FM decoding

FM decoding with SDR means taking a radio signal whose frequency is wiggled by audio (frequency modulation, FM) and using software on a computer to turn that back into sound, instead of using analog radio circuitry.

Introduction

In FM, the transmitter keeps the signal strength roughly constant but shifts the carrier frequency up and down according to the audio waveform.

An SDR (software-defined radio) stick or board converts the radio-frequency signal into digital samples (often complex I/Q samples) and sends them to your PC.

Your SDR software then does in software what a classic FM radio did with hardware: tuning, filtering, FM demodulation, and audio output.

A simple mental picture: your SDR grabs a chunk of spectrum around specific frequency, then your software “zooms in” on that station, strips away the carrier, measures how fast the signal’s phase/frequency is changing, and converts those variations into audio samples you can send to the sound card.

(source: payatu.com)

Quick tutorial

• run gqrx software defined radio
• set center frequency to one from 87,5 MHz - 108 MHz range
• select WFM / WBFM / FM decoding mode

another method could be :

• launch rtl_fm and redirect output to audio player

  rtl_fm -f 96.9M -M wfm -s 180k -E deemp | play -r 180k -t raw -e s -b 16 -c 1 -V1 - lowpass 16k

See also

• FM Broadcast Radio at sigidwiki.com
• Listening to FM using RTL-SDR and GQRX
• Decoding FM radio Signals using SDR
• RTL-SDR FM Receiver using GNU Radio