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TinyTUS - Tiny Transmission Using Sound

Small, modular C99 library for transmitting digital data over acoustic channels using FSK modulation. Targets desktop (Windows, Linux), WebAssembly, and embedded platforms.

Features

• GFSK modulation - configurable bits-per-symbol (1, 2, 4, 8), frequency range, sample rate and symbol rate
• Reed-Solomon ECC - configurable error correction percentage per frame
• DSS scrambling - optional data scrambling before encoding
• AGC - automatic gain control on the receive path
• Sync detection - marker-based start/end-of-frame detection
• Builds as an executable, static lib, DLL, or WASM module
• PortAudio, libusb and the WAV helper are all optional and conditionally compiled

Live Demo

Since this library is compilable also for the web (using WebAssembly), I have built a small static website which shows it's capabilities. You can check it out here: https://servacek.github.io/audiomodem/

Custom hardware

In order to use an radio transceiver for digital communication using this library a special piece of hardware has to be made, which enables the computer to control the PTT circuit of the radio. Here is the schematic of such hardware, I have designed and used during testing with my Allamat 47 handhold CB radios.

Note that the value of the resistor R4 depends on the specific radio type used. Or in case you need flexibility, it can be replaced with a suitable trimmer.

Building

Requirements: clang, llvm-lib (Windows), make. PortAudio and libusb prebuilt libraries are included under libs/.

make run    # build and run the main executable
make lib    # build as static library  -> bin/tinytus/lib/tinytus.lib
make dll    # build as DLL             -> bin/tinytus.dll
make web    # build for WebAssembly (requires emsdk, set EMSDK_PATH in Makefile)

Compiler flags can be overridden via dev.config in the repo root (gitignored):

# dev.config
CFLAGS = -O0 -g

Tests

make tests          # build and run all tests
make test <name>    # build and run one test,  e.g.  make test gfsk
make clean          # remove all build artifacts

Each test is compiled from tests/test_.c against the full source tree. Output goes directly to stdout/stderr.

Available tests:

make test gfsk           - full modulate/demodulate round-trip
make test ber            - BER at various SNR levels
make test fsk            - basic FSK / Goertzel demodulation
make test hamming        - Hamming ECC encode/decode
make test frame          - frame packing and unpacking
make test goertzel       - Goertzel frequency detection
make test dss            - data scrambling
make test sync_detector  - sync word detection
make test wav            - WAV file I/O
make test tlv            - modem profile TLV encode/decode
make test modem_profile  - default profile singleton and field checks

New test files dropped into tests/test_*.c are picked up automatically. See tests/helper.h for ok()/fail() helpers, ASSERT macros and section headers.

Quick Example

#include <tinytus.h>

void on_received(const uint8_t *bytes, uint8_t n) {
    for (uint8_t i = 0; i < n; i++) putchar(bytes[i]);
    putchar('\n');
}

int main(void) {
    start_listening(NULL, on_received);
    send_payload(NULL, (const uint8_t *)"Hello!", 6);
    while (1) {}
    stop_listening();
    return 0;
}

A more complete example is in examples/simplechat.c.

Configuration

Compile-time knobs are in src/const.h:

SAMPLE_BIT_DEPTH          16      sample depth (8 / 16 / 32)
SQUELCH_THRESHOLD         0.01    minimum signal power to attempt decode
MARKER_DETECTION_THRESHOLD 0.4    marker confidence threshold
LIB_PORTAUDIO_ENABLED     1       set to 0 to drop PortAudio
LIB_LIBUSB_ENABLED        1       set to 0 to drop libusb
LIB_WAV_ENABLED           1       set to 0 to drop the WAV helper

Platform Notes

• Windows - primary target; clang + MSVC CRT linkage
• Linux - builds with the same Makefile; linker flags may need adjustment
• WASM - set EMSDK_PATH in Makefile and run make web; PortAudio and libusb are disabled automatically