Study of Line Encoder

Theory:

A line code is a code used for data transmission of a digital signal over a transmission line. This process of coding is chosen so as to avoid overlap and distortion of the signal such as inter-symbol interference.

Types of Line Encoding:    

There are 4 types of Line encoding-

• RZ (Return to zero)
• NRZ (Not return to zero)
• AMI
• Manchester

All this can be Unipolar, Polar or Bipolar.

Unipolar Non-Return to Zero (NRZ):

In this type of unipolar signaling, a High in data is represented by a positive pulse called Mark, which has a duration T0 equal to the symbol bit duration. A Low in data input has no pulse.

Line encoding

Advantages:

The advantages of Unipolar NRZ are −

•          It is simple.
•          A lesser bandwidth is required.

Disadvantages:                                   

The disadvantages of Unipolar NRZ are −

•          No error correction was done.
•          The presence of low-frequency components may cause signal droop.
•         No clock is present.
•          Loss of synchronization is likely to occur (especially for long strings of 1s and 0s).

Circuit Connection:

NRZ waveform generator circuit

Experimental data:

Input signal

NRZ unipolar output

Unipolar Return to Zero (RZ):

In this type of unipolar signaling, a High in data, though represented by a Mark pulse, its duration T0 is less than the symbol bit duration. Half of the bit duration remains high but it immediately returns to zero and shows the absence of the pulse during the remaining half of the bit duration.

Advantages:

The advantages of Unipolar RZ are −

• It is simple.
• The spectral line present at the symbol rate can be used as a clock.

Disadvantages:

The disadvantages of Unipolar RZ are −

• No error correction.
• Occupies twice the bandwidth as unipolar NRZ.
• The signal droop is caused at the places where the signal is non-zero at 0 Hz. 

Circuit Connection:

RZ waveform generator circuit

Experimental data:

RZ  Unipolar

RZ bipolar

Discussion:

• Our input signal was a sinusoidal wave. The output of the line encoder is a square wave.
• The NRZ bipolar line coding denoted for a single bit time, the waveform did not return to 0 V. 
• On the other hand, RZ line coding works for a single bit time (half of a single bit time), the waveform returned to 0 V between data pulses.
• Data output in the experiment and the theoretical data was a little different.

Conclusion:

Conclusion Line coding schemes are used to convert analog to TTL and vice-versa. This is also used for various purposes like converting a binary signal to three-level or (+-) signal. This is used for error detection, etc without using any filter or modulation.