📡 Signal Decoder

Decode radio signals in your browser! Free, open-source web application for real-time decoding of RTTY (Radio Teletype / Baudot), CW (Morse code), and SSTV (Slow Scan Television) signals from microphone input. Works offline as a PWA. Forked from smolgroot/sstv-decoder.

🚀 Try the Live Demo - No Installation Required!

Works on Desktop • Mobile • Tablet | Chrome • Firefox • Safari • Edge

Quick Start

Visit → acesso.github.io/rtty-decoder
Allow microphone access when prompted
Play an RTTY, CW, or SSTV signal near your microphone
Watch the text or image decode in real-time!

No installation, no downloads, no setup - just open and decode!

Features

RTTY Decoding: Real-time Baudot/ITA2 radioteletype decoding from audio input
CW Decoding: Morse code decoder with automatic speed detection
SSTV Decoding: 15 modes — Robot36/72, Scottie S1/S2/DX, Martin M1/M2, PD50/90/120/160/180/240/290, Wraase SC2-180
Mode Auto-Detection: Automatically identifies the incoming signal type
Session Gallery: Review and save decoded sessions
Real-time Audio Processing: Captures microphone input using Web Audio API (auto-detects 44.1 kHz or 48 kHz)
Professional DSP Chain:
- FM demodulation with complex baseband conversion
- Kaiser-windowed FIR lowpass filtering
- Schmitt trigger sync detection
- Bidirectional exponential moving average filtering
Live Display: Progressive rendering with real-time spectrum visualization
Save Output: Export decoded images as PNG or text sessions
Signal Analysis: Real-time spectrum analyzer and signal strength indicator
Mobile-Responsive: Optimized for both desktop and mobile devices

Supported SSTV Modes

All 15 modes are fully implemented with VIS code auto-detection.

Mode	Resolution	Sync	Line Time	Total Time	VIS
Robot36	320×240	9ms	~150ms	~36s	8
Robot72	320×240	9ms	~300ms	~1m 12s	12
Scottie S2	320×256	9ms	~278ms	~1m 11s	56
Martin M2	320×256	5ms	~226ms	~58s	40
PD50	320×256	20ms	~406ms	~1m 44s	93
Scottie S1	320×256	9ms	~428ms	~1m 50s	60
Martin M1	320×256	5ms	~446ms	~1m 54s	44
PD120	640×496	20ms	~508ms	~2m 6s	95
PD160	512×400	20ms	~804ms	~2m 41s	98
PD180	640×496	20ms	~752ms	~3m 6s	96
PD90	320×256	20ms	~754ms	~3m 13s	99
Wraase SC2-180	320×256	5ms	~734ms	~3m 8s	55
Scottie DX	320×256	9ms	~1069ms	~4m 34s	76
PD240	640×496	20ms	~1018ms	~4m 13s	97
PD290	800×616	20ms	~954ms	~4m 54s	94

Color Encoding Overview

Family	Color Format	Notes
Robot36/72	Interlaced YUV	Standard for QSOs and ISS-style events
Scottie S1/S2/DX, Martin M1/M2	Sequential RGB/GBR	Clean colors, HF classic
Wraase SC2-180	Sequential RGB	High fidelity RGB
PD50/90/120/160/180/240/290	Dual-luma YUV	High resolution, ISS SSTV standard

RTTY Decoder

Real-time RTTY (Radio Teletype) decoding using Baudot/ITA2 encoding (5-bit characters).

RTTY Parameters

Parameter	Default	Range
Center Frequency	500 Hz	configurable
Carrier Shift	450 Hz	170 / 450 / 850 Hz typical
Baud Rate	50 baud	45 / 50 / 75 / 100 / 110
Stop Bits	1.5	1 / 1.5 / 2
Parity	None	none / even / odd
Reverse Shift	Off	for LSB/inverted signals

RTTY Signal Processing

Mark/Space detection: Dual Goertzel correlators with IIR lowpass smoothing
Baud clock: Edge-triggered (re-syncs on every mark→space or space→mark transition)
Character framing: Start bit → 5 data bits → optional parity → stop bit(s)
Shift state: Full LTRS/FIGS handling per ITA2 standard
Sessions: Each decoded transmission is stored as a named session

CW Decoder

Real-time CW (Morse code) decoder with a per-sample IIR pipeline, live Morse element visualisation, and dual-channel A/B decoding mode.

CW Parameters

Parameter	Default	Range	Notes
Center Frequency	700 Hz	100 – 1500 Hz	Quick-set input + spectrum drag
Bandwidth	90 Hz	30 – 500 Hz	Filter width; Q computed as `freq / bw`
Speed	20 WPM	3 – 70 WPM	Manual or adaptive (see below)
Squelch	Adjustable	0 – 100 %	Compared directly to FFT tone-bin level
A/B Mode	Off	—	Two independent decoders on separate frequencies

CW Signal Processing Pipeline

microphone → biquad bandpass (adjustable Q = center / bandwidth)
           → envelope detector  (fast attack ~3 ms, moderate release ~5 ms)
           → peak follower       (fast rise ~10 ms, slow fall ~300 ms)
           → 50 % / 25 % hysteresis vs peak → mark / space FSM
           → symbol buffer (max 6 elements — hard cap, overflow emits '?')
           → Morse table lookup → character / prosign output

