Also known as: FM radio, FM broadcast, WFM, Band II
Broadcast FM is the analog radio service that occupies the 88–108 MHz VHF band in most of the world, carrying audio by frequency modulation of a high-power carrier.1 Each station transmits a composite baseband signal — the stereo multiplex, or MPX — in which a mono sum, a stereo difference on a subcarrier, and low-rate data are stacked below 100 kHz and then frequency-modulated onto the RF carrier. It is the most familiar wideband FM signal on the spectrum and a common first target for software-defined-radio receivers.
How it works
At the transmitter, the left and right audio channels are matrixed into a sum (L+R) and a difference (L−R). The sum occupies 0–15 kHz so that mono receivers recover full audio simply by low-pass filtering. The difference is amplitude-modulated (double-sideband suppressed carrier) onto a 38 kHz subcarrier, and a low-level 19 kHz pilot tone — exactly half the subcarrier frequency — is added so the receiver can regenerate a phase-locked 38 kHz reference to demodulate it. A stereo decoder adds and subtracts the recovered sum and difference to reconstruct left and right. Above the stereo pair, a 57 kHz subcarrier — the third harmonic of the pilot — carries RDS digital data, and some stations add further subcarriers (SCA) at 67 or 92 kHz for private audio services.
Before modulation the audio is boosted at high frequencies by pre-emphasis (50 µs in ITU regions, 75 µs in North America); the receiver applies the complementary de-emphasis, trading the FM noise spectrum’s rising high-frequency hiss for an improved signal-to-noise ratio. The composite signal frequency-modulates the carrier with a peak deviation of ±75 kHz, and with Carson’s-rule bandwidth this gives an occupied width of roughly 180–200 kHz per station on a 200 kHz raster.
Relevance to SDR
Broadcast FM is a staple SDR target precisely because its wide deviation makes it loud and easy to demodulate: an RTL-SDR tuned to a strong local station and passed through a quadrature FM discriminator produces clear audio with almost no tuning finesse, which is why “listen to FM” is the canonical first SDR experiment. Recovering stereo requires locking a 19 kHz pilot loop and demodulating the 38 kHz DSB signal; recovering RDS requires a further subcarrier demodulator and differential decoder.
GopherTrunk is a trunked land-mobile scanner (P25, DMR, NXDN, TETRA and similar) and does not decode broadcast FM audio or RDS. The band is nonetheless relevant as context: it is a strong, ever-present signal an SDR front end must handle without overloading, and its multiplex structure is a clean illustration of the subcarrier and pre-emphasis ideas that recur throughout narrowband land-mobile FM.
Sources
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FM broadcasting — Wikipedia, for the 88–108 MHz band, the stereo multiplex with 19 kHz pilot and 38 kHz subcarrier, the 57 kHz RDS subcarrier, and ±75 kHz deviation. ↩