2 ms·

I don't think radios use a PLL to demodulate the FM audio: the signal has a huge "pilot" tone at 19kHz that you can match to get the first part of the spectrum,

I don't think radios use a PLL to demodulate the FM audio: the signal has a huge "pilot" tone at 19kHz that you can match to get the first part of the spectrum, mono audio (L+R channels), and at double that frequency you know you'll find the stereo part (L-R channels). Precise phase estimation is only necessary to decode the RDS digital data (station name, datetime, etc.).

lrasinen 2y ago
They do. First of all, the pilot is only required for decoding stereo and RDS. Mono FM does not use a pilot, so obviously there had to be a way to detect FM before stereo came along. I linked to a few of the approaches in a sibling (cousin?) comment. Second, the pilot is embedded in the decoded FM audio. You need to demodulate FM to get to it in the first place. If you look at the waterfall display in an SDR receiver, it might seem like the signal is already present in the original radio frequencies (especially during silent periods), but it's there only indirectly. If you have silence in an FM transmission (say 96.6 MHz), the only audio component present is the 19 kHz pilot signal, which causes the FM radio signal frequency to vary between 96.6 MHz ± k*19 kHz (not sure what's the value for k, but it's not 1). The sine likes to spend most of the time near the extreme values of its range; plot a histogram of a sine wave and you'll see peaks on either end. The waterfall is basically a histogram over frequencies so it gets those peaks as streaks on both sides of the main carrier frequency (plus smaller ones for other components in the signal).
- jhallenworld 2y ago
  The 19 KHz pilot tone is interesting: it's not filtered out and is often reproduced by the speakers if you have good ones. You can verify this by using an audio spectrum analyzer application on your phone. I've wondered if FM stereo drives pets nuts with its constant high-pitched tone.
kmbfjr 2y ago
It is not “huge”, it is no more than 10 percent and no less than 8 percent of the total modulation.
Johnythree 2y ago
Most cheap FM receivers definitely use a PLL to detect FM. Giving automatic tuning is just a side benifit.
- jhallenworld 2y ago
  Do you mean AFT (automatic fine tuning)? This has nothing to do with a PLL-based demodulator. Older receivers with ratio detectors or discriminators also had AFT- there is a low frequency feedback path from the demodulator to the local oscillator to make this work.

lrasinen 2y ago

They do. First of all, the pilot is only required for decoding stereo and RDS. Mono FM does not use a pilot, so obviously there had to be a way to detect FM before stereo came along. I linked to a few of the approaches in a sibling (cousin?) comment. Second, the pilot is embedded in the decoded FM audio. You need to demodulate FM to get to it in the first place. If you look at the waterfall display in an SDR receiver, it might seem like the signal is already present in the original radio frequencies (especially during silent periods), but it's there only indirectly. If you have silence in an FM transmission (say 96.6 MHz), the only audio component present is the 19 kHz pilot signal, which causes the FM radio signal frequency to vary between 96.6 MHz ± k*19 kHz (not sure what's the value for k, but it's not 1). The sine likes to spend most of the time near the extreme values of its range; plot a histogram of a sine wave and you'll see peaks on either end. The waterfall is basically a histogram over frequencies so it gets those peaks as streaks on both sides of the main carrier frequency (plus smaller ones for other components in the signal).

jhallenworld 2y ago
The 19 KHz pilot tone is interesting: it's not filtered out and is often reproduced by the speakers if you have good ones. You can verify this by using an audio spectrum analyzer application on your phone. I've wondered if FM stereo drives pets nuts with its constant high-pitched tone.

kmbfjr 2y ago

It is not “huge”, it is no more than 10 percent and no less than 8 percent of the total modulation.

Johnythree 2y ago

Most cheap FM receivers definitely use a PLL to detect FM. Giving automatic tuning is just a side benifit.

jhallenworld 2y ago
Do you mean AFT (automatic fine tuning)? This has nothing to do with a PLL-based demodulator. Older receivers with ratio detectors or discriminators also had AFT- there is a low frequency feedback path from the demodulator to the local oscillator to make this work.