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 The 20kHz limit?
Author: Kevin Bowman 
Date:   1999-08-12 15:06

Hiroshi wrote (in another thread):
As to CDs,they cut sounds above 20k Hz. There are machines to recover (Pioneer makes one of them) those ranges since we feel uncomfortable without those ranges of souds.
-------
Sorry, I had to take this issue out of the vibrato thread and slam it around a bit here. I don't wish to offend you, Hiroshi, but there are many myths surrounding recording, digital music, sampling, etc. that I must debunk.

First - there *may* be a psycho-accoustic effect on humans by frequencies over 20kHz and a *very* small percentage of people can hear frequencies up to 22kHz, albeit very faintly. But the fact remains that physical limit of the average (and most common) human ear is indeed 20kH.

Second - many audiophiles believe that digital recoding does not have the "warmth" of analog - this is partly true. I say digital recordings have more "brilliance"! This disparity is due not to the recording medium but the the advance in audio sensor technology (i.e. microphones). Since the advent of digital recording techniques, engineers have wanted to use the *entire* spectrum up to 20 kHz. Before digital, you would be very hard-pressed to find a microphone that had decent response above, say 18kHz. So what people started hearing with digital recordings was the frequencies they had never before heard from analog recordings - minus the "scratch", too.

Third - I've never heard of device to "recover" lost frequencies. Beside's, it's just not possible. Sampling an analog signal to a discreet medium (digital conversion) is a *lossy* process. There's absolutely no way to recover what the exact original values were. So if frequencies above 20kHz were not recorded, there's no way to "get them back". No matter what Pioneer's advertising hype might be.

It's my belief that recording technology has advanced sufficiently to bring near-real-life sound to listeners ears. But a recording will *never* be the same as hearing an orchestra, opera, big band, etc. live. There's so much involved in the recording process that can (and will) degrade the signal, even with the most expensive hi-tech equipment. Heisenberg's (sp?) uncertainty principle comes into play here which, basically, is this: the very act of observing something effect the outcome. In other words, you can't record sound without changing it.

Feel free to flame away (as I know this is a highly volatile subject) ...

Kevin Bowman


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 RE: The 20kHz limit?
Author: paul 
Date:   1999-08-12 15:28

Good points listed above Kevin.

I find it quite interesting that people can hear and feel tones slightly outside the normal hearing range by feeling it through their bones. That's how some deaf people can pick up the heavy beats of the drum for dancing at low frequencies. Ditto for very high frequency hearing tests and with some implants through the skull.

There is a lot more to Speech Pathology and the Physiology of the Ear/Hearing than this, obviously. However, through the years I've overheard and witnessed an entire certified Master's degree in Speech Language Pathology as I manage a videoconferecing network for education. I've also "been there and done that" enough to realize that there is much more value in experiencing live music than hearing a recording of it. I've even listened to the same band live verus recorded, even with 100% digital recordings. There is a difference, but the average person wouldn't necessarily pick it up. With the really good recordings, you have to actively listen to pick up the differences. Ditto for very good analog recordings of extremely good artists several decades ago. There is a reason why the artists from the 1930s through the 1960s preferred a certain kind of microphone for their performances and recordings. This particular mike could pick up tones and nuances that other mikes just didn't even register.

So, with all that said, you have some valid points. If I can, I prefer to attend live concerts. Failing that, I'll take a good 100% digital recording where available. Failing that, listening to the masters on analog to digital still has its merits and is still quite enjoyable.


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 RE: The 20kHz limit?
Author: Don Berger 
Date:   1999-08-12 17:51

Very interesting, long over-due discussion, Kevin and Paul. There is a short chapter, p 16, in "Clarinet Acoustics" by Lee Gibson on "the ear" and other chapters [none of which I've read] which along with Benade's research should interest at least a few of us. The mention of Heisenberg, and my thoughts from tussling with Einstein years ago, makes me long for youth [and "smart"] !!! Thanx, Don

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 RE: The 20kHz limit?
Author: Daniel 
Date:   1999-08-12 21:21



paul wrote:
-------------------------------
There is a reason why the artists from the 1930s through the 1960s preferred a certain kind of microphone for their performances and recordings. This particular mike could pick up tones and nuances that other mikes just didn't even register.



