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(in my following ramblings, I will only talk about pure sine waves if
not mentioned otherwise. Also, I'm talking about theory, not about real
converters or other real devices):
> Stephen - It is my understanding that the Nyquist theorum actually
> specifies the theoretical maximum frequency that can be represented
> digital signal. It says nothing of the quality of that
> There's a key issue that makes the nyquist theorum all important to
> conversion process - frequencies higher than the Nyquist freq. will
> themselves in the resultant digital signal as much lower frequency
> That noise is basically garbage and is nearly unpredictable.
Frequencies higher than the Nyquist frequencie will come out as a
so-called alias at a lower frequency (and incidentally, also at higher
frequencies). But not as noise, but as a pure sine wave. This signal is
> Furthermore, frequencies that are close to but still less than the
> frequency will be represented, however the amplitude of the
> will vary over time based on the _difference_ in frequencies. How
> output varies in amplitude is a more complex equasion - important
> that frequencies close to the Nyquist freq. (even though they are
> than) will be VERY distorted but will still be represented "perfectly
> in-tune". Example: 48KHz digital A/D converter could capture a 23KHz
> sine wave, and output a 23KHz sine wave whose amplitude varies at 1Khz
> would hear that 1Khz signal for sure!
Frequencies below Nyquist come through without any change.