Sampling Theorem (Figure 3-9a, 3-9b)


No aliasing occurs due to use of anti-aliasing filter.  However, the time-axis waveform does not appear as  a complete rectangular wave because the filter does not pass high frequency components and thus distorts the waveform.

Fig. 3-9a

Without an anti-aliasing filter, the time-axis waveform appears to be the original rectangular wave.  However, aliasing appears on a result of FFT processing showing frequency components that actually do not exist.

Fig. 3-9b