Home

Awesome

Application note on using rpitx

The recently released <a href="https://github.com/F5OEO/rpitx">rpitx</a> by Evariste, F5OEO allows us to transmit an RF signal over GPIO18 (pin 12) of a Raspberry Pi. The software can accept an I/Q signal as an input, so now a Pi can be used as a general purpose SDR transmitter.

This document is about using GNU Radio and csdr with rpitx.

Table of contents

...and actually you may distrub some important radio communication services. Please don't do this! You can get into trouble! You have been warned.

In addition to the squre wave output, the spectrum is also affected by the PWM-based amplitude modulation implemented in rpitx.

Using a Raspberry Pi as remote software defined radio peripheral from GNU Radio

To do this, you will need:

We will stream the I/Q signal from the PC to the Raspberry Pi via TCP.

On the Raspberry Pi, execute:

nc -l 8011 | sudo rpitx -i- -m IQFLOAT -f 28400

If you want it to restart everytime the connection is lost, use this command:

while true; do (nc -l 8011; dd if=/dev/zero bs=4096 count=30); done | sudo rpitx -i- -m IQFLOAT -f 28400

NFM modulator example for GNU Radio

On the PC, open GNU Radio Companion, and load the flow graph in this repo at gnuradio/nfm-rpitx.

NFM in GNU Radio

If you execute it, the I/Q signal will be streamed to the Raspberry Pi through the TCP socket.

Here is the good result if received with an RTL-SDR and GQRX:

NFM in GQRX

SSB modulator example for GNU Radio

Use the flow graph in gnuradio/ssb-rpitx. This will transmit an USB signal.

SSB in GNU Radio

The spectrum of the transmitted I/Q signal looks like this:

SSB in GNU Radio

After receiving it with RTL-SDR and GQRX:

SSB in GQRX

If you want LSB, you should double-click the "Band Pass Filter" block, and change:

SSB in GQRX

AM modulator example for GNU Radio

Use the flow graph in gnuradio/am-rpitx. This will transmit an AM signal.

AM in GQRX

The spectrum of the transmitted I/Q signal looks like this:

AM in GQRX

After receiving it with RTL-SDR and GQRX:

AM in GQRX

Using rpitx with csdr to modulate streaming input

<a href="https://github.com/simonyiszk/csdr">csdr</a> is a command line tool for simple DSP tasks. It can be used to build simple AM/FM/SSB receivers, and now transmitters as well, and is quite fast to setup.

You will need the dev branch of csdr for doing this.<br /> Setup instructions:

git clone https://github.com/simonyiszk/csdr.git
cd csdr
git fetch
git checkout dev
make && sudo make install

Note that it should be already done if you installed qtcsdr previously.

Modulate from raw audio file

These examples will use the raw audio file music48000.raw and speech48000.raw, which is present in this repo. You can get this file by:

git clone https://github.com/ha7ilm/rpitx-app-note
cd rpitx-app-note; ls  #There are your files.

We will play these files in a loop, you can stop it with Ctrl+C.

Generate AM modulation:

(while true; do cat music48000.raw; done) | csdr convert_i16_f | csdr gain_ff 1.0 | csdr dsb_fc | csdr add_dcoffset_cc | sudo rpitx -i- -m IQFLOAT -f 28400

Generate USB modulation:

(while true; do cat speech48000.raw; done) | csdr convert_i16_f | csdr dsb_fc | csdr bandpass_fir_fft_cc 0 0.1 0.01 | csdr gain_ff 2.0 | csdr shift_addition_cc 0.2 | sudo rpitx -i- -m IQFLOAT -f 28400

Generate LSB modulation:

(while true; do cat speech48000.raw; done) | csdr convert_i16_f | csdr dsb_fc | csdr bandpass_fir_fft_cc -0.1 0 0.01 | csdr gain_ff 2.0 | csdr shift_addition_cc 0.2 | sudo rpitx -i- -m IQFLOAT -f 28400

Generate NFM modulation:

(while true; do cat music48000.raw; done) | csdr convert_i16_f | csdr gain_ff 7000 | csdr convert_f_samplerf 20833 | sudo rpitx -i- -m RF -f 28400

Generate WFM modulation:

(while true; do cat music48000.raw; done) | csdr convert_i16_f | csdr gain_ff 70000 | csdr convert_f_samplerf 20833 | sudo rpitx -i- -m RF -f 28400

Modulate from microphone input source

Use this if you have a microphone connected to the Raspberry Pi via external USB audio device.<br /> You will have to get the correct ALSA device ID via arecord -L, it will be something like plughw:CARD=Device,DEV=0. You should enter as the argument of the -D switch of arecord.

