What is Bipolar Encoding?

The bipolar encoding scheme defines three voltage levels: positive, negative, and zero. In bipolar encoding, zero voltage represents binary 0, while binary 1 is represented by alternating positive and negative voltages.

The first 1 bit is represented by positive amplitude, the second 1 bit by negative voltage, the third 1 bit by positive amplitude, and so on. This alternation continues even when the 1 bits are not consecutive, ensuring a balanced signal without DC component.

There are three main types of bipolar encoding schemes:

AMI (Alternate Mark Inversion)

AMI stands for Bipolar Alternate Mark Inversion. It is the fundamental method of bipolar encoding. The term 'mark' originates from telegraphy and represents binary 1. AMI implements alternate inversion for each occurrence of binary 1.

In the bipolar AMI encoding scheme, 0 bits are represented by zero voltage levels, while 1 bits are represented by alternating positive and negative voltages. This ensures zero DC component and helps maintain synchronization, though long strings of zeros can still cause synchronization issues.

B8ZS (Bipolar 8-Zero Substitution)

B8ZS stands for Bipolar 8-Zero Substitution. This scheme is similar to bipolar AMI but addresses the synchronization problem caused by long sequences of zeros. Whenever eight or more consecutive zeros are encountered in the data stream, B8ZS substitutes a specific pattern based on the polarity of the previous 1 bit.

The substitution pattern creates intentional bipolar violations to maintain synchronization:

• If previous 1 was positive: 000+-0-+ (where + and - represent violations)

• If previous 1 was negative: 000-+0+- (where + and - represent violations)

HDB3 (High-Density Bipolar 3)

HDB3 represents High-Density Bipolar 3. It addresses the synchronization issue of consecutive zeros by substituting every four consecutive zeros with a specific bipolar pattern. The substitution pattern depends on two factors: the polarity of the last 1 bit and the number of 1s since the previous substitution.

Substitution Rules:

• Odd number of 1s since last substitution: 000V (violation in 4th position)

• Even number of 1s since last substitution: B00V (violation in 1st and 4th positions)

Comparison of Bipolar Encoding Schemes

Advantages of Bipolar Encoding

• Zero DC component: Alternating polarities eliminate DC bias

• Error detection: Bipolar violations can indicate transmission errors

• Bandwidth efficiency: Better spectral characteristics than unipolar encoding

• Synchronization: B8ZS and HDB3 variants maintain clock recovery

Conclusion

Bipolar encoding uses alternating positive and negative voltages to represent binary 1s, with zero voltage for 0s. Advanced variants like B8ZS and HDB3 solve synchronization issues through strategic zero substitution patterns, making them suitable for high-speed digital communication systems.