I have several recordings of jazz bands where they used several different brands and styles of mics so that every range of frequency would be picked up. I don't know enough about all the differen multi-thousand dollar mics there have been and are... or about audio engineering in general to really discuss this topic in detail... i just wanted to state that alot of recording studios used various mics so that the analog recordings (and i'm sure it's done today too for digital) would be as close to accurate in reproduction.

As for Analog playback... the frequency range you hear is partly determined by your equipment. Most phono cartridges are designed to reproduce 10hz to 20khz or 20hz to 25/30khz... tack on a preamp, an amp, and speakers... that's alot of variables to reduce or improve the quality of sound reproduction. I can't see why people would spend $10k on a turntable, $5 on a tonearm, $7k on a cartridge, $6k on a preamp, $5k-10k on an amp.. and then another lump sum on speakers...
My Pioneer PL-510, with a $70-$80 Shure cartridge, that i paid $85 for total works just fine... sure, there's probably SOME difference in the quality of sound from those multithougsand turntables... but not much.. plus most of my records aren't mint pristine condition like audiophile with theseveral thousand dollar setups would have...

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 RE: The 20kHz limit?
Author: Rick2 
Date:   1999-08-13 04:37

1) The 20kHz limit does not exist (except perhaps on Hiroshi's CD player). The upper limit on frequency depends on the nyquist frequency which is completely dependant on the sampling rate.

2) Kevin, I would use the word "crisp" instead of "brilliant." Analog recording is inherently imbedded in white noise. White noise is quite simple to remove from a digital recording. A band-pass filter does the job as white noise is an impulse in the frequency domain.

3) Regarding the recovery of lost frequencies, I tend to agree with you...with a however. There are several conceivable processes for doing just that, including on-the-fly coding of the signal as it's recorded, which saves (but possibly distorts) the details of the signal in the upper frequency. A visual example of this works better. Scan in a photograph, then save it as a jpeg file. Open it again and convert it to a bitmap, then save the bitmap as a jpeg. Keep repeating this and the details in the photo start disappearing because the jpeg coding is not an exact duplicate but a best guess. Another possibility is that an algorithm looks at a signal and compares it with a database in memory. Signals like "this" tend to have high freqency components like "that" so let's put it in. They cheated their way into a richer tone. In conclusion to this point, I must say that you are completely correct that digital recording is lossy. Nyquist frequency is always the hard limit to record without aliasing.

4) I must disagree with your final point. The recording industry has not kept up with the visual. The visual in virtual reality can completely fool your eyes, but the audio is lagging. Stereo still sounds like the orchestra pit is situated someplace between your ears. There is a lot of active research in the realm of 3D sound..i know because I did my masters thesis in this area. There exist 3D (binaural) recordings but you and I will hear the sounds coming from different locations because the geometry of our bodies (especially the pinna of your ear) are completely different. The mathematics behind this are so complicated that it takes a very long time to not only develop them, but also because they tend to be based on slowly converging infinite series', they take a very long time to compute numerically. Bottom line is that I can make a recording that will convince you that you are in a symphony hall. I simply can't tell you what section you will be sitting in.

By the way, you spelled Heisenberg correctly. For technicalities, I disagree with your statement regarding the uncertainty principle, but I'm not anally retentive enough to really care.

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 RE: The 20kHz limit?
Author: Wyatt 
Date:   1999-08-13 04:46

There's NOTHING better than being where the magic's happening. Short of that, most recording and replay equipment is better than our ears are, anyway.

