Based on the AMI code (alternating the levels of voltage when
transmitting "1"), this limits the maximum number of consecutive zeros transmitted
to three. The basic idea consists of replacing series of four bits that are to equal
to "0" with a code word "000V" or "B00V", where "V"
is a pulse that violates the AMI law of alternate polarity and is rectangular or some
other shape. The rules for using "000V" or "B00V" are as follows:
- "B00V" is used when up to the previous pulse, the coded signal presents a DC component that is not null (the number of positive pulses is not compensated for by the number of negative pulses).
- "000V" is used under the same conditions as above when up to the previous pulse the DC component is null.
- The pulse "B" ("B" for balancing), which respects the AMI alternancy rule, has positive or negative polarity, ensuring that two successive V pulses will have different polarity.
The HDB3 code has the following characterisitics:
- The timing information is preserved by embedding it in the line signal even when long sequences of zeros are transmitted, which allows the clock to be recovered properly on reception.
- The DC component of a signal that is coded in HDB3 is null.
