An E1 line coding method in which each block of four successive zeros is replaced by 000V or B00V, so that the number of B pulses between consecutive V pulses is odd. Therefore, successive V pulses are of alternate polarity so that no dc component is introduced. Note: B represents an inserted pulse conforming to the alternate mark inversion rule and V represents an AMI violation. HDB3 is similar to B8ZS used with T1.