Also known as: NRZI, non-return-to-zero inverted

NRZI (non-return-to-zero inverted) is a line code in which each bit is conveyed by whether the signal level changes, not by the level itself: conventionally a 0 causes a transition and a 1 causes none (or vice-versa).¹ It is used by AX.25/APRS and AIS.

NRZI encodes bits as the presence or absence of a level transition, which guarantees frequent edges for timing.

Overview

Because NRZI ties bits to transitions, combining it with bit stuffing (as HDLC does) guarantees regular edges, which keeps the receiver’s clock recovery locked even through long runs of identical data bits.

Sources

1. Non-return-to-zero — NRZI — Wikipedia, for the transition-based line-code definition. ↩

See also

• AX.25
• APRS
• Automatic Identification System (AIS)
• Frequency-shift keying (FSK)
• Clock recovery

Learn it step by step

• The demodulation pipeline