brabbelbox_rf

Implements the SDR part of brabbelbox project: Sticking gnuradio blocks together to do a simultaneous multi stream FM transmission.

System requirements

Gnuradio 3.8 (but should also run on 3.7)
osmocom compatible SDR, the code might be tuned for hackrf but should easily be portable

What does it do

Take N audio streams from a sound card (N being around 3..8)
Modulate them on adjustable frequencies in a 6..8 MHz band (even more if you throw enough computing power at it)

What still needs to be done

API to set frequencies
Care about low latency: Provide HackRF patches to lower buffer size
Use a gnuradio source that provides adaptive resampling (e.g. add zita-resampler to the gr-audio jack source)

How to use it?

When you open the gnuradio file polyphase_single.grc you have a not too complex flow graph:

variable aud_rate: Sets audio sampling rate (suggest to use 48 kHz everywhere)
variable mod_rate: Sets the sampling rate used for modulation (use at least 200 kHz for wide band FM), will also define the channel spacing
variable num_banks: Sets the number of available channels. mod_rate x num_banks sets the RF bandwidth that will be sent by your SDR. (Suggest to use 4..20 MHz with HackRF)
variable taps: fine tune low pass filter used in polyphase synthesizer. If you want to change something, you already know why and what :-)
variable taps_rs: fine tune low pass filter used in rational resampler (audio -> modulation resampler). Shouldn't need touching as well.
Audio source on the left: Multi channel audio source
resampler, WBFM transmit blocks: These out of the box gnuradio blocks perform really good and shouldn't need any modifications.
polyphase synthesizer: This block performs the magic of combining all the fm signals together.
- taps: The filter here is responsible to provide a clear channel separation without destroying the usable signal.
- Channel map: Define the relationship of the connected channels to the transmission frequencies. Channel 0 is transmitted on 0 Hz (e.g. center frequency), channel 1 on 0+mod_rate (e.g. 200 kHz), channel num_banks/2 is on both ends of the spectrum (so shouldn't be used) and num_banks/2+1 starts at the lower end, whereby num_banks-1 is on 0-mod_rate, e.g. -200 kHz.
osmocom sink: Set your SDR parameters here.
- hackrf likes to transmit a signal at least 2 MHz wide, better a bit wider
- use the device argument buffers=1 for a decreased latency
- set the frequency to your center frequency
- tune the transmission power with RF gain (0 or 14 dB TX amplifier) and IF Gain (0..42 dB in 1 dB increments)

Never ever connect your SDR to an antenna as this might be illegal in your country! Always use a dummy load or a direct connection to another SDR for receiving (make sure to use a low transmission power to not brick your second SDR).

Performance figures

30 banks, 3 channels use a bit more than 1 core of a Raspberry Pi 4B with full CPU load
- polyphase synthesizer uses ~75 % of one core
- each channel uses just under 10 % of one core additionally
- hack rf data transmission uses another 10 %