HDB3 (High-density bipolar-3 zeros) is a telecommunications line code mainly used in Japan and Europe (for example, in E-1 lines) and is based on AMI. It is also very similar to the B8ZS encoding used in T-1 lines.
Four Zero bits in a row are coded in the following way:
| Number of pulses since last replacement |
|---|
| last pulse | odd | even |
|---|
| negative | 000- | +00+ |
| positive | 000+ | -00- |