Encode FM recordings around the bandwidth the broadcast actually carried
LAME MP3 settings for analogue FM recordings, including the 15 kHz programme band, residual 19 kHz stereo pilot and a practical 15.5 kHz low-pass.
MP3 Settings for FM Radio Recordings
Why FM is a special case
Analogue FM stereo does not deliver full 20 kHz programme audio. The useful programme band is nominally limited to about 15 kHz, while the stereo multiplex uses a 19 kHz pilot and higher-frequency subcarrier components. A recording made from a tuner can therefore contain residual energy around 19 kHz even though it is not programme audio.
A practical 15.5 kHz compromise
For a normal FM tuner recording, 15.5 kHz is a useful source-aware low-pass starting point: it gives a little margin above the nominal 15 kHz programme band while beginning attenuation well before the 19 kHz stereo pilot. A stricter 15 kHz setting is appropriate when pilot rejection matters more than preserving the last fraction of the transition band.
| Use | Example |
|---|---|
| High-quality FM archive copy | -V2 --lowpass 15.5 |
| Efficient FM programme copy | --abr 160 --lowpass 15.5 |
| Strict pilot-conscious filter | -V2 --lowpass 15 |
Why not low-pass at 19 kHz?
A low-pass transition begins around the selected region rather than behaving like an ideal brick wall. Starting at 19 kHz is therefore too late if the purpose is to suppress a 19 kHz pilot. The cutoff should sit comfortably below the unwanted tone.
Tuner quality still matters
A high-quality tuner may already suppress the stereo pilot strongly. Source-aware low-pass filtering is still useful for recordings where residual pilot, tuner noise or digitisation artefacts remain, but it should not be assumed that every FM capture contains a strong 19 kHz tone.
References
The FM stereo multiplex structure and 15 kHz programme bandwidth are defined by broadcast standards. For practical encoding, LAME documents --lowpass as a way to reduce the amount of high-frequency data presented to the MP3 encoder.