Decoding MPX without pilot tone

[Otuxam3]

Hello there !

I saw the last update and actually thought about something to share here. After moving to a new location last year, I've started listening to local radios and I was surprised that one of them was broadcasting in stereo without a pilot tone! As much as it's weird and against probably all known specs in the FM stereo modulation world, I find it could be an interesting challenge to try and demux it anyway so here's my thought about the process...

Normally, to properly demodulate stereo and keep it in phase between M and S subcarriers, an initial calibration on the transmitter must be done, that's a thing. On the receiver side, being at 19.0000000000000kHz and 38.0000000000000kHz is nearly impossible due to several factors, having the center frequency of the stereo subcarrier being exactly the double of the frequency of pilot tone is of great help though, so a dozen Hz offset isn't a problem...

Now here is the trick, the pilot tone is totally absent from my recording, not even attenuated. How to recover stereo this way? A first idea was to just pick 38kHz but phase would be constantly rotating, resulting in weird effects. Second possibility : find the center frequency. Very few stations I had the chance to visualize the composite signal used lower side band for the stereo subcarrier and when they did, it was with a pilot tone. But in this current case, I got both side bands. There are methods described on internet about how to retrieve the center frequency of a DSB-SC signal. The last problem would still be the phase which would result in bad stereo separation, however it would be locked and a manual setting could allow for fine tuning.

I still hold the recording if you want to tinker with it, my local law allows for recording and sharing of public broadcasts.