Also known as: amplitude-shift keying, ASK
Amplitude-shift keying (ASK) is digital modulation that represents each symbol by a discrete amplitude of the carrier, while its frequency and phase stay fixed.1 It is the digital counterpart of analog amplitude modulation: where AM varies amplitude continuously with a message, ASK snaps it to a small set of levels chosen by the data. The two-level case — carrier on or off — is on-off keying, by far the most common form.
How it works
The transmitter multiplies the carrier by a data-dependent gain, producing bursts of different amplitude. A receiver recovers the bits with an envelope detector — it takes the magnitude of the signal and thresholds it — which needs no carrier-phase recovery, so ASK demodulators are simple and cheap. With M amplitude levels, each symbol carries log₂M bits; four levels give 2 bits per symbol, and so on.
The weakness of ASK is that amplitude is exactly what noise, fading, and gain drift attack most directly. A fluctuating channel moves the received levels around, so the decision thresholds must adapt, and multi-level ASK needs a good signal-to-noise ratio to keep the levels distinguishable. Because the information rides on amplitude, ASK also demands a linear transmit amplifier, unlike constant-envelope FSK, which tolerates efficient saturated amplifiers. These constraints keep pure ASK confined to short, benign links.
Variants
Binary ASK is OOK. Combining amplitude and phase into a two-dimensional grid produces quadrature amplitude modulation, which is really the joint amplitude/phase generalisation of ASK and PSK and is what carries high data rates in Wi-Fi, DVB, and cellular. So while pure multi-level ASK is rare on the air, its amplitude dimension lives on inside QAM.
Relevance to SDR
Most ASK you will meet in software radio is on-off keying in the license-free ISM bands: RFID tags, remote controls, wireless sensors, and simple telemetry links. Amplitude keying also appears in optical and infrared links, where turning a light source on and off is the natural channel. On a waterfall, ASK bursts show up as amplitude-modulated stripes whose brightness — not position — carries the data.
Demodulating ASK in an SDR is a matter of computing the IQ magnitude and slicing it against an adaptive threshold; no Costas loop or symbol-phase tracking is required. GopherTrunk targets trunked land-mobile systems that use frequency- and phase-keyed modulations, so it does not decode ASK telemetry itself; ASK is documented here because it anchors the amplitude branch of the modulation family and is a common first signal on the same low-cost dongles GopherTrunk runs on.
Sources
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Amplitude-shift keying — Wikipedia, for the definition, the on-off-keying special case, and multi-level ASK. ↩