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Return-to-zero describes a line code used in telecommunications signals in which the signal drops to zero between each pulse. This takes place even if a number of consecutive 0s or 1s occur in the signal. The signal is self-clocking. This means that a separate clock does not need to be sent alongside the signal, but suffers from using twice the bandwidth to achieve the same data-rate as compared to non-return-to-zero format.
The "zero" between each bit is a neutral or rest condition, such as a zero amplitude in pulse-amplitude modulation , zero phase shift in phase-shift keying , or mid-frequency in frequency-shift keying.That "zero" condition is typically halfway between the significant condition representing a 1 bit and the other significant condition representing a 0 bit.
Although return-to-zero contains a provision for synchronization, it still has a DC component resulting in “baseline wander” during long strings of 0 or 1 bits, just like the line code non-return-to-zero.