rpitx-app-note/README.md

Application note on using rpitx

Using with csdr to modulate streaming input

You will need the dev branch of csdr for doing this.
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.

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 is your file.

A raw audio file differs from a .wav file because it doesn't have any headers to store its parameters, just the samples after each other.

Generate AM modulation:

cat music48000.raw | csdr convert_i16_f | csdr gain_ff 1.0 | csdr dsb_fc | csdr add_dcoffset_cc | sudo rpitx -i- -m IQFLOAT -f 28400

• The part csdr gain_ff 1.0 can be changed to increase/decrease modulation.
• The -f 28400 at the end gives the transmit frequency in kHz.

Generate USB modulation:

cat speech48000.raw | 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:

cat speech48000.raw | 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

• It's the matter of the filter which sideband do we select:
  • csdr bandpass_fir_fft_cc -0.1 0 is the lower sideband
  • csdr bandpass_fir_fft_cc 0 0.1 is the upper sideband
• 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

Generate NFM modulation:

cat music48000.raw | csdr convert_i16_f | csdr gain_ff 7000 | csdr convert_f_samplerf 20833 | sudo rpitx -i- -m RF -f 28400