returntozero

Return-to-zero, often abbreviated as RZ, is a binary line coding scheme used in digital communications. It is also referred to as return-to-zero encoding. In standard RZ, each bit period is divided into two halves, and the signal returns to a baseline zero level within the bit period. The most common variant is unipolar RZ: a logical '1' is represented by a pulse of non-zero voltage during the first half of the bit period followed by zero voltage during the second half, while a logical '0' is represented by zero voltage for the entire bit period. Other conventions exist, such as polar RZ, where the non-zero pulse used for '1' has a fixed polarity, or alternating polarity to reduce direct current (DC) content.

Advantages of RZ include easier clock recovery and timing synchronization, since transitions occur within every bit period. It also performs better on AC-coupled links and is less susceptible to baseline wander than some non-return-to-zero schemes. The main drawback is a higher bandwidth requirement and greater energy consumption due to the extra transitions, compared with NRZ schemes where the signal may remain constant for long runs of identical bits.

Historically, RZ was widely used in early digital transmission systems and some fiber-optic links. In modern high-speed serial communications, NRZ, Manchester encoding, and other line codes have largely supplanted RZ for many applications, though RZ variants persist in certain legacy systems and in channel constraints where straightforward clock recovery is advantageous.