Also known as: amplitude modulation

Amplitude modulation (AM) encodes information by varying the amplitude of a carrier wave while its frequency stays fixed.¹ It is the oldest and simplest modulation scheme.

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

How it works

Louder audio produces larger swings in carrier height, creating two mirror-image sidebands around the carrier. Because noise is also an amplitude variation, AM is relatively noise-prone.

Relevance to SDR

AM survives where simplicity or a useful property matters — shortwave broadcast, and aviation VHF airband (where overlapping transmissions beat together so a controller notices). An SDR demodulates AM by tracking the envelope.

Sources

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

See also

• Modulation
• Frequency modulation (FM)
• Single sideband (SSB)
• Carrier wave

Learn it step by step

• Analog modulation — AM, FM, SSB