Lightweight Java library for Bell 202 / AFSK-1200 demodulation. Designed to run on desktop JVMs and Android (no native deps).

Most AFSK/AX.25 stacks are tied to native code or heavyweight DSP toolkits. afsk-java is pure-Java, fast enough for phones, and easy to embed in apps, services, and tests.

• 🎧 Demodulation pipeline: band-limit ➜ decimate ➜ complex/quadrature FM discriminator ➜ soft-slicer PLL ➜ NRZI/HDLC ➜ AX.25 frames
• 🧪 Optional CRC bit-flip recovery (1–2 bit search) — off by default; toggleable via constructor flag
• 🎚️ Works at arbitrary sample rates (you pass the rate); accepts mono float samples in $-1..1$
• 📦 Pure Java (no JNI), small footprint, Android-friendly
• 🔧 Pluggable callbacks for decoded frames; easy to wire to audio, WAV, or SDR frontends
• 🧰 Utilities for test vectors and WAV decoding (see src/test)

Artifact coordinates: io.github.dkaukov:afsk-java (available on Maven Central). (mvnrepository.com)

Gradle (Kotlin DSL)

dependencies {
  implementation("io.github.dkaukov:afsk-java:1.9")
}

Maven

<dependency>
  <groupId>io.github.dkaukov</groupId>
  <artifactId>afsk-java</artifactId>
  <version>1.9</version>
</dependency>

(See Maven Central for the latest version.) (mvnrepository.com)

import io.github.dkaukov.afsk.Afsk1200Demodulator;
import io.github.dkaukov.afsk.Ax25Frame;
import java.nio.FloatBuffer;

public class Demo {
  public static void main(String[] args) {
    final int sampleRate = 48000;        // use your real input rate
    final boolean enableCrcBitRepair = false; // optional; defaults may differ

    Afsk1200Demodulator demod = new Afsk1200Demodulator(sampleRate, enableCrcBitRepair,
        frame -> {
          // Called for each good AX.25 frame
          System.out.println(frame.toString());  // pretty print or hex dump
        },
        err -> {
          // Optional: receive detailed decode stats/errors if you want
          // e.g., symbol clock drift, CRC failures, recovered bits, etc.
        });

    // Feed mono float samples [-1..1] as they arrive:
    while (hasMoreAudio()) {
      FloatBuffer chunk = nextAudioChunk();
      demod.addSamples(chunk);
    }

    demod.close();
  }
}

try (WavReader wav = WavReader.open("input.wav")) {      // any simple WAV reader
  Afsk1200Demodulator demod = new Afsk1200Demodulator(
      wav.getSampleRate(), /*enableCrcBitRepair=*/false,
      (Ax25Frame f) -> System.out.println("AX25: " + f),
      null
  );

  float[] buf = new float[4096];
  int n;
  while ((n = wav.read(buf)) > 0) {
    demod.addSamples(buf, 0, n);
  }
  demod.close();
}

Notes

• The demodulator is sample-rate agnostic; pass the actual rate used by your audio source.
• Input must be mono. If you have stereo, mix down ((L+R)/2).
• For Android, capture audio via AudioRecord and feed the float buffer directly.

Key types (names shown for orientation; see Javadoc):

• Afsk1200Demodulator — high-level façade that handles the full AFSK→AX.25 pipeline
• Afsk1200Modulator — modulator

The API is evolving; minor changes may occur between releases. Check the Javadoc and tests for current usage patterns.

• Prefer 48 kHz or 44.1 kHz capture. Lower rates work; higher rates add CPU cost with little benefit.
• Keep audio levels conservative (avoid clipping). AFSK hates hard limiting.
• If you see frequent CRC fails but clear eye-diagrams, try enabling the CRC bit-flip search (at the cost of a little CPU).

• Android: wire AudioRecord ➜ Afsk1200Demodulator ➜ app UI/DB
• Desktop: WAV/ALSA/CoreAudio capture ➜ demod ➜ print/IGate/TNC-KISS bridge
• Testing: run demod over FLAC/WAV vectors in CI to catch regressions

• Optional AFSK modulator (for loopback tests)
• Soft-decision output for external Viterbi/EC experiments
• Tools for AFC and pre/de-emphasis compensation

PRs and issues welcome! Please include:

• Short description and a failing test (if bug)
• Before/after CPU and decode stats (if perf/quality)
• Audio snippets (if RF/decoding issue)

GNU GPL-3.0. See LICENSE for details.