HydroModem - Underwater Bell 103 modem implementation for low-power microcontrollers

HydroModem is a Bell 103 modem implementation to decode data from an always-on underwater hydrophone. Designed for low-power microcontrollers such as AVR ATmega328P, it can listen underwater long-term and implement the reliable trigger for an acoustic release in ocean research sensor prototypes.

Overview

This is an implementation of a Bell 103 modem decoder for Arduino devices with low-power microcontrollers such as ATmega328P. It uses a simple Goertzel algorithm to detect the power of two frequencies, compares them to determine mark-space bits, and then decodes these into 8-bit characters. This method can handle background noise and is used here to implement an acoustic underwater data transmit/receive mechanism. Low-power devices can be permanently placed underwater, listening for a wakeup message, and then take some kind of action like burn a nichrome wire for an acoustic release. It is expected that the modem will run on its own ATmega328P since there is no time remaining, and a co-processor like another ATmega328P will listen to the decoded characters and take action. HydroModem is an easy-to-use component that can be used for building low-cost devices to support research in the world's oceans.

This implementation supports a 2600 Hz sample rate on an ATmega328P at 16 MHz. Transmission is with sine wave tones at 1070 Hz and 1270 Hz. The sample rate must be at least double the highest frequency (Nyquist theorem) and during testing at low signal strength some sample rates were found to be more reliable than others due to aliasing. This implementation supports up to 3300 Hz, but beyond that requires a higher clock rate on the microcontroller, possibly using something like an ESP32S3. The input is read in via the Arduino ADC, which needs to be able to capture samples at the frequency required.

The ATmega328P must be run at 16 MHz for 2600 Hz sample rate, requires at least 4.0V for this clock speed according to the datasheet, and will draw 7 mA current or 0.028 W power. This is 1-2 orders of magnitude better than a Raspberry Pi running Linux, and capable of running for weeks/months on a battery underwater.

Sample audio

Sample audio generated by the modem tests, with image and video spectrogram of the FSK frequencies in use:

Arduino Uno 16MHz on Linux command-line

The modemrx.ino project can be built on the command-line and also loaded into the Arduino IDE on any supported platform.

# Download arduino-cli (run once)
./download-arduino-tools

# Download arduino libraries (run once)
./download-arduino-libs

# Build and upload the modemrx project via USB serial
make -C modemrx upload

# Read characters decoded from the modemrx project
make -C modemrx read

Listen and record audio on Linux via Arduino ADC

The recordwav.ino project allows you to listen to or record the audio at the ADC, making debugging with Linux tools easier.

# Download arduino-cli (run once)
./download-arduino-tools

# Download arduino libraries (run once)
./download-arduino-libs

# Build and upload the recordwav project via USB serial to record from the ADC
make -C recordwav upload

# Save audio to disk for offline testing, adjust gain and center for SparkFun SPH8878 microphone
./recordwav/save_arduino_sparkfun_sph8878.sh

# Listen to live audio on speakers, adjust audio gain and center for SparkFun SPH8878 microphone
./recordwav/live_arduino_sparkfun_sph8878.sh

Linux testing of modem

The modem code also builds on Linux with the ability to encode to and decode from WAV files. Automated regression tests are provided to ensure that different sample rates all work after changes are made. Tests also verify that HydroModem's Bell 103 encoding is compatible with MiniModem http://www.whence.com/minimodem/ on Linux.

# Download Debian/Ubuntu dependencies (run once)
./linux-test/apt-get-deps

# Build Linux binaries
make -C linux-test

# Generate WAV files using test strings
./linux-test/bell103_goertzel_wav_encoder

# Decode WAV files and recover test strings
./linux-test/bell103_goertzel_wav_decoder

# Run test suite that verifies encoding and decoding at various sample rates.
make -C linux-test test

Connecting a microphone or hydrophone

This is currently the software needed to read audio from the ADC pin on an Arduino Uno. Currently it is easiest to test by connecting a microphone like the SparkFun SPH8878 and testing it in air listening to the sample audio from a phone transmitter. Once this is working, you then need to connect a hydrophone to the ADC pin. The design of a suitable hydrophone and how to connect it is future work. Make sure that your microphone does not exceed the voltage specs of the Arduino ADC.

Future work is to provide a circuit schematic and PCB layout for a custom dual ATmega328P design with input from a piezo microphone. The modem also needs to be tested in water and frequencies tuned for best performance.

Source code

The HydroModem source code is available on GitHub under GPLv3 at https://github.com/waynepiekarski/hydro-modem

GPLv3 License

HydroModem: Underwater Bell 103 modem implementation for low-power microcontrollers
Copyright (C) 2024-2025 Wayne Piekarski wayne@tinmith.net
https://waynepiekarski.net/projects/hydro-modem.htm

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program.  If not, see http://www.gnu.org/licenses/

Generate modem audio samples

The source code includes a HTML5 sample web-bell103-encoder.htm to generate audio. It is also available here to use on a mobile phone and can transmit the audio via Bluetooth or a headphone jack to a hydrophone transmitter underwater.

Modem audio generator

Log