Signal Types in Digital Signal Processing – Software Engineering
Before we start to look at discrete signals in more detail, it’s worth briefly reviewing the different categories of signals.
Continuous-time signals:
This is the type of signal produced when we talk or sing, or by a musical instrument. It is ‘continuous’ in that it is present at all times during the talking or the singing. For example, if a note is played near a microphone then the output voltage from the microphone might vary as shown in Fig. 2.1. Clearly, continuous does not mean that it goes on for ever! Other examples would be the output from a signal generator when used to produce sine, square, triangular waves etc. They are strictly called continuous-time, analogue signals.
Discrete-time signals:
These, on the other hand, are defined at particular or discrete instants only- they are usually sampled signals. Some simple examples would be the distance travelled
by a car, atmospheric pressure, the temperature of a process, audio signals, but all recorded at certain times. The signal is often defined at regular time intervals.
For example, atmospheric pressure might be sampled at the same time each day (Fig. 2.2), whereas an audio signal would obviously have to be sampled much
more frequently, perhaps every 10/Is, in order to produce a reasonable representation of the signal. (The audio signal could contain frequency components of up to
20 kHz and so, to prevent aliasing, it must be sampled at a minimum of 40 kHz, i.e. at least every 25/~s.)
Digital signals
This term is used to describe a discrete-time signal where the sampled, analogue values have been converted to their equivalent digital values. For example, if the
pressure values of Fig. 2.2 are to be automatically processed by a computer then, clearly, they will first need to be changed to equivalent voltage values by means
of a suitable transducer. An ADC is then needed to convert the resulting voltages to a series of digital values. It is this series of numbers that constitutes the digital
signal.
Category: Software Engineering
