The source of a signal is modelled as a system with an acoustic ousput. A system which models an operation on a signal is a filter, which can be thought of as a black box with one or more inputs and one or more outputs.
An analog system or continuous-time system processes continuous-time signals, while a digital system or discrete-time system processes discrete-time signals.
An analog-to-digital converter, ADC, samples a continuous-time signal, generally at regular intervals, and converts it to a sequence of discrete-time values. The sampling frequency or sampling rate must be at least twice as high as the highest frequency to be measured, or an effect known as the `aliasing effect' occurs: a phantom signal appears in the output. For example, if the frequency to be measured is an exact multiple of half a wavelength, then the signal will appear to have the same invariant amplitude.
Conversely, a digital-to-analog converter, DAC, converts a digital signal to an analog signal.
A digital signal processor, DSP, is a dedicated computer, often on a single chip, for performing programmable operations and transformations on digital signals.
A digital signal processing system has the structure shown in the Figure.
Figure 1: Block diagramme of a digital signal processing system.
