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# 2 Signal processing

Signal processing is a branch of electronics concerned not just with the properties of signals, but also with the properties of the devices and systems that carry the signals. The objective of signal processing is to optimise the recovery of some particular aspect of the signal that is of most interest, or to optimise the use of a communication medium.

Signal processing usually means filtering a signal. This could be to reduce or remove interference; it might mean changing the signal so that the communication channel can be used more economically or more efficiently; or it might mean processing a signal so that it can be sampled satisfactorily in an analogue-to-digital converter.

## What is filtering?

In the context of electronic signals, filtering means altering the signal so that some aspects of the signal are removed while other parts of the signal remain. In this section you will learn about the difference between ‘ideal’ filters and real filters. You will also look at different types of filters and their characteristics and a type of graph used to show the frequency-dependent gain of a filter.

One of the most common uses of filters is to reduce the intrusion of unwanted signals, or noise, into wanted signals. A common example of filtering is radio and television tuning. Here, the antenna picks up multiple transmissions being broadcast on different frequencies; the tuning circuit ideally passes the wanted broadcast to the output and blocks all the others, which are on different frequencies from the wanted broadcast.

Filters can be analogue or digital. Analogue filters use components such as inductors, capacitors, resistors, and sometimes operational amplifiers (op-amps). Digital filters are basically computers and achieve their filtering effects through mathematical operations on the sample values of a digitised signal. You will look at these more closely in Section 3. In this section, however, you will focus only on analogue filters.

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