An analog signal has continuous and varying value over any time interval, with infinite resolution in time and magnitude. A simple analog signal is a sine wave that is described using amplitude and time as shown in Figure 8.3:

Analog signals are used for control operations of analog devices, such as controlling the speed of a DC motor (a DC motor is an analog device). To change the speed of motor we simply vary the resistance in its current path due to which current available at motor changes and hence the speed.

Let's consider that we use a battery as a source of analog signal for controlling the speed of the motor. During the operation, the output voltage and current of battery drops over a period of time, due to which the change in resistance will not change the speed of the motor in the same proportion as it was doing so earlier. Due to this varying nature of an analog signal, we will never be able to achieve the accuracy and consistency in speed control. One
more disadvantage of using analog signaling in our case is that it is prone to noise, which will distort the signal and reduce the quality of transmission.

Having discussed the issues with analog signaling, it is still possible to control the operation, but it requires precision analog circuits which are bulky, costly, and consume a lot of power, which is not ideal for many use cases.