With respect to aliasing, an analog to digital converter contains an analog low-pass filter which reduces the amplitude of the signal content above the Nyquist

With respect to aliasing, an analog to digital converter contains an analog low-pass filter which reduces the amplitude of the signal content above the Nyquist frequency prior to digitization. This effectively eliminates alsiasing in the digitized signal. In terms of more complex signals, or frequency "beats", the same principles apply. If two high frequency (above-20kHz) signals interact in such a way to create an audible (below-20kHz) beating, the beating will be accurately recorded by the ADC.