HDB3 code

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The HDB3 code ( High Density Bipolar ) is a line code from the field of telecommunications .

HDB3 coding of a serial data stream

Short definition

The HDB3 code is based on the AMI code , although long zero sequences can lead to loss of synchronization . So-called code violation rules are used in the HDB3 code to avoid loss of synchronicity. The fourth zero in a row is replaced by a one in the wrong polarity.

Replacement table

Number of pulses since last replacement
Last pulse Odd Straight
negative 000− +00+
positive 000+ −00−

Trivial explanation

HDB3 means that four zeros are never transmitted in a row. If four zeros have to be transmitted, the AMI code rule is violated and a zero with a "false one" is transmitted. A wrong one means that it has the "wrong" sign (namely the same as the last one). In itself that would be enough to enable the transmission of any number of zeros and ones in a row. With certain combinations of zeros and ones, however, the DC voltage component of the signal shifts. HDB3 is used to counteract this. Put simply, the HDB3 code works like this:

Channel

The sender remembers in which direction (plus or minus) the last one went. If the next "wrong one" (i.e. four zeros) has to be transmitted, the above replacement table is used.

These four sequences correspond to four times zero .

receiver

The receiver evaluates a positive or negative voltage as one. If there is now a violation of the AMI code rule (two ones with the same polarity), this wrong one is rated as zero. However, if such a violation does not come after three zeros, but after two, the first one, which was actually the correct one on the line, is also rated as zero (the second false zero as well).

advantages

disadvantage

  • As with the AMI code, a line driver is required that can generate the three different levels. Compared to a simple binary transmission, a correspondingly more complex receiving circuit is necessary.

application

The HDB3 line code is used on PCM30 transmission links , among other things .