kopia lustrzana https://github.com/ha7ilm/rpitx-app-note
240 wiersze
11 KiB
Markdown
240 wiersze
11 KiB
Markdown
# Application note on using rpitx
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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.
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This document is about using [GNU Radio](#using-a-raspberry-pi-as-remote-software-defined-radio-peripheral-from-gnu-radio) and [csdr](#using-rpitx-with-csdr-to-modulate-streaming-input) with *rpitx*.
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**Table of contents**
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* [Using a Raspberry Pi as remote software defined radio peripheral from GNU Radio](#using-a-raspberry-pi-as-remote-software-defined-radio-peripheral-from-gnu-radio)
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* [NFM modulator example for GNU Radio](#nfm-modulator-example-for-gnu-radio)
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* [SSB modulator example for GNU Radio](#ssb-modulator-example-for-gnu-radio)
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* [AM modulator example for GNU Radio](#am-modulator-example-for-gnu-radio)
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* [Using rpitx with csdr to modulate streaming input](#using-rpitx-with-csdr-to-modulate-streaming-input)
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* [Modulate from raw audio file](#modulate-from-raw-audio-file)
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* [Modulate from microphone input source](#modulate-from-microphone-input-source)
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* [Modulate from audio streamed from remote computer](#modulate-from-audio-streamed-from-remote-computer)
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* [Using ADPCM codec to decrease network usage while streaming](#using-adpcm-codec-to-decrease-network-usage-while-streaming)
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* [Transmitting amateur radio digital modes from remote computer](#transmitting-amateur-radio-digital-modes-from-remote-computer)
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<br /><br />
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![Warning](/images/bpf-warning.png?raw=true)
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<br /><br />
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The output of the Raspberry Pi GPIO 18 is a square wave, which contains harmonics.<br />
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For example, if you transmit on 28 MHz, you will also transmit on:
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* 28 × 3 = 84 MHz
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* 28 × 5 = 140 MHz
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* 28 × 7 = 196 MHz<br/>...
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...and actually you may distrub some important radio communication services. Please don't do this! You can get into trouble! You have been warned.
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In addition to the squre wave output, the spectrum is also affected by the PWM-based amplitude modulation implemented in *rpitx*.
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* Right now it is more useful to use these examples for e.g. demonstrations on SDR transmitters<br />(as most of them use the PWM mode).
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* If you want to connect this to a real antenna, you should use a **very good band-pass filter**!
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* Please check your signal output on a proper spectrum analyzer before ever using this on the air!
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* In addition, only transmit if you have a proper license to do so.
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## Using a Raspberry Pi as remote software defined radio peripheral from GNU Radio
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To do this, you will need:
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* a PC running GNU Radio on Linux,
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* a Raspberry Pi with *rpitx* installed,
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* both connected to the same network.
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We will stream the I/Q signal from the PC to the Raspberry Pi via TCP.
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On the Raspberry Pi, execute:
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nc -l 8011 | sudo rpitx -i- -m IQFLOAT -f 28400
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* This will listen on TCP port 8011 for the I/Q signal.
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* The center frequency of the transmitter will be 28400 kHz.
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If you want it to restart everytime the connection is lost, use this command:
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while true; do (nc -l 8011; dd if=/dev/zero bs=4096 count=30); done | sudo rpitx -i- -m IQFLOAT -f 28400
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### NFM modulator example for GNU Radio
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On the PC, open GNU Radio Companion, and load the flow graph in this repo at `gnuradio/nfm-rpitx`.
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![NFM in GNU Radio](/images/grc-nfm.png?raw=true)
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If you execute it, the I/Q signal will be streamed to the Raspberry Pi through the TCP socket.
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* Note that the frequency translation is there on intention. On some unkown reason, the spectrum is quite distorted if our signal is centered at DC.
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Here is the good result if received with an RTL-SDR and GQRX:
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![NFM in GQRX](/images/gqrx-nfm.png?raw=true)
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### SSB modulator example for GNU Radio
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Use the flow graph in `gnuradio/ssb-rpitx`.
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This will transmit an **USB** signal.
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![SSB in GNU Radio](/images/grc-ssb.png?raw=true)
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The spectrum of the transmitted I/Q signal looks like this:
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![SSB in GNU Radio](/images/grc-ssb-running.png?raw=true)
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After receiving it with RTL-SDR and GQRX:
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![SSB in GQRX](/images/gqrx-ssb.png?raw=true)
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If you want **LSB**, you should double-click the "Band Pass Filter" block, and change:
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* *Low Cutoff Freq* to -3000
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* *High Cutoff Freq* to -300
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![SSB in GQRX](/images/grc-ssb-bpf.png?raw=true)
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### AM modulator example for GNU Radio
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Use the flow graph in `gnuradio/am-rpitx`.
