I am currently studying encoding techniques. The scrambling technique HDB3 is introduced as follows:

  • High-Density Bipolar-3 Zeros
  • Based on bipolar-AMI
  • String of four zeros is replaced with sequences containing one or two pulses.
  • A sequence of four consecutive zeros is encoded using a special 'violation' bit. This bit has the same polarity as the last 1-bit that was sent using the AMI encoding rule.
  • The violation bit alternates between + and - pulses for every consecutive group of four zeros. enter image description here

And the following example is given:

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I don't understand the even/odd stuff. What does the "parity of the +/- bits since previous V" mean, and how does one ascertain the even/odd values (that is, how can we tell if it's supposed to be "even" or "odd", as shown by the arrows) as in the example?


The point of scrambling is to remove as much DC bias from the electrical signal as possible. DC prevents long links, causing noise and interfering with the use of transducers/pulse transformers for power-free data transmission.

The parity is for the number of +/- symbols, an even or odd number.

  • +- = two +/- symbols, even parity
  • +00+ = two +/- symbols, even parity
  • - = one symbol, odd

and so on.

Four consecutive 0000 require a 'violation bit' V that resolves the problem. "Since previous V" simply means since the last 0000 problem that required resolution.

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