What is analog signal and digital signal?

Answer 1

"Analog" means something that essentially resembles something else; an analog signal has the same smoothly varying structure as a real-world signal such as a sound, for example. "Digital" means composed of digits, in other words, of numbers. The real world is not composed of numbers (although if you examine it on a sufficiently small scale, it turns out to be quantized, which means that in a sense, the real world is digital after all) but you can approximate the real world if you use enough numbers to do so. The simplest example of this (in case my explanation is not already clear) is in sound recording technology. The old-fashioned vinyl record album contains a groove in which the shape of the vinyl reflects the pattern of a sound wave. No numbers are involved. A needle riding in that groove will physically move the way a sound wave moves. Then you have a digital medium such as a compact disc. It records a series of numbers which can be translated by a computer into a sound.

Answer 2

An analog signal can change continuously over a wide range of values. A digital signal can only assume a limited number of discrete (fixed) values.

The amplitude of an analog signal can be represented by a decimal number whereas the amplitude of a digital signal can be represented only by a binary number. For example, the output of an analog circuit could vary continuously between 0 volts and 5.0 volts while the output of a digital circuit could be only 0 volts or 5 volts. Theoretically an analog signal can vary infinitely within its operating range and a digital signal can only be at one of two states. The two states of a digital signal are typically called on and off, or high and low, or 1 and 0.

An analog signal is in some way a direct analogyof the information it represents. There are many ways it can be a direct analogy: the amplitude (AM), the frequency (FM), the phase (PM), the pulse rate (PRM), the pulse width (PWM), shaft rotation rate, etc. No matter what form of analog representation is selected, an analog signal is always a continuously varying signal (i.e., no discontinuity as it changes values).

A digital signal is a number (or sometimes a symbol) standing for the value of the information it represents. These numbers (or symbols) are usually represented in binary code; but there are still some mechanical digital devices that use decimal (e.g. odometer) and the Morse Code uses non-binary pulse coded symbols. Since digital signals are represented by coded numbers (or symbols), they are discontinuous signals when they change value (i.e., they jump from the old to the new value with no intermediate values).