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HB1PMS > TECH 28.11.20 12:05l 183 Lines 7964 Bytes #999 (0) @ WW
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Subj: FM-RDS transmitter using the Raspberry Pi
Path: DBO595<DBX320<DBX233<VB1BOX<NL3VKL<DX3RRR<HB1BBS
Sent: 201127/1914Z 13483@HB1BBS.ZL.NLD.EU BPQ6.0.19
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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.
It is compatible with both the Raspberry Pi 1 (the original one) and the
Raspberry Pi 2, 3 and 4.
PiFmRds has been developed for experimentation only. It is not a media
center, it is not intended to broadcast music to your stereo system. See the
legal warning.
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.
Pi-FM-RDS also depends on the Linux rpi-mailbox driver, so you need a recent
Linux kernel. The Raspbian releases have this starting from August 2015.
Important. The binaries compiled for the Raspberry Pi 1 are not compatible
with the Raspberry Pi 2/3, and conversely. Always re-compile when switching
models, so do not skip the make clean step in the instructions below!
Clone the source repository and run make in the src directory:
git clone https://github.com/ChristopheJacquet/PiFmRds.git
cd PiFmRds/src
make clean
make
Important. If make reports any error, then no pi_fm_rds executable file is
generated (and vice versa). So any error must be fixed before you can
proceed to the next steps. make may fail if any required library is missing
(see above), or it could be a bug on a specific/newer distribution. In this
case, please file a bug.
If make reports no error (i.e. the pi_fm_rds executable gets generated), you
can then simply 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 an audio file as
follows:
sudo ./pi_fm_rds -audio 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] [-audio file] [-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.
-audio specifies an audio 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: -audio sound.wav. The
supported formats depend on libsndfile. This includes WAV and Ogg/Vorbis
(among others) but not MP3. Specify - as the file name to read audio data on
standard input (useful for piping audio into Pi-FM-RDS, see below).
-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!'.
-ctl specifies a named pipe (FIFO) to use as a control channel to change PS
and RT at run-time (see below).
-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 receivers 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...
Piping audio into Pi-FM-RDS
If you use the argument -audio -, Pi-FM-RDS reads audio data on standard
input. This allows you to pipe the output of a program into Pi-FM-RDS. For
instance, this can be used to read MP3 files using Sox:
sox -t mp3 http://www.linuxvoice.com/episodes/lv_s02e01.mp3 -t wav - | sudo
./pi_fm_rds -audio -
Or to pipe the AUX input of a sound card into Pi-FM-RDS:
sudo arecord -fS16_LE -r 44100 -Dplughw:1,0 -c 2 - | sudo ./pi_fm_rds -
audio -
Changing PS, RT and TA at run-time
You can control PS, RT and TA (Traffic Announcement flag) at run-time using
a named pipe (FIFO). For this run Pi-FM-RDS with the -ctl argument.
Example:
mkfifo rds_ctl
sudo ./pi_fm_rds -ctl rds_ctl
Then you can send “commandsö to change PS, RT and TA:
cat >rds_ctl
PS MyText
RT A text to be sent as radiotext
TA ON
PS OtherTxt
TA OFF
...
Every line must start with either PS, RT or TA, followed by one space
character, and the desired value. Any other line format is silently ignored.
TA ON switches the Traffic Announcement flag to on, any other value switches
it to off.
Warning and Disclaimer
PiFmRds is an experimental program, designed only for experimentation. It is
in no way intended to become a personal media center or a tool to operate a
radio station, or even broadcast sound to one's own stereo system.
In most countries, transmitting radio waves without a state-issued licence
specific to the transmission modalities (frequency, power, bandwidth, etc.)
is illegal.
Therefore, always connect a shielded transmission line from the RaspberryPi
directly to a radio receiver, so as not to emit radio waves. Never use an
antenna.
Even if you are a licensed amateur radio operator, using PiFmRds to transmit
radio waves on ham frequencies without any filtering between the RaspberryPi
and an antenna is most probably illegal because the square-wave carrier is
very rich in harmonics, so the bandwidth requirements are likely not met.
I could not be held liable for any misuse of your own Raspberry Pi. Any
experiment is made under your own responsibility.
73 Henk.
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