Analogue and digital signals:

Analogue (continuous) information is made available in virtually all aircraft equipment.  Figure 1 shows the analogue signal created by a variable resistor.  In the circuit +0V is present at the output "A" when the potentiometer is at position 1 and +5V when at position 2.  These values would represent either a 1 (+5V) or a 0 (+0V).  However, it can be seen from the graph of the analogue signal that it does produce distinct values of +5V and +0V as the potentiometer moves from one end to the other.

Analogue Signal Representation

A digital signal is one that contains two distinct values (1 and 0).  Figure 2 shows a digital signal being produced by use of a switch.  With the switch in the open position, +0V will be present at point (logic 0).  When the switch closes, +5V will be present at point (logic 1).  Digital signals are often considered to be either "ON" or "OFF" (logic 1 or 0).

Signals in analogue form can be processed using operational amplifiers and other devices in various configurations and ultimately converted to an observable output by a suitable output device. 

Systems, which are completely analogue, are limited in the accuracy that can be achieved both physically and economically, they also suffer from error and distortion for various reasons such as non-linearity, drift, crosstalk, noise etc.  Digital systems, especially since the advent of integrated circuits, offer improvements over analogue systems in most respects, thus modern processing systems employ fixed analogue and digital circuitry (hybrid systems) in which, of course, conversion from one form to the other must take place at certain points within the system. 

Hybrid systems are more common than all digital systems presumably because of the simplicity of analogue transducers, and the nature of the information to be processed lends itself more readily to analogue representation.  For example it would be difficult to digitize an audio signal without converting it from changing air pressure to an electrical analogue by means of a microphone (transducer).