MPX-Tool bug ?

It’s not a bug in MpxTool, it’s a bug in FM Stereo 🙂.

What you’re seeing IS the RDS signal being demodulated into audio, and this happens with EVERY radio. The reason is a simple matter of bandwidth.

The stereo subcarrier is demoduated by frequency-shifting it down 38 kHz, so that 38 kHz becomes 0 (DC) on the output, and 23/53 kHz becomes 15 kHz on the output.

In 99.99% of MPX decoders (inside radios), real-world filter limitations mean that the decoding goes outside this range, so even the 19 kHz pilot appears on the output.

In MpxTool I designed a brick-wall filter which completely stops the 19 kHz pilot, and completely passes 18 kHz and below. So, in MpxTool, everything between 20 and 56 kHz gets wrapped down into the audio, for 18 kHz frequency response on FM.

The pilot has no bandwidth — it’s a pure tone at 19 kHz, so it doesn’t get through. The RDS signal, on the other hand, has bandwidth, and has information between 54 and 60 kHz. Thus, the 54 – 56 part gets into the audio at 16 – 18 kHz, just like in a regular radio. It’s just the nature of FM.

There IS a way around it though!

MpxTool is (afaik) the only mpx decoder in the world which allows you to choose to decode the whole stereo signal, only the lower sideband, or only the upper sideband.
You can select this with the Mono/Stereo switch — it also has LSB and USB.

If you choose LSB, MpxTool will decode the entire stereo image from just the 20 – 38 kHz part, and ignore anything above. With this option enabled, NO rds leaks through into the audio. As far as I know, there are no drawbacks to this method, other than being a highly non-standard approach to decoding the MPX signal.

///Leif