Analog & Digital Quantities - Difference

Analog Quantities:

All the processes and phenomenon in real world are continuous. One can consider the example of rise in temperature of a room, increasing day light at the dawn, change in speed of a car and many other daily life processes. All these processes change from one initial condition to the last one in a steady and continuous manner. No one can say it was 0^oC just a second (or more precisely, just a moment/ instant) before and now it is about 10^oC or something else. Temperature may vary in very short duration but it will change by passing through all the rational and irrational numbers between two points. In the case just mentioned, increasing of temperature from 0^oC to 10^oC is an analog process because temperature will increase from 0^oC to 1^oC in steps of very small size which cannot be measured by human senses and also by the most highly précised and accurate instruments. Therefore, it is said that in changing from one limit to other, analog quantities pass through the series of infinite steps of approximately zero size.

Another example, which is more obvious and easily understandable, is of speedometer of a car. Speedometer in cars show the deflection of needle which is proportional to its speed. This needle, for example, will never move from 30 km/ h to 40 km/h in a single step. Rather it will mover in a steady manner and will pass through all the steps between these two limits. The same phenomenon repeats when the car decreases its speed.

On the basis of these examples, and many other such process in real world, one can discover that the important factor in analog quantities is that they vary over infinite and continuous range of values while changing from one point to the other.

Digital Quantities:

On the other side, digital quantities are discrete or discontinuous. While considering the above mentioned example of change in temperature, digital system will take information (correct value of temperature) at certain defined times and display it in the form of digits and symbols on some display. If temperature changes from 0^oC to 10^oC in 10 minutes, the digital system will take information samples at certain times and display them. Time duration after which the sample needs to be taken is defined by the manufacturer of the digital system and it may vary. Let us consider the following table of temperature readings taken by certain digital system.

Sample Number	Time value (minutes)	Temperature Value (oC)
1	0	0
2	2	2
3	4	3.5
4	6	6.5
5	8	9
6	10	10

This table does not give any information about the change in temperature between 0 – 2 minutes time. It only shows that the temperature was 0^oC at start of measurement (i.e; 0 minutes) and was 2^oC after 2 minutes time. This sort of data is called discrete or discontinuous and systems providing such data are called digital systems. Although the temperature is analog quantity, it is represented in a discrete manner here.

Similar explanation can be given for the example of car speedometer. Sensor which senses the speed of a car can take samples after certain fixed time duration and the digital system can show it by the symbols and digits on display.

These, and many other like these, examples can help us to discover that the important factor in digital quantities is that they vary in discrete steps of predefined time duration while changing from one point to the other.

Advantages of Digital Representation

Despite of the fact that real world is mainly analog, nearly all the systems in modern world are digital in nature due to the invention and implementation of fast microprocessors and systems based on it. Digital systems: 
-a- Are easier to design and can be fabricated using nano – technology.
-b- Are advantageous when processing and transmitting the data.

-c- Can store huge amount of data (information) at lesser space and with least chances of errors.
-d- Can reproduce data with greater accuracy and precision than analog systems.
-e- Are less prone to noise, and 
-f- Have better readability due to no ambiguity as values are written in the form of symbols on displays.

Although in most precise way, it is said that analog systems have better accuracy while measuring some physical quantity but in modern world, accuracy of digital systems can be increase, and even taken near to the level of accuracy of analog systems, by increasing the resolution of it. This will be discussed in later posts.

References: 
-1- Ronald J. Tocci and Neal S. Widmer, Digital Systems Principles and Applications, 7^th Edition, Prentice-Hall, New Jersey.
-2- Thomas L. Floyd, Digital Fundamentals, 7^th Edition, Prentice-Hall, New Jersey.

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Posted by M. Shahzad at 12:46 PM