HDB2

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High density bipolar code (HDB2) - one of the transmission and pseudorandom codes. The HDB-2 has the features needed to perform the transmission despite the constraints imposed by the transmission channel and its associated equipment. This code has three code states despite the fact that the information signal has only two states. In addition, it also contains linear elements whose duration is half the characteristic period T. Three levels make the signal has convenient spectral properties, reducing the level of interference and improving the transmission range and quality. Frequent changes in the signal state (even if the signal state remains unchanged) provide good synchronization properties.

Signal properties:

The code converts element "1" to positive and negative pulses alternately, and element "0" to zero signal level unless there is a zero sequence of length greater than two. In this case, every third zero is replaced by a polarization impulse corresponding to the last pulse polarity (disturbance of the alternating rule). The polarization is changed from pulse to pulse (no long sequence replays with zero pulse sequences with the same polarity), which prevents the unwanted low frequency portion of the spectrum from being highlighted.

Defective code:

How the decoder works The error decoder detects errors using the code properties that result from its creation. Since the 1s are encoded with alternating polarization, the error will be reported when the two poles are detected with the same polarity. In addition, no zeros of length greater than two can occur in the code - otherwise, the decoder will signal an error.

Disadvantages of decoder:

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