What are Analog and Digital Signals
A signal is basically a variation of a physical parameter with time. This parameter can be anything from the temperature of your room to the rate of your heart beat, from the velocity of your car to the intensity of light falling upon your retinas. Similarly an electrical signal is nothing but a variation in an electrical quantity such as a voltage, current etc. with time.
An electrical signal can easily be processed and transmitted over long range of distances and that is the reason that many of the physical quantities such as sound, picture, video etc. are first of all converted in to the electrical realm with the use of a transducer and then are transmitted through electrical channels to remote users and reproduced there for entertainment, educational, scientific or purely communicational purposes.
Electrical signals can be divided in to two main types:-
Analog Signals Digital Signals
An analog signal is one that varies continuously within a range and can take any one out of the infinite number of values within that range at each instant of time. All signals found in nature are analog. When an analog signal is to be observed, the accuracy of the observation depends on the resolution of the measuring instrument. To clarify the point, let there be an analog signal whose value is 3.28 Volts at a particular instant of time. Now the instrument must have a scale resolution of up to 2 decimal points in order to accurately measure this value. If the instrument used can measure the value up to only 1 decimal point, then the value will most probably be measured as 3.3 Volts.
If all signals in nature are analog, then what is a digital signal? A digital signal is nothing but the representation of an analog signal in terms of a discrete number of levels. To make the point clear, let there be a signal that varies between 0 to 2 volts continuously. Let us then divide this range in to four levels represented by values 0 volts (level 1), 0.5 Volts (level 2), 1.0 Volts (level 3), 1.5 Volts (level 4). Now all values of the signal in observation are represented by the lowest nearby level. E.g. 1.2 V will be represented as level 3, 1.8 will be level 4 etc. Thus the analog signal is now represented by a number of discrete levels and is thus converted in to the digital domain. Each of these levels are stored and transmitted by using a finite number of digits.
In a nutshell, the reason for the existence and wide popularity of digital domain is that digital signals can be more reliably transmitted over long distances because of their inherent immunity to noise. Also digital equipments used for processing and storage of these signal are much more cheap, powerful and sophisticated than their analog counterparts.