Also known as: AM radio, AM broadcast, medium wave, MW, long wave, LW
Broadcast AM is the oldest mass radio service, carrying audio by amplitude modulation of a carrier on the long-wave (148–283 kHz) and medium-wave (roughly 530–1710 kHz) bands.1 The audio rides directly on the carrier’s amplitude, so the transmitted envelope is a scaled copy of the modulating waveform — which means a receiver can recover the sound with nothing more than a diode and a capacitor, the property that made AM the foundation of early radio.
How it works
An AM transmitter multiplies a constant-frequency carrier by (1 + m·audio), where the modulation index m stays below 1 to avoid over-modulation distortion. The result is a carrier flanked by two mirror-image sidebands, each spanning the audio bandwidth; a double-sideband AM signal is therefore about twice the audio bandwidth wide, and broadcast channels are spaced 9 kHz (ITU regions) or 10 kHz (the Americas) apart, limiting fidelity to a few kHz of audio.
Because the envelope directly represents the audio, the classic envelope detector — a diode that rectifies the RF and a resistor–capacitor network that smooths it — recovers the sound without any local oscillator or phase reference. This simplicity is AM’s defining virtue and its weakness: the same detector responds to any amplitude change, so static crashes, fading, and electrical noise pass straight through. A synchronous detector, which regenerates a phase-locked carrier and multiplies it back against the signal, improves noise and fading performance at the cost of complexity. Some AM stations add a low-level subcarrier (AMSS or C-QUAM stereo pilot tone) above the audio.
Relevance to SDR
AM is a natural SDR demodulation exercise: take the magnitude of the complex baseband IQ samples and you have the envelope, no carrier recovery required — the software equivalent of the diode. Long- and medium-wave AM also showcase propagation: by day the ground wave reaches perhaps a few hundred kilometres, while after dark the sky wave refracts off the ionosphere and distant stations pile in, making MW DXing a popular pastime for direct-sampling SDRs that reach down to the low kHz.
GopherTrunk targets VHF/UHF trunked land-mobile systems and does not decode broadcast AM. The mode remains useful background: envelope detection is the conceptual root of the amplitude-shift-keying and magnitude-based detectors that appear elsewhere in radio, and the MW band is a demanding test of an SDR front end’s dynamic range at night when many strong signals coexist.
Sources
-
AM broadcasting — Wikipedia, for the long-wave and medium-wave bands, amplitude modulation with double sidebands, 9/10 kHz channel spacing, and envelope detection. ↩