Understanding HDB (High Density Bipolar) Coding

High-Density Bipolar or HDB Coding is used mainly for Bandwidth compression. Using this coding system, we eliminate a large number of consecutive zeros and replace them with certain codes.

Here, HDB3 and HDB4 will be discussed.  Follow each step and you should have no problems understanding the process.

Bit pattern:   10100001100000100000101

Step 1: For HDB3, (3+1)=4 zeros and for HDB4, (4+1)=5 zeros will be replaced with code.



Step 2: If the number of 1's before (n+1) number of zeros are:

• EVEN: replace with 100V
• ODD: replace with 000V

Step 3: For HDB3,
                                         10100001100000100000101
                                         10100001100000100000101
                  Final code:   101000V11100V01000V0101

Note: The first code is 000V because the number of 1's before it are 3 (odd), the second code is 100V as the number of 1's before it is 2 (even) and the third code is 000V as the number of 1's before it is 1 (odd).

          For HDB4,
                                         10100001100000100000101
                                         10100001100000100000101
                  Final code:     101000011100V1000V101

Note: The first code is 100V because number of 1's before it are 4 (even) and the second code is 000V as the number of 1's before it is 1 (odd).
Draw the wave shape as shown above for HDB3.

For further assistance, check Slides on Line codes: Digital Communication Slides Collection