Jon, (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 with a > digital signal. It says nothing of the quality of that representation. > There's a key issue that makes the nyquist theorum all important to the A/D > conversion process - frequencies higher than the Nyquist freq. will manifest > themselves in the resultant digital signal as much lower frequency noise. > 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 predictable. > Furthermore, frequencies that are close to but still less than the Nyquist > frequency will be represented, however the amplitude of the representation > will vary over time based on the _difference_ in frequencies. How much the > output varies in amplitude is a more complex equasion - important thing is > that frequencies close to the Nyquist freq. (even though they are still less > than) will be VERY distorted but will still be represented "perfectly > in-tune". Example: 48KHz digital A/D converter could capture a 23KHz pure > sine wave, and output a 23KHz sine wave whose amplitude varies at 1Khz - you > would hear that 1Khz signal for sure! Frequencies below Nyquist come through without any change.