I understand why sampling below the Nyquist rate will result in degradation due to aliasing, but in one of their examples they get significant signal degradation already when sampling a 15 kHz sinusoid at a rate of 32 kHz. That is above the Nyquist rate, so why is the signal degraded?
When they go on to sample an 18 kHz sinusoid at a rate of 32 kHz, i.e. below the Nyquist rate, the frequency plot shows the expected aliasing, producing a spike at 14 kHz. But the time domain plot still shows something unexpected, because it does not show a 14 kHz sinusoid (as I would expect after reconstruction) but some other significantly degraded waveform.
So there seems to be some source of degradation other than the sampling rate. Could someone clarify? Is it related to the fact that a DFT (specifically an FFT) is used rather than an ideal DTFT?