nodem

A pure JavaScript library for encoding and decoding AX.25 packet radio frames as AFSK audio. Runs in browsers and Node.js with no build step and no dependencies.

AX.25 is the Layer 2 protocol used by amateur radio packet networks including APRS. nodem converts between AX.25 frame objects and Float32Array audio samples, making it straightforward to transmit and receive packet radio signals via a sound card or the Web Audio API.

Live Demo

axchat.kenbarbour.com is a browser-based packet radio chat application built with nodem. It encodes and decodes AX.25 frames in real time directly in the browser using the Web Audio API — no plugins, no install required.

Why nodem?

• Pure JavaScript — no native addons, no compile step, no dependencies
• Works everywhere — Node.js and browsers via the Web Audio API
• Fast — decoding speed comparable to Direwolf, which is written in C
• Accurate — decodes 911 of 1000 packets on TNC Test CD Track 2
• Tested — 100% test coverage

Installation

npm install @kenbarbour/nodem

Usage

import { encoder, decoder } from '@kenbarbour/nodem';

const config = {
  sampleRate: 44100,  // Hz
  baudRate:   1200,   // baud
  markFreq:   1200,   // Hz — binary 1
  spaceFreq:  2200,   // Hz — binary 0
  onPacket: (packet) => console.log(packet),
};

// Encoder: packet object → Float32Array of audio samples
const encode = encoder(config);
const samples = encode({
  source: { callsign: 'N0CALL', ssid: 0 },
  dest:   { callsign: 'APRS',   ssid: 0 },
  frame:  { type: 'U', subtype: 'UI' },
  data:   'Hello, World!',
});

// Decoder: stream Float32Array chunks → packets via onPacket callback
const decode = decoder(config);
decode(float32ArrayChunk);

Performance

Benchmarked against the TNC Test CD:

On the same hardware both run at roughly 85–88× realtime.

License

MIT

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