This is detailed instruction how to decode RDS information broadcasted by an FM radio station. MNM4SDR controls demodulation and streaming from remote Raspberry Pi. But all of this is possible because of other great software such as:
VB-Audio Hi-Fi Cable
The legitimate question is why do we need virtual audio card at all? Because, RDS Spy requires 192kHz sample rate audio on its input but unfortunately many audio cards cannot provide it. The picture bellow shows how audio playback and recording for the VB-Audio Hi-Fi cable should be set.
RDS Spy must be configured as it is on the picture. Select "Configure" from the main menu then "Select RDS Source" (Configure=>Select RDS Source) and press "Sound card" button then choose Hi-Fi Cable output and check Direct RDS/MPX (192kHz)
MNM4SDR and VLC
As MNM4SDR hasn't got built-in media player I use VLC for the purpose but you can use what ever player you want that support FLAC Free lossless Audio Codec. I've tested smplayer also and it works but the switching output audio is, in my opinion, less convenient. As it is shown at picture. It is only important to select Sample rate 192000 and Stream rate 192000 on the MNM4SDR. But as FLAC is Losless Audio Compression you can set Stream rate lower, in my experience 128000 bit/s could also work but BER is going to be higher and synchronization is lost more often.
You can hear audio on the VLC now but in order to decode RDS data you must redirect audio to the VB-Audio Hi-Fi cable input as it is shown by the picture, and then switch back to the speaker/default If you like.