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In Reply to: RE: blame it on Nyquist posted by Ralph on May 02, 2008 at 10:30:16
"It seems that the application of Nyquist to audio is a bit of a myth. Perhaps more of a marketing scam to get engineers who didn't want to do their math on board with the idea of digital audio."
Nyquist's theorem is valid for **periodic** signals. It falls down for music signals or any signal that is not periodic, especially for momentary or transient signal components whose frequency spectra approach or even exceed half the medium's sample rate.
"If you look into test equipment, you will find that the test equipment manufacturers use a more stringent standard: they scan at 10X the highest frequency to be measured/displayed. That is because they want waveform accuracy, which Nyquist does not give you."
I'm not sure of the "10x" thing...... But I personally think "waveform fidelity" is the key to superior digital audio playback. Image (photograph) decimating/interpolating software has used subjective (visual) perception to choose the ideal filters, and time-resolute filters such as Lanczos3, which don't conform to Nyquist's theorem like sinc filtering, have often been chosen as the most-ideal for the application.
"If you look into the better digital recorders nowadays, you will see a lot of them using higher scan frequencies as well. My Alesis Masterlink can scan at 88.2KHz, allowing for a much better sounding CD as a result."
All CDs have a sample rate of 44.1 kHz, and although higher-resolution masters can help in producing a superior-sounding CD, it's not the primary reason for such result.
"What you have to get is that if Nyquist were real, there would be no sonic improvement by using more advanced scan techniques."
The "advanced scan techniques" is actually the digital filter application. As stated earlier, the time-resolute filters (spline, Lanczos, bicubic, etc.) yield superior sonic results.
"Occam's Razor tells us that in the face of this, Nyquist's application to digital audio is phony."
I wouldn't necessarily use "Occam's razor" here..... It's the simple fact that Nyquist's theorem works only with steady-state periodic signals. Any "signals and systems" textbook would state this explicitly. With rare exceptions (like string or wind notes of long duration and constant intensity), there is hardly anything in music that's periodic, hence Nyquist's theorem is not necessarily the ideal application.
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