Pi-FM-RDS

FM-RDS transmitter using the Raspberry Pi

This program generates an FM modulation, with RDS (Radio Data System) data generated in real time. It can include monophonic or stereophonic audio.

It is based on the FM transmitter created by Oliver Mattos and Oskar Weigl, and later adapted to using DMA by Richard Hirst. Christophe Jacquet adapted it and added the RDS data generator and modulator. The transmitter uses the Raspberry Pi's PWM generator to produce VHF signals.

NEW: now supports any sample rate for audio files, and generates FM-Stereo signals!

How to use it?

Pi-FM-RDS, depends on the sndfile library. To install this library on Debian-like distributions, for instance Raspbian, run sudo apt-get install libsndfile1-dev.

Then clone the source repository and run make in the src directory:

git clone https://github.com/ChristopheJacquet/PiFmRds.git
cd PiFmRds/src
make

Then you can just run:

sudo ./pi_fm_rds

This will generate an FM transmission on 107.9 MHz, with default station name (PS), radiotext (RT) and PI-code, without audio. The radiofrequency signal is emitted on GPIO 4 (pin 7 on header P1).

You can add monophonic or stereophonic audio by referencing a WAV file as follows:

sudo ./pi_fm_rds -wav sound.wav

To test stereophonic audio, you can try the file stereo_44100.wav provided.

The more general syntax for running Pi-FM-RDS is as follows:

pi_fm_rds [-freq freq] [-wav file.wav] [-ppm ppm_error] [-pi pi_code] [-ps ps_text] [-rt rt_text]

All arguments are optional:

• -freq specifies the carrier frequency (in MHz). Example: -freq 107.9.
• -wav specifies a WAV file to play as audio. The sample rate does not matter: Pi-FM-RDS will resample and filter it. If a stereo file is provided, Pi-FM-RDS will produce an FM-Stereo signal. Example: -wav sound.wav.
• -pi specifies the PI-code of the RDS broadcast. 4 hexadecimal digits. Example: -pi FFFF.
• -ps specifies the station name (Program Service name, PS) of the RDS broadcast. Limit: 8 characters. Example: -ps RASP-PI.
• -rt specifies the radiotext (RT) to be transmitted. Limit: 64 characters. Example: -rt 'Hello, world!'.
• -ppm specifies your Raspberry Pi's oscillator error in parts per million (ppm), see below.

By default the PS changes back and forth between Pi-FmRds and a sequence number, starting at 00000000. The PS changes around one time per second.

Clock calibration (only if experiencing difficulties)

The RDS standards states that the error for the 57 kHz subcarrier must be less than ± 6 Hz, i.e. less than 105 ppm (parts per million). The Raspberry Pi's oscillator error may be above this figure. That is where the -ppm parameter comes into play: you specify your Pi's error and Pi-FM-RDS adjusts the clock dividers accordingly.

In practice, I found that Pi-FM-RDS works okay even without using the -ppm parameter. I suppose the receiver are more tolerant than stated in the RDS spec.

One way to measure the ppm error is to play the pulses.wav file: it will play a pulse for precisely 1 second, then play a 1-second silence, and so on. Record the audio output from a radio with a good audio card. Say you sample at 44.1 kHz. Measure 10 intervals. Using Audacity for example determine the number of samples of these 10 intervals: in the absence of clock error, it should be 441,000 samples. With my Pi, I found 441,132 samples. Therefore, my ppm error is (441132-441000)/441000 * 1e6 = 299 ppm, assuming that my sampling device (audio card) has no clock error...

Warning and Diclaimer

Never use this program to transmit VHF-FM data through an antenna, as it is illegal in most countries. This code is for experimental purposes only. Always connect a shielded transmission line from the RaspberryPi directly to a radio receiver, so as not to emit radio waves.

I could not be held liable for any misuse of your own Raspberry Pi. Any experiment is made under your own responsibility.

Tests

Pi-FM-RDS was successfully tested with all my RDS-able devices, namely:

• a Sony ICF-C20RDS alarm clock from 1995,
• a Sangean PR-D1 portable receiver from 1998,
• a Samsung Galaxy S2 mobile phone from 2011,
• a Philips MBD7020 hifi system from 2012,
• a Silicon Labs USBFMRADIO-RD USB stick, employing an Si4701 chip, and using my RDS Surveyor program,
• a “PCear Fm Radio”, a Chinese clone of the above, again using RDS Surveyor.

Reception works perfectly with all the devices above. RDS Surveyor reports no group errors.

CPU Usage

CPU usage is as follows:

• without audio: 9%
• with mono audio: 42%
• with stereo audio: 54%

CPU usage increases dramatically when adding audio because the program has to upsample the (unspecified) sample rate of the input audio file to 228 kHz, its internal operating sample rate. Doing so, it has to apply an FIR filter, which is costly.

Design

The RDS data generator lies in the rds.c file.

The RDS data generator generates cyclically four 0A groups (for transmitting PS), and one 2A group (for transmitting RT). get_rds_group generates one group, and uses crc for computing the CRC.

To get samples of RDS data, call get_rds_samples. It calls get_rds_group, differentially encodes the signal and generates a shaped biphase symbol. Successive biphase symbols overlap: the samples are added so that the result is equivalent to applying the shaping filter (a root-raised-cosine (RRC) filter specified in the RDS standard) to a sequence of Manchester-encoded pulses.

The shaped biphase symbol is generated once and for all by a Python program called generate_waveforms.py that uses Pydemod, one of my other software radio projects. This Python program generates an array called waveform_biphase that results from the application of the RRC filter to a positive-negative impulse pair. Note that the output of generate_waveforms.py, two files named waveforms.c and waveforms.h, are included in the Git repository, so you don't need to run the Python script yourself to compile Pi-FM-RDS.

Internally, the program samples all signals at 228 kHz, four times the RDS subcarrier's 57 kHz.

The FM multiplex signal (baseband signal) is generated by fm_mpx.c. This file handles the upsampling of the input audio file to 228 kHz, and the generation of the multiplex: unmodulated left+right signal (limited to 15 kHz), possibly the stereo pilot at 19 kHz, possibly the left-right signal, amplitude-modulated on 38 kHz (suppressed carrier) and RDS signal from rds.c. Upsampling is performed using a zero-order hold followed by an FIR low-pass filter of order 60. The filter is a sampled sinc windowed by a Hamming window. The filter coefficients are generated at startup so that the filter cuts frequencies above the minimum of:

• the Nyquist frequency of the input audio file (half the sample rate) to avoid aliasing,
• 15 kHz, the bandpass of the left+right and left-right channels, as per the FM broadcasting standards.

The samples are played by pi_fm_rds.c that is adapted from Richard Hirst's PiFmDma. The program was changed to support a sample rate of precisely 228 kHz.

References

• EN 50067, Specification of the radio data system (RDS) for VHF/FM sound broadcasting in the frequency range 87.5 to 108.0 MHz

History

• 2014-04-14: new release that supports any sample rate for the audio input, and that can generate a proper FM-Stereo signal if a stereophonic input file is provided
• 2014-04-06: initial release, which only supported 228 kHz monophonic audio input files

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© Christophe Jacquet (F8FTK), 2014. Released under the GNU GPL v3.