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Return-to-zero (RZ orRTZ) describes aline code used intelecommunicationssignals in which the signal drops (returns) to zero betweenpulses.[1] This takes place even if a number of consecutive 0s or 1s occur in the signal. The signal isself-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 tonon-return-to-zero format.
The "zero" between each bit is a neutral or rest condition, such as a zero amplitude inpulse-amplitude modulation (PAM), zerophase shift inphase-shift keying (PSK), or mid-frequency infrequency-shift keying (FSK).That "zero" condition is typically halfway between thesignificant 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 aDC component resulting in “baseline wander” during long strings of 0 or 1 bits, just like the line codenon-return-to-zero.
Return-to-zero, inverted (RZI) is a method of mapping for transmission. The two-level RZI signal has apulse (shorter than aclock 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 theIrDA 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 theARINC 429 bus.
Other line codes that have 3 states:
Line coding (digital baseband transmission) | ||
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| Basicline codes | ||
| Extended line codes | ||
| Optical line codes | ||