Unipolar encoding
Unipolar encoding is a line code. A positive voltage represents a binary 1, and zero volts indicates a binary 0.
Its drawbacks are that it is not self-clocking and it has a significant DC component. The DC can be reduced by one-half by using return-to-zero, where the signal returns to zero in the middle of the bit period. With a 50% duty cycle each rectangular pulse is only at a positive voltage for half of the bit period. This is ideal if one symbol is sent much more often than the other and power considerations are necessary, and also makes the signal self-clocking.