NOTE: libmodem is a work in progress, the implementation is not yet complete

A(FSK) modem library for APRS and packet radio.

libmodem provides a reusable platform for experimenting with AFSK/FSK modulation and demodulation, and for building and evaluating new demodulators.

libmodem includes the components needed to implement a complete software modem: AX.25 and FX.25 bitstream encode/decode, an audio interface with support for WASAPI, ALSA, and Core Audio, I/O routines for communication over sockets and serial ports, and a collection of modulators and demodulators.

The library is designed to be modular: you can swap bitstream encoders, audio interfaces, modulators, and demodulators to configure different pipelines. Components are intentionally loosely coupled so pieces can be reused independently, down to small functions.

libmodem is cross platform and is supported natively on Windows, Linux and OSX. The build system and code is seamless to integrate on all platforms.

libmodem is fully testable, and is very extensivly tested, with thousands of lines of test code and over 50 tests, including FFTs for frequency accuracy, using Direwolf to test the modulator, and using the WA8LMF TNC Test CDs and Direwolf generated bitstreams.

A complete, end to end reference modem implementation is included, with support for 300, 1200, 2400, and 9600 bps operation and AX.25 plus FX.25 framing. The modem is configured as a composable pipeline: modulators and demodulators are wired to named audio streams, data streams, and PTT streams, and each stage can be swapped independently.

The configuration supports many to many routing. A single modulator can transmit to multiple audio outputs at the same time, for example a live sound card and a WAV renderer, while ingesting packets from multiple inputs such as TCP, serial, stdin, or built in test sources. Audio streams can be shared across pipelines and reused as many times as needed. The audio layer supports native devices (WASAPI and ALSA), TCP audio transport with separate audio and control ports, WAV input and output for repeatable testing, and null or synthetic audio sources for demos and validation.

On the data side, the modem can expose multiple interfaces concurrently, including multi client TCP servers, serial KISS, stdout, rotating log and JSON file streams, and an external dynamic library interface for custom integration. Output formats are selectable per stream and can be mixed across consumers, including KISS, TNC2 style text, JSON, AX.25 hex or binary, raw bitstream, and telemetry augmented variants where supported. This makes it easy to do workflows like writing decoded bitstreams to disk for analysis, streaming decoded frames to multiple clients, or generating synthetic traffic using random and repeat packet sources.

Transmit control is equally flexible. PTT can be driven over serial using RTS or DTR with configurable polarity, via GPIO on platforms like Raspberry Pi with optional pre and post delays, or through a plugin library so you can integrate with custom keying hardware. Modulator parameters such as mark and space frequencies, TX delay and tail, gain, preemphasis, and leading and trailing silence are all configurable, and the same base modulator can be cloned and specialized via inheritance for variants like a WAV only renderer or a multi output transmitter.

• Modulator
  • Support for AFSK and FSK modulation
  • Supported baud rates: 300, 1200, 2400, and 9600
  • Coherent modulation
  • Modulator interface enables swappable modulators and extensibility
• Audio interface
  • Support for native Windows, Linux and OSX audio: WASAPI, ALSA, and Core Audio
  • Sound over TCP/IP is supported for both capture/render with separate audio and control ports
    • Control port can be used to control the volume over a TCP interface
  • WAV file input and output streams
  • Null and synthetic audio sources for demos, validation, and stress testing
  • Modem can automatically control volume level
  • Extensibility allows for seamless integration of new audio interfaces into the modem
• PTT
  • Serial PTT via RTS or DTR with configurable polarity
  • GPIO PTT support with configurable pin numbering and optional pre and post delays on Raspberry Pi platforms
  • External plugin library PTT interface for custom keying hardware
    • Enables hamlib or PTT control over CAT
  • TCP PTT control
• Formatting
  • Supports AX.25 and FX.25 encoding and decoding
• Pipeline
  • Pluggable pipeline design where bitstream encoders, audio interfaces, modulators, and demodulators can be swapped independently
  • Data stream support for multiple concurrent interfaces
    • Multi client TCP servers with arbitrary number of clients
    • Serial interface
    • stdin and stdout data streams
    • Rotating log streams and rotating JSON file streams with size and flush policies
    • External dynamic library data stream interface for custom integrations
  • Multiple selectable data formats per stream
    • KISS, TNC2 text, APRS text, APRS JSON, AX.25 hex, AX.25 binary, raw bitstream, telemetry streams
    • Telemetry augmented variants for supported formats
• Configurable from a single JSON configuration file
  • JSON file support comments, and inheritance of configurations
• Cross platform support with native Windows, Linux, and macOS builds

• Modular library-first design for experimenting with AFSK and FSK modulation and demodulation and for building and evaluating new demodulators
• Readability, reuse and simplicity even at the cost of some code duplication
• Core building blocks for a complete software modem, including AX.25 and FX.25 bitstream encode and decode
• Full testability with test suite with 50 plus tests, including FFT based frequency checks and Direwolf and WA8LMF test vector based validation
• Complete end to end reference modem implementation with support for 300, 1200, 2400, and 9600 bps operation and AX.25 plus FX.25 framing
• State of the art demulator for 1200 bps AFSK based on the WA8LMF TNC Test CD benchmark

• bitstream.h/bitstream.cpp - Bitstream encoding and decoding including AX.25 and FX.25. Contains all the routines needed to encode an APRS packet or a AX.25 frame into a bistream, or decode a bitstream into a AX.25 frame or APRS packet.
  • Contains all bitstream manipulation routines including NRZI encoding/decoding, bit stuffing/unstuffing, CRC calculation and verification, and AX.25 address field encoding/decoding, FX.25 Reed-Solomon encoding/decoding.
  • Supports bit by bit processing for streaming operation.
  • libcorrect is used for Reed-Solomon coding.
  • Customizable packet type support, allowing the same packet type to be used among different libraries.
  • Has no coupling to other code in the project.