#AM:
arecord -c1 -r48000 -D plughw:CARD=Device,DEV=0 -fS16_LE - | csdr convert_i16_f | csdr gain_ff 1.0 | csdr dsb_fc | csdr add_dcoffset_cc | sudo rpitx -i- -m IQFLOAT -f 28400
#USB:
arecord -c1 -r48000 -D plughw:CARD=Device,DEV=0 -fS16_LE - | csdr convert_i16_f | csdr dsb_fc | csdr bandpass_fir_fft_cc 0 0.1 0.01 | csdr gain_ff 2.0 | csdr shift_addition_cc 0.2 | sudo rpitx -i- -m IQFLOAT -f 28400
#LSB:
arecord -c1 -r48000 -D plughw:CARD=Device,DEV=0 -fS16_LE - | csdr convert_i16_f | csdr dsb_fc | csdr bandpass_fir_fft_cc -0.1 0 0.01 | csdr gain_ff 2.0 | csdr shift_addition_cc 0.2 | sudo rpitx -i- -m IQFLOAT -f 28400
#NFM:
arecord -c1 -r48000 -D plughw:CARD=Device,DEV=0 -fS16_LE - | csdr convert_i16_f | csdr gain_ff 7000 | csdr convert_f_samplerf 20833 | sudo rpitx -i- -m RF -f 28400
#WFM:
arecord -c1 -r48000 -D plughw:CARD=Device,DEV=0 -fS16_LE - | csdr convert_i16_f | csdr gain_ff 70000 | csdr convert_f_samplerf 20833 | sudo rpitx -i- -m RF -f 28400

Modulate from audio streamed from remote computer

First, start the transmitter:

#AM:
nc -l 8011 | csdr convert_i16_f | csdr gain_ff 1.0 | csdr dsb_fc | csdr add_dcoffset_cc | sudo rpitx -i- -m IQFLOAT -f 28400
#USB:
nc -l 8011 | csdr convert_i16_f | csdr dsb_fc | csdr bandpass_fir_fft_cc 0 0.1 0.01 | csdr gain_ff 2.0 | csdr shift_addition_cc 0.2 | sudo rpitx -i- -m IQFLOAT -f 28400
#LSB:
nc -l 8011 | csdr convert_i16_f | csdr dsb_fc | csdr bandpass_fir_fft_cc -0.1 0 0.01 | csdr gain_ff 2.0 | csdr shift_addition_cc 0.2 | sudo rpitx -i- -m IQFLOAT -f 28400
#NFM:
nc -l 8011 | csdr convert_i16_f | csdr gain_ff 7000 | csdr convert_f_samplerf 20833 | sudo rpitx -i- -m RF -f 28400
#WFM:
nc -l 8011 | csdr convert_i16_f | csdr gain_ff 70000 | csdr convert_f_samplerf 20833 | sudo rpitx -i- -m RF -f 28400

The Raspberry Pi will listen on TCP port 8011, and wait for connection.

Then, on the remote computer, execute:

arecord -fS16_LE -r48000 -c1 - | nc raspberrypi.local 8011

You should replace raspberrypi.local with the IP address of the Raspberry Pi (unless avahi config is the default).

Using ADPCM codec to decrease network usage while streaming

Let's see an example for this on the NFM modulator. Execute on the Raspberry Pi:

nc -l 8011 | csdr decode_ima_adpcm_u8_i16 | csdr convert_i16_f | csdr gain_ff 7000 | csdr convert_f_samplerf 20833 | sudo rpitx -i- -m RF -f 28400

On the PC, execute this:

arecord -fS16_LE -r48000 -c1 - | csdr encode_ima_adpcm_i16_u8 | nc raspberrypi.local 8011

Transmitting amateur radio digital modes from remote computer

On the Raspberry Pi, run this to transmit USB:

nc -l 8011 | csdr convert_i16_f | csdr dsb_fc | csdr bandpass_fir_fft_cc 0 0.1 0.01 | csdr gain_ff 2.0 | csdr shift_addition_cc 0.2 | sudo rpitx -i- -m IQFLOAT -f 28400

On your PC, run:

arecord -fS16_LE -r48000 -c1 - | nc raspberrypi.local 8011

If you have Ubuntu on your PC, you are likely to have PulseAudio.

Run pavucontrol and set the recording source of arecord to "Monitor of" your audio output.

pavucontrol

On the PC, run fldigi, and start to transmit:

fldigi

If you don't want to listen to the transmitted signal on your PC:

sudo modprobe snd-aloop

This will create an ALSA loopback interface. You also have to make the appropriate changes in pavucontrol:

pavucontrol