Modified AMI code

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

Short definition

The HDB3 code is based on the AMI code, in which, however, may cause long zero sequences for synchronization loss. To avoid loss of synchronism so-called code violation rules are used when HDB3 code. For the fourth consecutive zero is replaced by a one in the wrong polarity.

Replacement table

Trivial explanation

HDB3 means that never four zeros are transmitted consecutively. If four zeros must be transmitted, the AMI code rule is violated and transmit a zero with a "wrong one". Wrong fuel in this case means that it has the "wrong" sign ( which is that same as the last one). In itself, this would be sufficient to allow the transfer of any number of zeros and ones in a row. For certain combinations of zeros and ones, however, displaces the DC component of the signal. To counter HDB3 is used. In simple terms, the HDB3 code works as follows:

Transmitter

The transmitter remembers which direction (positive or negative ) was the last one. Now, when the next " wrong one" ( ie four zeros) must be transferred, the procedure according to the substitution table above.

These four sequences correspond to four times zero.

Receiver

The receiver measures 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 false unity is taken as zero. However, where such a violation does not by three zeros, but after two, the first one, which was actually to be the right one on line when zero is counted (the second false zero too).

Benefits

  • Reliable clock recovery from the signal, without a scrambler must be used.
  • A low dc voltage component of the signal

Disadvantages

  • Such as the AMI- code is a line driver is needed that can produce three different levels. Compared with a simple binary transmission according to a more complex receiving circuit is necessary.

Application

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

  • Line code
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