What is the difference between an analogue and a digital signal

An analogue signal is an electronic copy of the original sound wave or light spectrum. It is what you see on an oscilloscope screen when you plug in a microphone. It might look something like diagram1:

Diagram 1 an analogue signal

Diagram an analogue signal with interference 2




The major problem with analogue signals is that they pick up interference or noise. This can come from poor connections, from other signals or from nearby electrical machinery. When that happens the signal might look like diagram 2 and it will sound distorted or mushy. If the signal carries video then you might see a lot of colour or white flecks that distorts the picture.

To get around this we convert the analogue signal to a digital signal before we transmit it and then convert it back in the receiver. A sample of the analogue signal is taken at intervals as in diagrams 3 and 4.

If you would like to learn more about digital signal processing there are three notes sheets in PDF format which you can download here:

Bandwidth and bits per sample   Digital signals - bandwidth and sampling   Digital signals - Noise and sampling    How a cd works    Analogue and digital (as above)

Diagram 3 - the analogue signal is sampled

Diagram 4






The length of each line is then converted into a number. The number is written in binary and transmitted as a series of pulses, perhaps like this in diagram 5.

Diagram 5 a binary number converted into a signal





The receiver then converts these numbers back in a reverse of the process. As you can see, the wave in diagram 6 is similar but not exactly the same.

Diagram 6

If the sampling was done much more often then the signal transmitted and received would be a very accurate copy- as you can hear and see from a CD or digital TV. There is a lot more information to send but the reconstructed signal is almost perfect.

It is the square wave signal that is transmitted. Even if it picks up interference

the number can still be recognised and reproduced exactly, so the digital signal is not distorted by electrical interference.