Sampling Theory

The dual of the Poisson Summation Formula is the continuous-time aliasing theorem, which lies at the foundation of elementary sampling theory [264, Appendix G]. If $x(t)$ denotes a continuous-time signal, its sampled version $x(nT)$ , $n\in\mathbb{Z}$ , is associated with the continuous-time signal

$$x_d(t) \isdefs x(t)\psi_T(t) \isdefs x(t)\sum_m\delta(t-mT).$$ (B.60)

where $T$ denotes the (fixed) sampling interval in seconds. The sampled signal values $x(nT)$ are thus treated mathematically as coefficients of impulses at the sampling instants. Taking the Fourier transform gives

$$\begin{eqnarray*} X_d(f) &=& \hbox{\sc FT}_f(x\cdot\psi_T) \eqsp X\ast \Psi_T\\ &=& \frac{1}{T}X\ast \psi_{1/T} \eqsp f_s\sum_{k=-\infty}^{\infty}X(f-kf_s) \end{eqnarray*}$$

where $f_s\isdef 1 f_s\isdef 1/T$ denotes the sampling rate in radians per second. Note that $X_d(f)$ is periodic with period $f_s$ . We see that if $X(f)$ is bandlimited to less than $f_s$ radians per second, i.e., if $X(f)=0$ for all $\vert f\vert\geq f_s/2$ , then only the $k=0$ term will be nonzero in the summation over $k$ , and this means there is no aliasing. The terms $X(f-kf_s)$ for $k\neq 0$ are all aliasing terms.