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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:
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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.
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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:
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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.
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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