A problem called aliasing occurs when a signal to be
sampled contains energy at frequencies above the sampling Nyquist
frequency. The next figure illustrates how aliasing would occur when
the sampling rate is much too low for the frequency of an input
signal. The solid curve represents the analog signal at a
comparatively high frequency. Circles show where samples were taken at
a relatively low sampling rate. The dotted line illustrates the
apparent frequency of the sampled waveform, completing about two
cycles in the period that the original signal completed 20 cycles.
Obviously, aliasing has the effect of producing sounds of lower
frequency from sounds that are higher in frequency than the Nyquist
frequency. Once aliasing has occurred, it is absolutely impossible to
distinguish a component generated by aliasing from one that was
actually present in the input signal. This effect is one of the most
common sources of distortion in digitized waveforms. Fortunately, most
modern computer hardware for digitizing sound has builtin filters
which are tuned to remove sound energy at frequencies beyond the
Nyquist frequency for whatever sampling rate is being used.