Wyatt

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 RE: The 20kHz limit?
Author: Mark Charette 
Date:   1999-08-13 11:55

Wyatt wrote:
-------------------------------
Short of that, most recording and replay equipment is better than our ears are, anyway.
------
Read A. Benade's seminal tome 1st, and then come back to me and say that with a straight face :^) There's lots we _know_ about acoustics and psycho-acoustics, and a lot more we just guess at.

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 RE: The 20kHz limit?
Author: Kevin Bowman 
Date:   1999-08-13 14:20

See? I knew this topic would be good for a can of worms!

Rick2 wrote:
---
1) The 20kHz limit does not exist (except perhaps on Hiroshi's CD player). The upper limit on frequency depends on the nyquist frequency which is completely dependant on the sampling rate.
---
Right - isn't the sampling rate on commercial CD's 44.1kHz? That means the highest reproducable frequency is 22.05kHz, assuming you have an ideal LP filter on back end of your DAC.

Again, Rick2:
---
2) Kevin, I would use the word "crisp" instead of "brilliant." Analog recording is inherently imbedded in white noise. White noise is quite simple to remove from a digital recording. A band-pass filter does the job as white noise is an impulse in the frequency domain.
---
Huh? White noise, by definition, is uniform across all frequencies. A pure sine wave shows up as an impulse in the freq. domain. White noise translates to a (nearly) straight line (DC, if you will) in the freq. domain. Hum, and sometimes pop, can be eliminated with a band-reject filter. The noise floor that exists on analog recordings can be greatly reduced (below the hearing threshold) by expanding the dynamic range of the recording medium so that the source signal can ride far above the noise floor.

One myth about digital recording is that "there is no noise". Noise exists in a different form: quatization error. Fortunately, this can be greatly reduced (to *near* nothing) by oversampling and by good Low Pass Filters on playback equipment (reduces high-frequency aliasing, or "foldback").

Rick, I see your point about "guessing" at restoring lost signal - but at best, it's only a guess. It's not the real thing, IMO.

Lastly, I know Heisenburg's Uncertainty Principle (HUP) was really meant to apply at the quantum level - but I see it's effects in many fields. If you measure, sample, or observe a phenomenon, you will affect the phenomenon - even if to a very small degree. This is my firm belief. I see it happening in computer programming all the time - some code is not acting right, so you run it on the debugger and "poof", it works fine - or the other way around. Just one example, but I beleive it applies to a lot of other fields too. My conclusion? You can never really "capture" the real thing - it'll always be the real thing changed in some way by the capturing process. Not much proof - just my opinion.

Now my brain hurts :)

Kevin Bowman


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 RE: The 20kHz limit?
Author: Ginny 
Date:   1999-08-13 16:08

What limits are there on minidiscs and things like audio recorder (other than my dinky computer speakers :-)) software? Are they about the same as CDs? Midi stuff, would not be limited by mics, but by speakers.


I save some music stuff on my zip drive...of course I'm not an audiophile, too many cans of worms for my taste.

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 RE: The 20kHz limit?
Author: Mark Charette 
Date:   1999-08-13 16:34

Ginny,
Most audio cards that people buy are pretty lousy in A-D (analog to digital) conversions - the preamps are non-linear, the A-D is 16 bit, no oversampling, etc. - but -

I use my computer to burn CDROMs of my LPs. They're not as good as the LPs, but I can play them in the car or on the road.

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 RE: The 20kHz limit?
Author: Rick2 
Date:   1999-08-14 04:00

Kevin,

Of course you are correct with regard to the sine wave being an impulse. The desert heat must be getting my brain overheated. Still, a great deal of noise can be filtered. My DSP training has been dormant too long and unfortunately it shows.

With regard to comercial CD recorders/players, I don't follow them, so I don't know whether marketing has forced higher sampling rates down the engineers' throats yet.

Regarding the guess approach to restoration of a lost signal, it's not just your humble opinion, it's a solid fact that it is a guess.

By the way, the best example of affecting a phenomenon by the act of measuring it is light particle-wave duality. It takes the form you try to measure it with. If you try to measure both at the same time, it will only occur in one form. It's truly fascinating.

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