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Introduction: Digital signal is sequence of discrete, discontinuous voltage pulses. It is used to map data bits by various encoding techniques such as NRZ, RZ, biphase etc.

### What is RZ encoding ?

In this line encoding type, the binary data is represented by pulse having transition rather than constant level voltage. In RZ, transition happens exactly at the center of the bit period. Hence it is known as 'Return to Zero'. Further the rules of unipolar, polar and bipolar signaling applies before encoding the binary data as per RZ encoding technique.

As shown, using unipolar RZ encoding, binary '1' is mapped using positive pulse of amplitude 'V' for half bit period and returns to zero for next half bit period. Binary '0' is mapped by absence of pulse for entire bit period.
Example of Unipolar RZ encoding :
INPUT : [1 0 1 0 0 1 1 1 0] , Bit period : Tb
OUTPUT : [+ve pulse (over Tb/2 time duration) DC (Tb/2), DC (over Tb duration), +ve pulse (Tb/2) DC (Tb/2), DC (Tb) , DC (Tb), +ve pulse (Tb/2) DC (Tb/2) , +ve pulse (Tb/2) DC (Tb/2) , +ve pulse (Tb/2) DC (Tb/2) , DC (Tb) ]

As shown, using polar RZ encoding, binary '1' is mapped using positive pulse of amplitude 'V' in the first half and return to zero in the second half where as binary '0' is mapped using negative pulse of amplitude 'V' in the first half bit period and return to zero in the next half bit period.
Example of Polar RZ coding :
INPUT : [1 0 1 0 0 1 1 1 0]
OUTPUT : [+ve pulse (Tb/2) DC (Tb/2), -ve pulse (Tb/2) DC (Tb/2), +ve pulse (Tb/2) DC (Tb/2) , -ve pulse (Tb/2) DC (Tb/2), -ve pulse (Tb/2) DC (Tb/2), +ve pulse (Tb/2) DC (Tb/2), +ve pulse (Tb/2) DC (Tb/2), +ve pulse (Tb/2) DC(Tb/2), -ve pulse(Tb/2) DC (Tb/2) ]

As shown, using Bipolar RZ encoding, binary '1' is mapped using positive pulse and negative pulse of amplitude 'V' based on its alternate positions during first half bit period unlike NRZ. For the next half bit period it remains at zero DC level. The binary '0' is mapped by absence of pulse for entire bit period.
Example of Bipolar RZ line coding :
INPUT : [1 0 1 0 0 1 1 1 0]
OUTPUT : [+ve pulse (over 'Tb/2') DC ('Tb/2'), DC (over 'Tb'), -ve pulse ('Tb/2') DC ('Tb/2'), DC ('Tb'), DC ('Tb'), +ve pulse ('Tb/2') DC ('Tb/2') , -ve pulse ('Tb/2') DC ('Tb/2') , +ve pulse ('Tb/2') DC ('Tb/2'), DC ('Tb')]

### Benefits or advantages of RZ encoding

Following are the benefits or advantages of RZ encoding:
➨It is simple line coding technique.
➨In polar RZ and bipolar RZ, no low frequency components are present.
➨Bipolar NRZ/RZ signaling waveform occupies lower bandwidth than unipolar NRZ and polar NRZ waveforms.
➨Signal drooping does not happen in Bipolar coding. Hence this line coding is suitable for data transmission over AC coupled lines.
➨Single error detection is possible using this line coding technique.

### Drawbacks or disadvantages of RZ line coding

Following are the drawbacks or disadvantages of RZ line coding:
➨Signal droop occurs when signal is non-zero at 0 Hertz.
➨Unipolar/Polar RZ occupy twice bandwidth than Unipolar/polar NRZ respectively.
➨No error correction.
➨No clock is available.
➨Loss of synchronization occurs due to long string of ones and zeros in the binary data.