AudioMoth

Project AudioMoth
Status	In progress
Contact	bertrik
Last Update	2020-04-14

What

I have an AudioMoth board, and plan to improve its software:

• https://www.openacousticdevices.info/
• https://github.com/OpenAcousticDevices/AudioMoth-Project source code

Ideas:

• general clean-up
  • fix white space, remove many of the empty lines (earlier CR/LF mistake?), like these https://github.com/bertrik/AudioMoth-Firmware-Basic/blob/master/main.c#L154
  • fix sprintf statements, like these https://github.com/bertrik/AudioMoth-Firmware-Basic/blob/master/main.c#L167
  • fix complicated and non-portable casts (sensitive to endianness), like this https://github.com/bertrik/AudioMoth-Firmware-Basic/blob/master/main.c#L161
• update tool-chain to newer version
  • update gcc
  • attempt to make it possible to compile with platformio
  • document it and upstream it when it works
• extend functions
  • add trigger functionality, maybe a specific bat trigger (high pass filter)?
  • crest (instantaneous vs average) trigger?
  • save files in sub-directories (per day) instead of a flat filesystem?

Hardware

Frequency response

See https://circuithub.com/projects/OpenAcoustics/AudioMoth

The microphone is a SPM0408LE5H. Can't find much about its ultrasonic performance. The official graph shows a hint of resonance above 10k so it does have ultrasonic sensitivity. For frequencies higher than the resonance peak the response of a typical microphone drops off steeply with 6 dB/octave. I have been able to record some pipistrelle bats with it (40 - 50 kHz).

The processor is EFM32WG380F256, link

Software

My fork on github: https://github.com/bertrik/AudioMoth-Project

Suggested trigger extension

A bat specific trigger is desireable because this way we only get "interesting" files.

The idea is that bat signals are, in general:

• ultrasonic, frequency above 10 kHz
• organised in short pulses, e.g. 8 pulses per second of 5 ms each for a pipistrelle

Challenges:

• crickets, sound is also at least partly ultrasonic, overlapping with the calls of some of the larger bats and with social bat calls in general

Proposed algorithm: 1) Apply a high-pass filter at about 10 kHz, this needs not be super-sharp filter. Perhaps just a simple 1-pole IIR (easy to calculate). 2) Calculate the energy in each millisecond by adding the squared amplitude of the high-pass filtered signal over this millisecond. Perhaps we can also just calculate variance here and forget about step 1). 3) Calculate a running average energy over several (say 500) of these milliseconds values. 4) If the millisecond energy exceeds the average energy by a factor X (the sensitivity of the trigger), consider the trigger to be active.

Running the tools

Running the AudioMoth tools (configuration tool, flash tool, etc) requires the --no-sandbox option (at least on my system), for example

 ./AudioMothConfigurationAppSetup1.2.5.AppImage --no-sandbox

IDE for development

A description of all steps to get the firmware compiled is written here: https://github.com/OpenAcousticDevices/AudioMoth-Project/wiki/AudioMoth

Compiling

platformio

It compiles and produces a firmware.bin file, but it's not recognized by the flashing tool as a valid file.

Use this platformio.ini file:

 [platformio]
 src_dir = src,fatfs_src
 include_dir = inc,fatfs_inc
 [env:default]
 framework = zephyr
 platform = siliconlabsefm32
 board = efm32wg_stk3800

Install platformio:

 sudo apt install python3-pip
 sudo pip3 install platformio
 sudo pip3 install -U PyYAML

Compile it:

 pio run