Bounds on the output of a linear system are developed and used to reexamine the nature of the truncation error e
Nin the sampling expansion of a band-limited signal

. It is shown that if

has finite energy, then the maximum of

is bounded by the mean-square value of the error

in the approximation of

by a truncated Fourier series. If

has finite power, then the mean-square value of e
Nis bounded by the maximum of

.