ly on line coding, which changes source symbols into another form for transmission. The operation of line encoding transforms a digital message into a new sequence of symbols. Decod- ing is the opposite process that converts the encoded sequence back into the original message (Figure 4.16). Consider a computer terminal with a keypad. Each key represents a dis- crete digital symbol. Uncoded transmission would require as many different waveforms as there are keys, one for each key (or more, one for shift, one for Alt, and one for Alt Gr). Alternatively, each symbol can be an encoder into a binary code word consisting of a number of binary digits for binary transmission. Introduction to Telecommunications Network Engineering Binary Binary message Encoded line signal message Encoder Decoder (+ modulator) (+ demodulator) Line coding: Extracts the dc-content from the message; Adds synchronization information into the line signal; Increases information data rate through the channel; Changes the spectral shape of the message so that it suits the channel better Figure 4.16 Line coding. 4.4.1 Purpose of Line Coding One purpose of line coding is to make the form of the spectrum of a digital signal suitable for a certain communication media. The line codes usually have no dc content (direct current, frequency component at 0 Hz). We want to get rid of the dc that does not transmit any information but wastes power. Another reason for line encoding is to help to synchronize the receiver. In digital transmissions the receiver must be synchronized with the transmit- ter in order to receive the information when each new symbol arrives. For this the data should be transmitted in a form that contains synchronization information SO that there is no need to transmit additional clock or timing signals. The systems that use only line coding, but not modulation, are called baseband transmission systems. The spectrum of the line signal is still in the frequency range of the original message's "baseband." In radio systems both coding and m