Return-to-zero


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.

Return-to-zero in optical communication

Return to zero, inverted

Return-to-zero, inverted is a method of mapping for transmission. The two-level RZI signal has a pulse if the binary signal is 0, and no pulse if the binary signal is 1. It is used by the IrDA serial infrared physical layer specification. Required bandwidth for this kind of modulation is: BW = R.

Bipolar return-to-Zero (bipolar RZ)

For bipolar return-to-zero, a binary one is encoded as +V volts, a binary zero is encoded as -V volts, and 0 volt is used to provide padding and separation between bits.
Bipolar return-to-zero encoding is used by the ARINC 429 bus.