MPX-DX

It is possible to identify FM-DX stations using the 19 KHz MPX Signal. It already appears when the signal ist rather weak and you only need a few seconds to get a result. Every stereo station has such a 19 KHz signal within the demodulated FM audio spectrum. It varies from transmitter to transmitter. When it's the same on one network, the signal is probably retransmitted. You can use the free SpecLab software to measure the peaks as exact as 1/1000 Hz.

German: Wer auf UKW Stationen identifizieren will, kann das komfortabel mit dem RDS-Signal. Das wird allerdings erst bei einem recht guten Signalpegel dekodiert. Eine Methode, die erheblich schneller zum Ergebnis kommt, ist das MPX-Signal.

MPX - what is that?

Free software like SpecLab is excellent to get exact peak frequencies.

SpecLab-Settings

• Quick Settings -> Image Cancelling DCeceiver -> 48 KHz broadband display
• Peak macro button (left panel) -> click right -> "peak at "+str("###0.00 Hz",peak_f(18950,19050))
• FFT input size: 131072
• internal average FFTs: 10

I use the SDR# software with Airspy HF+ Discovery receiver. The plugin "MPX-Output" can send the full MPX signal (192 KHz sample rate) to Virtual Audio Cable (VAC). Use this as input.

Precision

Coming down to 1/1000 Hz requires stable reception or a longer observation time and a stable MPX carrier. Most transmitters provide that.

I recommend to drive the internal FFT average up to 100 rather than increasing the FFT length. The minimum IF bandwith would be about 40 KHz. Use more if possible.

Distortion and dopplers

Watching the spectrum of a 200 km distant but stable station reveals some interesting development in the demodulated audio. From time to time blurs and the result of dopplers appear in the spectrum.

Long time conclusions 2007 to 2026

All offsets have changed within 19 years. At least, they seem stable over some months or years. Precision has improved. Most stations are within a 0.1 Hz deviation now. I found only one unstable network. Various RSH transmitters drift between 18998.9 to 19001.1 Hz in a minute cycle.

Here some results received here in Handewitt, North Germany. Columns are sortable by clicking on their headers. This helps to check locations and networks whether they have similar deviations. Most of them do not.