RDS clock recovery from pilot tone?

Question: Could the 19 kHz stereo pilot tone be used for more robust RDS clock recovery?

Short answer

Yes, in some cases.

Long answer

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, ignoring the standard, so there may also be a frequency difference.
• Some stations don't even have a pilot tone, if they don't transmit in stereo.

Measurements

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

Station	Location	d_freq	d_phase
Ouï FM	Paris FR	0	+20°
Fun Radio	Paris FR	0	+90°
Radio Nova	Paris FR	0	-45°
Notre Dame	Paris FR	0	+20°
NRJ	Paris FR	0	-20°
Sud Plus	Paris FR	0	+90°
RFM	Paris FR	0	+60°
MARC	Paris FR	0	+90°
Vikerraadio	Tallinn EE	0	0°
Yle Yksi	Espoo FI	0	-45°
Radio Nova	Espoo FI	0	-45°
Business FM	Helsinki FI	0	+20°
Lähiradio	Helsinki FI	0	-20°
Iskelmä	Espoo FI	0	+60°

RDS not locked to pilot

Station	Location	d_freq	d_phase
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

Station	Location	d_freq	d_phase
Europe 1	Paris FR	n/a	n/a
France Info	Paris FR	n/a	n/a
Järviradio	Helsinki FI	n/a	n/a

Possible solutions

I've been envisioning a command-line option to enable a "specialized demodulator" that locks a PLL to the pilot tone. Later the option could be used for other enhanced demodulation methods, too, like something similar to the time integrating PI detector in RDS Spy.