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Line Coding
Line Coding is the process of converting binary data, a sequence of bits, to a digital signal. For example, data, text, number, graphical image, audio and video that are stored in computer memory are all sequence of bits. Line coding converts a sequence of bits to a digital signal.
4.1 Line Coding
Some Characteristics
Line Coding Schemes
A digital signal can have a limited number of values. However, only some of these values can be used to represent data; rest are used for other purposes as we shall see shortly.
Signal Levels: The number of values allowed in a particular signal.
Data Levels: The number of values used to represent data.
Pulse Rate: It defines the number of pulses per second. A pulse is the minimum amount of time required to transmit a symbol.
Bit Rate: It defines the number of bits per second.
Relation between the two: If a pulse carries only 1 bit, the pulse rate and the bit rate are the same. If the pulse carries more than 1 bit, then the bit rate is greater than the pulse rate. So we have a formula to calculate bit rate in relation with pulse rate:
A Self-Synchronizing digital signal includes timing information in the data being transmitted. This can be achieved if there is the signal that alerts the receiver to the beginning, middle, or end of pulse. If the receiver’s clock is out of synchronization, these alerting points can reset the clock.
The polarity of a pulse refers to whether it is positive or negative.
Unipolar encoding uses only one voltage level. It is named so because it uses only one polarity. This polarity is assigned to one of the two binary states, usually the 1. the other state, usually the 0, is represented by zero voltage.
Polar encoding uses two voltage levels, one positive and one negative.
Polar encoding is classified as follows:
Non return to Zero (NRZ): In it, the value of the signal is always either positive or negative. It is classified in two categories as follows:
1) In NRZ-L the level of the signal is dependent upon the state of the bit.
2) In NRZ-I the signal is inverted if a 1 is encountered.
Return to Zero (RZ): It uses three values: positive, negative, and zero. In it signal changes not between bits but during each bit. A one bit is represented by positive-to-zero transition in the halfway of bit and a 0 bit by negative –to-zero transition.
Disadvantage:
It requires two signal changes to encode 1 bit and therefore occupies more bandwidth.