A EU digital network transmission protocol allowing information and control data to be embedded in the transmission bit stream.

The HDB3 code is a bipolar signaling technique (i.e. relies on the transmission of both positive and negative pulses). It is based on AMI, but extends this by inserting violation codes whenever there is a run of 4 or more 0's. This and similar (more complex) codes have replaced AMI in modern distribution networks.

The encoding rules follow those for AMI, except that a sequence of four consecutive 0's are encoded using a special "violation" bit. This bit has the same polarity as the last 1-bit which was sent using the AMI encoding rule.

The purpose of this is to prevent long runs of 0's in the data stream which may otherwise prevent a DPLL from tracking the centre of each bit. Such a code is sometimes called a "run length limited" code, since it limits the runs of 0's which would otherwise be produced by AMI.