# What does even/odd mean in HDB3 scrambling, and how does one ascertain these values?

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.

And the following example is given:

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?

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