Digital Signal Encoding Formats

Contents

• Non Return to Zero"NRZ"
• Non Return to Zero Inverted "NRZI"
• Multilevel binary
• Biphase
• Scrambling Techniques

NRZ- Non Return to Zero

One normally thinks of a digital signal as being a string of discrete amplitude rectangular pulses. That is the way it comes from a data source.

0 <==> high level

1 <==> low level

Require time coordination "synchronization": As all pulses have equal duration and there is no seperation between them.

Contain non zero DC component that is difficult to transmit, carries no information.

Back to Top

Non return to Zero Inverted "NRZI"

Invert on ones, a transition (low-to-high or high-to-low) at the beginning of bit time denotes a binary 1

for that bit time. No transition indicates binary 0.

Reliable to detect in presence of noise.

Not polarity sensitive.

Needs less power to transmitt.

Long string of 0s. (disadvantage)

Back to Top

Multilevel Binary

Bipolar-AMI

Binary 0 is represented by no line signal and a binary 1 is presented by a positive or nagative pulse.

Pseudoternary

In that case the binary 1 is represented by no line signal and a binary 0 is presented by a positive or nagative pulse.

No DC component.

Easy to synchronize.

Easy to detect one lost pulse.

Long strings of 0s (Bipolar-AMI) or long strings of 1s (pseudoternary).

Needs more power to transmitt.

Back to Top

Biphase

Manchester code

0s reprsent a transition from high to low in middle of pulse interval and 1s represents a transition

from low to high in pulse of intervals.

Differential Manchester

0s represents transition at the beginning of pulse interval while 1s represents absence of transition at the beginning of pulse interval.

Reciever can synchronize on transitions.

No DC component.

Bandwidth is wider than the bandwidth for multilevel binary code.

Back to Top

Scrambling Techniques

Widly used in long-distance applications.

B8ZS- Bipolar with 8-zeros substitution

If last pulse before 8 0s is positive, then encode as 000+-0-+

If last pulse before 8 0s is negative, then encode as 000-+0+-

HDB3 high-density bipolar-3 zeros

Replacing any string of four zeros by three zeros followed by violation that is the same polarity as previous transition.

Back to Top

References

Telesystem Winter 97, Period 3