Weird MPX

[bristolpirates]

Here's the MPX audio from a legal station...



Here's a weird one from a pirate I found recently...



Looks like it's got some attempt at RDS going on at 57k, but it doesn't appear to be modulated and it doesn't decode in either SDR# or RDS Spy. There is also a pilot at 95k that to my (limited) knowledge shouldn't be there.

Anybody got a clue what might be happening here?

Could it be an unwanted harmonic of the stereo pilot or RDS pilot?

[radio-berlin]

Pictures not there mate

[bristolpirates]

Should be ok now.

[Shedbuilt]

Looks like it may just be odd order harmonics of the pilot (3rd and 5th being the visible ones). The 4th harmonic at 76KHz may be suppressed by the same mechanism as the 2nd (38KHz suppressed carrier). Or simply because the pilot is a 19KHz square wave (19KHz and its odd order harmonics).
Encoder with no LPF ?

[pjeva]

Most probably, pilot is square wave and mixed directly at the output of the coder. This is why you should not use simple designs... [DISAPPOINTED FACE]


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[Hyperadio879]

Most probably, pilot is square wave and mixed directly at the output of the coder. This is why you should not use simple designs... [DISAPPOINTED FACE]


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Agree. Pilot is mixed direct. This causes bad harmonics.

[bristolpirates]

Cheers, that makes sense.

Anybody know if this kind of thing is common with Chinese rigs? I'm guessing that's all these guys are using.

[teckniqs]

What station is this? I don't think those Chinese things will allow you to run RDS as they are audio input only and not MPX input hence they're in Stereo.

[shuffy]

What station is this? I don't think those Chinese things will allow you to run RDS as they are audio input only and not MPX input hence they're in Stereo.

It's not doing RDS though - that 57KHz signal is the 3rd harmonic of the pilot. Looks like a square wave pilot! Actually it looks just like the output from one of those BH1417's or whatever they're called. The modulating signal on the above trace is pretty narrow bandwidth too, which makes it look a bit crappier than perhaps it really is... or am I being kind?

[teckniqs]

Ahh that makes sense, yeah I didn't for one minute think a Chinese ipod with a power amp was capable.

[sinus trouble]

TBH? I actually doubt the accuracy of the SDR? It could be picking up noise from a number of devices either in your surroundings?

[shuffy]

It could be, but the top trace from the "real" station is pretty spot on (that's how RDS should look around 57KHz) and also the harmonics from the 19KHz pilot on the bottom trace are appearing on the odd multiples like a square wave would.

[bristolpirates]

The stereo difference modulation at 38k is thin just because it was at the point I pressed print screen, it's normal width if you see it moving.

[NOYB]

I agree with Shuffy. The harmonics at 57 and 95 kHz appear to be exactly that - harmonics of the 19kHz pilot. A proper modulated RDS signal typically looks like the 57kHz signal on the legal station - a more spread signal without a peak. I'd be surprised if the RDS station name appears on the average radio with the pirate signal as shown. Even if it appears on some radios (which I doubt) does it appear on all radios? In my experience RDS is quite fussy - you can have a professional RDS encoder, but unless it is close to 0, 90 or -90 degrees phase relationship with the pilot, then many radios will not be able to interpret the RDS signal. Worse still, many auto-seek funtions on car radios scan past stations without an interpretable RDS signal, however strong the RF signal.