SDRPlayBatDetector

Hat tip to TechMinds for outlining the art of the possible here.

Now have more insight as to when these fascinating beasties visit my back garden.

Installation

Tested on Pi Debian Bookwork 64bit 2024-03-15.

I connected one of these to one of those for this.

Some experience will be needed with Linux, GnuRadio and a sensor / SDR receiver pair that works in the range 40 to 120 kHz or so.

sudo apt-get update upgrade
sudo apt-get install cmake gnuradio python3-paho-mqtt ffmpeg
pip3 install -r requirements.txt

Then follow the instructions here to install required APIs:

https://www.sdrplay.com/api/

https://github.com/fventuri/gr-sdrplay3

Design

This sketch uses an asistant python program called zmqpubsink.py which looks for bat activity as defined by the config file and

• Sends out frequency shifted audio to a remote location.
• Creates a csv file of events for further analysis and visualisation.
• Optionally sends events to your messaging server.

Configuration file

Edit the config file to adjust the following

Key	Notes
sample_rate	Suggest leaving, may need to subsequently tweak the decimation and interpolation values on the graph.
decimation	Default works well.
fft_resolution	Ditto.
fft_frame_rate	Ditto.
audio_conversion_gain	Ditto.
start_freq	Where to start looking for bats.
end_freq	If you need to increase this, you will need to increase the sample rate and then maybe tweak the decimation and interpolation values on the graph.
freq_bin_range	The fft is re-aggregrated based on this size to allow for more detailed but not over the top event generation.
trigger_gain_threshold	Spike in received power required to generate an event.
trigger_abs_threshold	So not use event if power is less than this
retrigger_seconds	Do not generate more than one event in this 'comparison against rolling average' interval.
bin_count_threshold	Must see activity over this number of bins to be a valid event
mqtt_ip_address	Optional messaging server.
mqtt_username	Ditto.
mqtt_password	Ditto.
mqtt_topic	Ditto.

Starting and stopping

bash start.sh

bash stop.sh

Example output

A real-time csv file is created with event information:

timestamp,timestamp-utc,start_freq,start_power,end_freq,end_power,number_of_bins_threshold_exceeded

pi@ShedPi:~/Documents/SDRPlayBatDetector $ tail -f SDRPlayBatDetector.csv 
2024-07-11 21:30:23,1720729823.81406,51592,-60,55732,-68,2
2024-07-11 21:30:28,1720729828.13749,55010,-60,75000,-73,4
2024-07-11 21:30:37,1720729837.46326,51836,-53,55000,-62,2
2024-07-11 21:32:28,1720729948.55296,53301,-51,97441,-74,4
2024-07-11 21:32:31,1720729951.10158,60615,-64,75977,-65,3
2024-07-11 21:32:38,1720729958.09749,51348,-41,95732,-68,6

Configuration

Edit the gnuradio sketch source block if you have a different source than the rspdx-r2.

For remote audio place you remote IP and port in the UDP sink block and run this at the remote end:

ffplay -f f32le -ar 25000 -fflags nobuffer -nodisp -i udp://127.0.0.1:50243 -af "volume=2.0"

Note that in the line above, 25000 comes from sample_rate/decimation in the ini file.

Notes

To visualise output a venv is required so not to mess up the dependancy sensitive GnuRadio. To set this up:

python -m venv venv
source venv/bin/activate
pip3 install plotnine
exit

Then to subsequently use and create events-by-timeofday.jpg and events-by-frequency.jpg:

bash ./visualise.sh

Enjoy!