Stereo FM receiver and RDS decoder¶

In this example, we propose a stereophonic FM broadcasting receiver and RDS (Radio Data System) decoder. Several python script for the radio demodulation and the RDS protocol decoding are provided. For studying purpose, we also provide GRC flowgraphs of the radio demodulator.

Listening to a local station¶

Download and extract the archive rds.tar.gz

tar -xvzf rds.tar.gz

Open the extracted folder and execute the program

cd rds
./fm.py -c <card> -s <sample_rate>

Where :

• card is the kind of card you are usingn : ursp for NI/Ettus USRP, rtlsdr for dongles based on the RTL2832u ;
• sample_rate is the sample rate at which you want your SDR receiver to operate.

Opening the flowgraphs in gnuradio-companion¶

The flowgraphs are located in the radio/grc folder. It contains four files :

• radio.grc : the main flowgraph performing the audio and RDS demodulation (needs fm_receiver.grc to be built) ;
• fm_receiver.grc : separates the L+R (Left plus Right), L-R and RDS channels of the MPX multiplex (needs fm_audio_demod and rds_demod to be built) ;
• fm_audio_demod.grc : produces the stereo audio output ;
• rds_demod.grc : produces the raw RDS bits output.

One would would notice the matlab script "gen_matched_filter_taps.m" at the root of this archive. This scripts allows to calculate the taps of the finite impulse response of the matched filter specified by the RDS standard for any sampling rate and filter length. After calculing new filter taps fitting your needs (e.g. if you want to execute the flowgraph at a different sample rate), they should be put into the variable "mathed_filter_taps" of the flowgraph "rds_demod".