Signals
What is a signal?

Definition of a signal, as used in this material, is a function that comprehensively describes information about the behavior of a phenomenon.

In the physical world, any quantity exhibiting variation in time (such as voice) or variation in space (such as an image) is potentially a signal that might provide information on the status of a physical system, or convey a message between observers, among other possibilities. Real signal is always interferenced with noise.

Real signal – signal+noise

In the area of signal processing and electrical engineering, two types of signal are distinguished – an analog and a digital signal.

## Analog Signal

An analog signal is any continuous signal and as such can have any value.

Analog signal has infinitely many values in time and in amplitude. It represents how characteristic feature or phenomenon is varying in time.

Typical analog signal is electrical signal or day temperature varying in time. For some experiments or analysis processes deterministic (can be described by mathematical formulas) or stochastic (behavior is sporadic, random and cannot be predicted) signals are used.

## Digital Signal

A digital signal is represented by a sequence of discrete (usually predefined) values.

Digital signal has finite number of samples in a particular time points. Simple example how to get discrete signal is sampling of continuous (or analog) signal. An example of digital signal can be air temperature measured only each five minutes, or ones and zeroes used in computing.

All processes in the nature are analog by design (think of a chart showing temperature over time, or speed of a car over time).

So the main advantage of processing of an analog signal is that we do not lose any information. However digital values are much easier to process and work with (think of a music CD – how easily it can be converted to a MP3, digital signal is less vulnerable to noise).

Analog signal is more difficult to process and work with (think of a vinyl gramophone record – good quality, but not as convenient to work with as a CD). On the other hand with digital signal we lose certain amount of information via process called sampling and quantization (think of a table showing temperature in each hour of the day – 24 values).

Example of analog and digital signal