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This will transmit an **AM** signal.
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* One thing that can be screwed up is **gain**. I had to play with the AGC to get it right.
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![AM in GQRX](/images/grc-am.png?raw=true)
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The spectrum of the transmitted I/Q signal looks like this:
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![AM in GQRX](/images/grc-am-running.png?raw=true)
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After receiving it with RTL-SDR and GQRX:
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![AM in GQRX](/images/gqrx-am.png?raw=true)
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## Using rpitx with csdr to modulate streaming input
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<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.
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You will need the *dev* branch of *csdr* for doing this.<br />
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Setup instructions:
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git clone https://github.com/simonyiszk/csdr.git
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cd csdr
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git fetch
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git checkout dev
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make && sudo make install
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Note that it should be already done if you installed *qtcsdr* previously.
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### Modulate from raw audio file
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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:
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git clone https://github.com/ha7ilm/rpitx-app-note
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cd rpitx-app-note; ls #There are your files.
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We will play these files in a loop, you can stop it with Ctrl+C.
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**Generate AM modulation:**
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(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
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* The part `csdr gain_ff 1.0` can be changed to increase/decrease modulation.
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* The `-f 28400` at the end gives the transmit frequency in kHz.
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**Generate USB modulation:**
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(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
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**Generate LSB modulation:**
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(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
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* It's the matter of the filter which sideband do we select:
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* `csdr bandpass_fir_fft_cc -0.1 0` is the lower sideband
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* `csdr bandpass_fir_fft_cc 0 0.1` is the upper sideband
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* I have experienced that if the SSB signal is in the center of the I/Q signal, then it is not transmitted correctly. The solution was to shift it. So the exact frequency should be around: `rpitx frequency + 48000*0.2 = rpitx frequency + 9600 Hz`
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* If you want to get some compression on the speech, you can try `csdr fastagc_ff` instead of `csdr gain_ff 2.0`.
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**Generate NFM modulation:**
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(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
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**Generate WFM modulation:**
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(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
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* Frequency deviation is set much higher with `csdr gain_ff 70000`, that's the only difference between this and NFM.
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### Modulate from microphone input source
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Use this if you have a microphone connected to the Raspberry Pi via external USB audio device.<br />
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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`.
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#AM:
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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
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#USB:
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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
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#LSB:
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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
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#NFM:
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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
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#WFM:
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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
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### Modulate from audio streamed from remote computer
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First, start the transmitter:
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#AM:
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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
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#USB:
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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
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#LSB:
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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
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#NFM:
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nc -l 8011 | csdr convert_i16_f | csdr gain_ff 7000 | csdr convert_f_samplerf 20833 | sudo rpitx -i- -m RF -f 28400
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#WFM:
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nc -l 8011 | csdr convert_i16_f | csdr gain_ff 70000 | csdr convert_f_samplerf 20833 | sudo rpitx -i- -m RF -f 28400
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The Raspberry Pi will listen on TCP port 8011, and wait for connection.
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Then, on the remote computer, execute:
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arecord -fS16_LE -r48000 -c1 - | nc raspberrypi.local 8011
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You should replace `raspberrypi.local` with the IP address of the Raspberry Pi (unless avahi config is the default).
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### Using ADPCM codec to decrease network usage while streaming
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Let's see an example for this on the NFM modulator. Execute on the Raspberry Pi:
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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
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On the PC, execute this:
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arecord -fS16_LE -r48000 -c1 - | csdr encode_ima_adpcm_i16_u8 | nc raspberrypi.local 8011
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### Transmitting amateur radio digital modes from remote computer
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On the Raspberry Pi, run this to transmit USB:
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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
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On your PC, run:
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arecord -fS16_LE -r48000 -c1 - | nc raspberrypi.local 8011
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If you have Ubuntu on your PC, you are likely to have PulseAudio.
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Run `pavucontrol` and set the recording source of `arecord` to "Monitor of" your audio output.
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![pavucontrol](/images/pavucontrol.png?raw=true)
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On the PC, run `fldigi`, and start to transmit:
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![fldigi](/images/fldigi.png?raw=true)
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If you don't want to listen to the transmitted signal on your PC:
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sudo modprobe snd-aloop
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This will create an ALSA loopback interface. You also have to make the appropriate changes in `pavucontrol`:
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![pavucontrol](/images/pavucontrol-loop.png?raw=true)
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