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RDS clock recovery from pilot tone?
Question: Could the 19 kHz stereo pilot tone be used for more robust RDS clock recovery?
Yes, in some cases.
Currently Redsea does not use the pilot tone in any way; it is discarded by filtering.
The RDS standard says that the subcarrier should be phase-locked to the third harmonic of the stereo pilot tone. However, there are problems in practice:
- The phase difference would also have to be determined, and there's a lot of variation in the wild.
- Some stations don't lock the RDS to the pilot, so there may also be a frequency difference.
- Some stations don't even have a pilot tone, if they don't transmit in stereo.
Here's a random selection of measurements of FM signals around Europe. They show the phase angle or rotation frequency of the resulting RDS constellation when the subcarrier is downmixed with an oscillator locked to the third harmonic of the pilot tone. The zero-point of the phase angle is arbitrarily set to that of Vikerraadio.
RDS locked to pilot
|Ouï FM||Paris FR||0||+20°|
|Fun Radio||Paris FR||0||+90°|
|Radio Nova||Paris FR||0||-45°|
|Notre Dame||Paris FR||0||+20°|
|Sud Plus||Paris FR||0||+90°|
|Yle Yksi||Espoo FI||0||-45°|
|Radio Nova||Espoo FI||0||-45°|
|Business FM||Helsinki FI||0||+20°|
RDS not locked to pilot
|RTL2||Paris FR||–0.7 Hz||rotating|
|I LUV 2K||Helsinki FI||-1.0 Hz||rotating|
|Bassoradio||Helsinki FI||-1.0 Hz||rotating|
RDS only, no pilot tone
|Europe 1||Paris FR||n/a||n/a|
|France Info||Paris FR||n/a||n/a|