.
Their experiments put a limit on the possibility that a physiological
non-linearity can make ultrasonic harmonics perceptible.  They find
that such a non-linearity does not exist at ultrasonic sound pressure
levels below 80dB.
All commercial recordings tested by the author as of 6/1/03 contained
either no ultrasonic information, or ultrasonic harmonics at levels
more than 40dB below the fundamentals. 
Our experiments suggest that the most important source of audible
intermodulation for ultrasonics is the electronics, not in the
transducers.
Some consumer grade equipment makes a tacit admission of the
inaudibility of frequencies above 22kHz by simply not reproducing
them.  Yet the advertising for these products claims the benefits of
â??higher resolution.â?
Even assuming ultrasonics are audible, loudspeaker directivity creates
an unusually tiny sweet spot, both horizontally and vertically."

More at http://world.std.com/~griesngr/ , at the end.

Nyquist was wrong?!

Reply #76
The lecture is available at http://world.std.com/~griesngr/intermod.ppt

Among other interesting things, it says and gives some examples of how most DVD-A and SACD the writer tested were just resampled from a 48 KHz master. In other words, they had no content at all over 22-24 KHz. And of the few he found had content over 48 KHz, it was in quite small amounts (40 dB below the fundamental, as much). It also shows ultrasonic noise characteristic of SACD, but somewhat surprisingly, just in one of the SACD disks tested.There are some spectrum graphs showing all this.

Also, it says that high directivity of speakers at ultrasonic frequencies, together with high attenuation of those frequencies along the way to the inner ear, makes reception of those signals possible just at a very small spot at the listening location.

There are many other explanations and test results over intermodulation, hearing internal working and such.

Nyquist was wrong?!

Reply #77
I know I'm attracting flamage here, but in trying to establish exactly what is "good enough", we have to remember that the process of audio-quality refinement will inevitably reach further into the realm of intangibles. There are perceptible differences, noticeable differences, and obvious differences. Obvious differences are the ones we can talk about with any degree of confidence, e.g. "wow, that 128kbit CBR file really does ring!". Noticeable differences are those we might not be able to put into words but that still affect our appreciation of the sound's quality. There are still perceptible differences that we may not be entirely conscious of, and it's this region we're starting to stray into by comparing CD-audio with DVD-A, SACD, or even the original analog signal. The sad truth is that it's almost unavoidable that differences must exist that are not justifiable within our current, limited models of the ear and its interaction with sound. The fact that serious audio enthusiasts (and I mean that in the best possible sense  ) are still today talking in terms of absolute frequency/tone response when discussing human hearing shows exactly how limited our knowledge is.

We must refrain from pooh-poohing the notion that the ear is nonlinear, because to date there is simply no good reason to suppose that our models are complete. The ear is a living subsystem, and the most essential defining feature of living systems is that their responses are nonlinear!

To summarise: just because you haven't discovered the mechanism yet, it doesn't mean it 's not there.

 

Nyquist was wrong?!

Reply #78
Quote
just because you haven't discovered the mechanism yet, it doesn't mean it 's not there.

Yes, but it means that it it doesn't affect us.
There is a difference between a fact for which we don't know the explanation, and the absence of fact ! This board's Terms of Service are strict : 8. Any statement about sound quality must be supported by the author responsible for such statements by a double blind listening test demonstrating that he can hear a difference.
This way, we have a fact to begin with, then, we can search an explanation. Without blind test results, we consider that we have no fact to explain.

For DVD-A or SACD quality, we are currently gathering facts, but they are difficult to reproduce in order to confirm them : the japanese  Oohashi et al. experiment, and Listen's results.

Griesinger's experiment, that Nika Aldrich and I have confirmed, doesn't necesseraly dismiss them, because we listen for intermodulation between continuous tones, while in Oohashi et al.'s experiment, the recording used was a gamelan's recording, that is a kind of metallophone. The negative result in Griesinger's experiment is that if a 90 dB (for example) ultrasound doesn't intermodulate, then the 50 dB harmonic of a music instrument certainly won't.
But we forgot that this 50 dB measured value is an average value, while the gamelan has high frequency content mostly on attacks, thus, since the high frequency content is present only for very short times, its level must be very high in order to give a 50 dB level after averaging.
In order to get the instant level, the frequency analysis should be done with the shortest possible FFT window, so as to see how loud the ultrasounds can get, compared to the other frequencies present at the same time. As long as the FFT window is longer than the attack duration, the instant level of some high frequencies may be higher than measured, if they are shorter.

 
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