> It was that precise signal how my doubts with Nyquist were raised initially. I just simplified it to a constant 20kHz signal for acedemic purposes, knowing full well no DAC can use more than a half second of data. Then, I saw the mirage of dawn in the possibility of the reconstruction filter, only to find out it was analog which made me doubt the fidelity of anything above 15.58kHz once again (increasing skepticism with frequency of course until 22.05kHz & then decreasing once again like a octave band of error). However, the fact it can be digitally done for upsampling gives me hope that an infinite XO make this band vanish. But, then again, you'll most likely be at the mercy of a FFT's accuracy. Ah, once more into the breech ...
