Also known as: on-off keying, OOK
On-off keying (OOK) is the simplest form of amplitude-shift keying: a binary scheme that transmits data by switching a carrier fully on to send a one and off to send a zero.1 Because the presence or absence of RF energy carries the bit, OOK needs no phase reference and only a trivial transmitter, which is why it dominates cheap short-range devices in the ISM bands.
How it works
An OOK transmitter gates the carrier with the data stream, typically by keying a simple oscillator on and off. The receiver does not need to recover carrier phase or even frequency precisely; it only measures envelope power and thresholds it. A basic non-coherent detector rectifies and low-pass filters the signal — an “envelope detector” — then compares the result against a decision level to recover each bit. This makes OOK receivers as cheap as a superregenerative stage or a single detector diode, and lets them run on a coin cell.
The trade-off is efficiency and robustness. Because the transmitter is silent during zeros, average power depends on the data pattern, and a fixed threshold drifts as signal strength changes. Practical OOK protocols therefore use DC-balanced line codes — often Manchester coding or pulse-width symbols — so that every bit contains a transition and the receiver can track a running average for its slicing threshold. OOK is also spectrally wasteful and sensitive to narrowband interference, since a burst of noise in an “off” interval can be mistaken for an “on.”
Variants
OOK is the degenerate two-level case of ASK; using more amplitude levels yields multi-level ASK. Related pulse schemes encode data in timing rather than a fixed on/off grid: pulse-width and pulse-position variants are common in remote-control chips because they tolerate loose timing references. Morse continuous wave telegraphy is historically the original OOK — a hand key switching the transmitter on and off.
Relevance to SDR
OOK is one of the first signals a newcomer decodes with a cheap receiver, because the demodulation is so forgiving. A huge population of consumer devices in the 315 MHz, 433.92 MHz, 868 MHz, and 915 MHz ISM bands use it: remote keyless entry fobs, garage-door and gate openers, tire-pressure monitoring sensors (often mixing OOK with FSK), wireless doorbells, weather stations, and many home-automation remotes. On a waterfall these appear as short, bursty vertical stripes that blink on and off with the data.
Decoding OOK in software usually means taking the magnitude of the IQ
stream, thresholding it, and measuring pulse widths to recover the underlying symbols —
the approach used by generic tools like the rtl_433 family. GopherTrunk is a trunked
land-mobile scanner focused on P25, DMR, NXDN, and TETRA, so OOK telemetry devices fall
outside its decode chain; OOK is covered here as background for the amplitude-modulation
family that GopherTrunk’s users frequently encounter on the same hardware.
Sources
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On–off keying — Wikipedia, for the definition of OOK as binary ASK and its use in low-cost ISM-band devices. ↩