The effects of sampling in the time and frequency domain. (a) A continuous time domain signal, (b) the corresponding frequency spectrum of (a) with bandwidth, f_b, (c) the discrete-time domain signal after multiplication of (a) with an impulse train, and (d) the corresponding frequency spectrum of (c), where the frequency spectrum of the continuous time domain signal is repeated at integer multiples of the sampling frequency, f_b. These frequency spectrum replicas are referred to as images (only the first two positive images are shown, but they continue to ±∞). f_N is the Nyquist frequency.