If you choose LSB, MpxTool will decode the entire stereo image from just the 20 – 38 kHz part, and ignore anything above. You can select this with the Mono/Stereo switch - it also has LSB and USB. 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. Thus, the 54 – 56 part gets into the audio at 16 – 18 kHz, just like in a regular radio. The RDS signal, on the other hand, has bandwidth, and has information between 54 and 60 kHz. The pilot has no bandwidth - it’s a pure tone at 19 kHz, so it doesn’t get through.

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

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

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