This Topic begins by discussing the mathematical model of perfect sampling, namely multiplying the signal (that is to be sampled) by an impulse train. Using this model, the Topic discusses how to consider what the impact of sampling a signal is, and proposes to consider the resulting change in the spectrum of the sampled signal compared with the spectrum of the original signal. Undertaking this analysis, the relationship between these spectra is derived, which therefore leads to the standard Nyquist sampling rate. The origin of this result is described in detail using the mathematical tools developed so far in this course.