Return-to-zero (RZ or RTZ) describes a line code used in telecommunications signals in which the signal drops (returns) 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 (PAM), zero phase shift in phase-shift keying (PSK), or mid-frequency in frequency-shift keying (FSK). 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 (RZ) 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, inverted (RZI) is a method of mapping for transmission. The two-level RZI signal has a pulse (shorter than a clock cycle) if the binary signal is 0, and no pulse if the binary signal is 1. It is used (with a pulse 3/16 of a bit long) by the IrDA serial infrared (SIR) physical layer specification. Required bandwidth for this kind of modulation is: BW = R(data rate).

For bipolar return-to-zero (bipolar RZ), 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.

Other line codes that have 3 states:

• Hybrid ternary code
• Bipolar encoding
• MLT-3 encoding
• 4B3T

• Savard, John J. G. (2018) [2006]. "Digital Magnetic Tape Recording". quadibloc. Archived from the original on 2018-07-02. Retrieved 2018-07-16.

v t e Line coding (digital baseband transmission)
Main articles	Unipolar encoding Bipolar encoding On-off keying Mark and space
Basic line codes	Return to zero (RZ) Non-return-to-zero, level (NRZ/NRZ-L) Non-return-to-zero, inverted (NRZ-I) Non-return-to-zero, space (NRZ-S) Manchester Differential Manchester/biphase (Bi-φ)
Extended line codes	Conditioned diphase 4B3T 4B5B 2B1Q Alternate mark inversion Modified AMI code Coded mark inversion MLT-3 encoding Hybrid ternary code 6b/8b encoding 8b/10b encoding 64b/66b encoding Eight-to-fourteen modulation Delay/Miller encoding TC-PAM
Optical line codes	Carrier-suppressed return-to-zero Alternate-phase return-to-zero
See also: Baseband Baud Bit rate Digital signal Digital transmission Ethernet physical layer Pulse modulation methods Pulse-amplitude modulation (PAM) Pulse-code modulation (PCM) Serial communication Category:Line codes