Alternate-Phase Return-to-Zero (APRZ) is an optical line code.
[citation needed] Return-to-zero can be seen as a special case of APRZ in which ΔΦ=0, while Carrier-Suppressed Return-to-Zero (CSRZ) can be viewed as a special case of APRZ in which ΔΦ=π (and the duty cycle is 67%, at least in the standard form of CSRZ).
APRZ can be used to generate specific optical modulation formats, for example, APRZ-OOK, in which data is coded on the intensity of the signal using a binary scheme (light on=1, light off=0).
The characteristic properties of an APRZ signal are those to have a spectrum similar to that of an RZ signal, except that frequency peaks at a spacing of BR/2 as opposed to BR are observed (where BR is the bit rate).
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