Data can either be analogue or digital. Digital quantities have values which jump from one to the next without any 'in-between' value. An on/off switch on a radio is a device that could be thought of as digital because there is no state between 'on' and 'off '. Some quantities can vary over a whole range of values. Temperature is an example, since it can be 10, 10.1, 10.01, 10.001°C, and so on. In fact, temperatures can be an infinite number of values. Quantities which have an infinite number of values are called analogue quantities. For analogue quantities to be processed by a digital computer they need first to be converted into digital quantities using an analogue to digital converter.
There are two types of computers: digital computers and analogue computers. Most people use digital computers; in fact analogue computers are quite rare. Analogue computers are used mainly for the control of processes in factories and for some forms of modelling. Digital computers like to be given their information in the form of numbers (i.e. in the form of binary digits). All information eventually has to be turned into groups of binary digits (0s and 1s).
Provided you have a multimedia computer system and have a microphone, then you can play a recording or a live sound into the sound card of your computer. The sound card listens to the sound and takes a measurement of the height of the signal many times a second. The measurement is called the sound sample.
Sound is an analogue waveform and for a computer to be able to process, store and play it back, it has to be turned into a digital signal.
This graph shows the waveform of an analogue sound signal being sampled at times t1, t2, t3, etc. At time t1 the signal lies between 3 and 4, although it is nearer to 4 so it is given the value 4. At time t2 the signal's sampling level is 3.
The sampling rate is the number of times the sample is measured per second and it is measured in kilohertz (kHz). The higher the sampling rate, the better the sound reproduction. There are implications in having perfect sound: the higher the quality of the sound, the larger the file will be. For a three minute high quality recording the file can be as large as 30 MB.
Analogue to digital conversion (ADC) and vice versa
In chemical factories, the processes used to make chemicals are usually controlled with the help of computers. Often, a batch of chemicals must be kept at a precise temperature. The signal from a temperature sensor is usually a continually changing analogue signal and this must be converted into digital form before it can be processed by a computer. This conversion is done with an analogue-digital converter (ADC). The computer may then control the process by outputting a digital signal, through a digital-analogue converter (DAC) to an electric motor which can open or close a valve.
Modems are used to enable data to be passed from one place to another using communication links (telephone wires, optical cables, radio links, etc.).
A modem (modulator-demodulator) is used to convert the binary digits from a digital computer into an analogue signal that may be passed along ordinary telephone wires. The modem at the other end of the wire converts the analogue signal back into a digital one that the computer is able to understand.
Say whether the following devices would be analogue or digital:
(a) a mercury-in-glass thermometer
(b) the floor indicator on a lift
(c) a radio tuning dial
(d) the display showing the track that is being played on your CD player
(e) a speedometer on a car.
More and more devices which previously had analogue displays, now have digital displays. Why do you think this is?
A car fuel injection system counts revolutions to measure the engine speed and uses an electronic sensor to measure the engine temperature.
(i) Indicate why the engine speed is the digital signal.
(ii) What extra piece of equipment does the analogue temperature signal need and what does it do?