RDS Spy finally supports RDS decoding from MPX or RDS signal connected directly to the sound card.
Choosing that option the entire connection becomes as simple as on the picture:

The RDS Spy provides complete DSP processing of the input signal, including 57 kHz band-pass filtering, subcarrier synchronization and demodulation via Costas Loop equivalent structure, RDS clock regeneration, biphase symbol decoding and differential decoding of RDS data. Thus the RDS Spy entirely removes a need of any external RDS processing circuit.

If your FM tuner does not provide MPX output and no such modification is possible, use the audio line output instead. Although typical FM tuner suppresses RDS signal on the audio output, there are usually some RDS signal residues remaining so the RDS decoding is still possible. Connect both audio channels, right to right, left to left.

Please note that the direct RDS decoding  is applicable only if your sound card supports 192 kHz sampling rate for recording. Most of sound cards do not provide this key facility. Follow the sound card’s documentation or visit the forum. SDR users may connect thru a VAC (Virtual Audio Cable) as well.

You may still decode RDS data with standard non-192 kHz sound card if you connect RDS Clock and Data signals into the Line-in socket. For complete list of RDS sources available click here.