Editing Return-to-zero

{{Short description|Line code in which the signal value returns to zero between each pulse}}
{{other uses|Return to Zero (disambiguation)}}
{{noref|date=April 2018}}
[[Image:RZcode.png|framed|The binary signal is encoded using rectangular [[pulse-amplitude modulation]] with polar return-to-zero code]]

'''Return-to-zero''' ('''RZ''' or '''RTZ''') describes a [[line code]] used in [[telecommunications]] [[signals]] in which the signal drops (returns) to zero between [[Pulse (signal processing)|pulses]].  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 (waves)|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 in optical communication==
===Return to zero, inverted===
Return-to-zero, inverted (RZI) is a method of mapping for transmission.  The two-level RZI signal has a [[Pulse (signal processing)|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&nbsp;= R(data rate).

==Bipolar return-to-zero (bipolar RZ)==
{{main|bipolar encoding}}

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.

==See also==
Other line codes that have 3 states:
* [[Hybrid ternary code]]
* [[Bipolar encoding]]
* [[MLT-3 encoding]]
* [[4B3T]]

==References==
{{Reflist}}

==Further reading==
* {{cite web |title=Digital Magnetic Tape Recording |author-first=John J. G. |author-last=Savard |date=2018 |orig-year=2006 |work=quadibloc |url=http://www.quadibloc.com/comp/tapeint.htm |access-date=2018-07-16 |url-status=live |archive-url=https://web.archive.org/web/20180702234956/http://www.quadibloc.com/comp/tapeint.htm |archive-date=2018-07-02}}

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[[Category:Encodings]]
[[Category:Line codes]]