Radio communication is a system in which electromagnetic waves are used for the transmission of information, instead of connecting lines. Communication is the transfer of information from one location (transmitting station) to another location (receiving station). Information or intelligence means any thing from which some meaning or sense can be derived.
Telecommunication covers various methods used for communication of information at a distance. Telecommunication includes Telegraphy. Telephony, Facsimile. Television, Telemeter, Radar, Internet, etc. In all these systems the information is carried out in the form of electric signals. In this chapter only telegraphy has been described.
Telegraphy is used to transfer information in the written form. It is a combination of two words. "Tele" means at a distance and "Graph" means to write. Hence telegraphy is to write at a distance. In the early days of civilization, there were various methods of transferring information (messages) over long distances these methods went on improving with the passage of time and today we have the most modern methods of communication. Telegraphy was the first electrical communication system that allowed the people to communicate with each other beyond the limited ranges of voice and vision. Telegraph depends upon the transmission of electrical signals, which are arranged according to some definite code. The information to be sent is first converted into some form of code for ease of transmission and reception. Codes are symbols that represent units of information understandable by both sender and receiver.
Telegraphy Code Types
In the unequal length code all the characters are represented by marks and spaces, and the time of transmission is not the same (equal). There is no limit to the number of characters combinations possible from this code. The most common unequal length code is the Morse code.
In the equal length code each character consists of equal number of elements, each having the same duration, which is also the basic time unit of the system. Five-unit code is an equal length code and is most extensively used in machine telegraphy.