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Also known as: amplitude modulation

Amplitude modulation (AM) encodes information by varying the amplitude of a carrier wave while its frequency and phase stay fixed.1 It is the oldest and simplest modulation scheme, dating to the first voice broadcasts around 1900, and it remains in daily use for shortwave and aviation because the receiver can be trivially cheap.

the envelope carries the message
AM varies the carrier's amplitude in step with the message; the dashed envelope is the audio.

How it works

A pure carrier at frequency fc carries no information — it is a single spectral line. AM multiplies that carrier by (1 + m·s(t)), where s(t) is the message scaled to ±1 and m is the modulation index. Louder audio produces larger swings in carrier height. In the frequency domain this multiplication shifts a copy of the message spectrum up to sit on either side of the carrier, producing two mirror-image sidebands:2 the upper sideband runs from fc to fc + fmax and the lower sideband is its reflection below the carrier. A 3 kHz audio bandwidth therefore occupies about 6 kHz of RF. Both sidebands carry the same information, so classic AM is inherently a double-sideband scheme that wastes half its bandwidth and, because the carrier itself carries no message, most of its transmitted power.

The great virtue is demodulation: because the message rides directly on the envelope, a receiver only needs an envelope detector — a diode, a capacitor, and a resistor — to recover the audio. No frequency reference or phase lock is required, which is why the cheapest possible radio can hear AM. The great weakness is that noise, interference, and fading are themselves amplitude disturbances, so they add directly to the recovered audio; AM has no inherent noise immunity the way FM does. Keeping the modulation index at or below 1 (100%) avoids overmodulation, where the envelope pinches to zero and the recovered audio distorts badly and splatters energy into adjacent channels.

Variants

Because full-carrier double-sideband AM is spendthrift, several relatives trim it down. Single-sideband (SSB) suppresses the carrier and one sideband, cutting bandwidth in half and putting all power on the information — the choice for long-distance HF voice. Double-sideband suppressed-carrier (DSB-SC) removes only the carrier. Vestigial sideband (VSB) transmits one full sideband plus a filtered stub of the other, a compromise used by analog television video. The digital cousins are amplitude-shift keying, which switches between discrete amplitude levels, and its two-level special case on-off keying, the modulation of Morse CW and many low-cost remote controls.

Relevance to SDR

AM survives where simplicity or a useful property matters. Shortwave broadcast still uses full-carrier AM so any household set can tune it. Aviation VHF airband (108–137 MHz) deliberately keeps AM because when two aircraft transmit at once their carriers beat together, producing an audible heterodyne that warns the controller of a collision — a safety property FM’s capture effect would hide. An SDR demodulates AM by computing the magnitude of the complex IQ baseband (a synchronous or envelope detector), which needs no PLL and is one of the first modes any SDR tutorial implements. GopherTrunk is a digital trunking decoder and does not target broadcast or airband AM voice, but the same envelope math appears wherever it must measure signal amplitude.

Sources

  1. Amplitude modulation — Wikipedia, for the definition, modulation index, sidebands, and uses of AM. ↩

  2. Sideband — Wikipedia, for how modulation creates upper and lower sidebands and the resulting occupied bandwidth. ↩

See also

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