Bandpass filter: Standard biquad, Q computed per-render so bandwidth_Hz stays constant as the center frequency is dragged
Squelch: Applied per audio buffer by comparing the FFT magnitude at the tone bin against the user's visual threshold — the squelch line on the spectrum canvas is the exact gate the decoder uses
Adaptive speed tracker: Always runs in the background regardless of manual/adaptive mode; the estimated WPM is shown as a live suggestion even when manual mode is active
Manual WPM override: When adaptive mode is off, the user sets a fixed WPM; toggling adaptive off pre-fills the input with the last detected speed
6-element cap: No valid Morse character exceeds 6 elements (the longest prosigns, e.g. <SK> = ...-.-, are exactly 6); any sequence longer than this is flushed immediately as ? to prevent noise from stalling the decoder
SNR display: Noise floor tracked via a separate slow-fall IIR; real-time dB readout colour-coded (green ≥ 15 dB, amber 6–15 dB, red < 6 dB)

Morse Display (live visualiser)

A real-time Morse element display sits inside the Audio Analysis panel and reflects the decoder's internal symbol buffer directly:

Dots (blue circles) and dashes (green pills) appear with a spring-bounce animation as each element is received
Receiving indicator: An amber pulsing dot shows while a tone is actively being measured (before dot vs dash is determined)
Character flash: When the decoder resolves a complete symbol, the decoded character blooms large in the centre, holds for ~1.4 s, then fades — colour and glow match the channel (blue for Ch A, orange for Ch B)
Recent strip: The last 10 decoded characters are shown in a fading history row with their Morse pattern underneath
Element display is driven by stats.partialSymbol (decoder source of truth) to avoid React render-batch ordering bugs

A/B Mode (dual-channel)

Enable A/B Mode to run two independent CW decoders simultaneously on different frequencies — useful for monitoring both sides of a QSO or two nearby stations.

Each channel (A = blue, B = orange) has its own center frequency, squelch gate, and Morse visualiser
Decoded text from both channels is interleaved in the output panel with distinct colours
Channel B can be enabled/disabled mid-session without restarting audio capture
The spectrum shows labelled A and B markers; both can be dragged independently
The spectrogram overlay highlights both filter bands simultaneously

CW How to Use

Click Start Decoding and allow microphone access
Drag the CF (or A) marker on the spectrum to the CW tone peak, or type the frequency in the Center input
Adjust Bandwidth — narrow (50–80 Hz) for clean signals, wider (150–300 Hz) for drifting or noisy ones
Drag the SQL line just above the noise floor to gate noise from decoding
Set speed: leave adaptive off and type the known WPM, or enable Adaptive WPM to let the decoder track the sender automatically — the live detected WPM is always visible as a suggestion even in manual mode
For a two-station QSO, enable A/B Mode and drag the B marker to the second tone

Technology Stack

Next.js 15: React framework with App Router
TypeScript: Type-safe development
Web Audio API: Real-time audio capture and processing
- ScriptProcessorNode (Chrome, Firefox, Edge)
- requestAnimationFrame polling (Safari, iOS)
Canvas API: Progressive image rendering
Tailwind CSS: Utility-first styling

Testing

The project includes comprehensive unit tests for the core SSTV decoding algorithms.

Running Tests

# Run all tests
npm test

# Run tests in watch mode (for development)
npm run test:watch

# Run tests with coverage report
npm run test:coverage

Test Framework

Jest - Test runner with TypeScript support
@testing-library/react - React component testing utilities
jsdom - DOM environment for tests

Getting Started

Prerequisites

Node.js 18+ installed
A modern web browser with microphone access

Installation

Clone the repository:

git clone https://github.com/acesso/rtty-decoder.git
cd rtty-decoder

Install dependencies:

npm install

Run the development server:

npm run dev

Open http://localhost:3000 in your browser

Building for Production

npm run build
npm start

How to Use

Select a mode from the top tab bar (RTTY / CW / SSTV), then click Start and allow microphone access when prompted.

RTTY

Configure: Set center frequency, carrier shift, and baud rate to match the incoming signal (defaults work for most HF RTTY: 50 baud / 450 Hz shift / 500 Hz center)
Start: Click "Start" — the decoder locks on to the first valid start bit it finds
Read text: Decoded characters appear in the session panel in real-time
Sessions: Each received text block is saved as a named session; switch between them in the session list
Reverse shift: Toggle "Rev" if the mark/space tones appear inverted (common when receiving on LSB)

CW

Tune: Adjust the tone frequency slider to match the CW note you want to decode (700 Hz is a common CW sidetone)
Squelch: Raise the squelch threshold until background noise stops producing output, then lower it until the signal decodes cleanly
Start: Click "Start" — decoded characters appear as the decoder tracks speed automatically
Speed: WPM is calculated adaptively; no manual entry needed

SSTV

Mode: Leave mode on Auto to let VIS code detection select the mode automatically, or pick a specific mode from the selector
Start: Click "Start Decoding" to begin capturing from the microphone
Receive: Play or tune to an SSTV signal — the image builds progressively on the canvas
Monitor: Use the spectrum analyzer and SNR indicator to optimise audio levels
Save: Click "Save Image" to download the decoded image as a PNG
- Filename: sstv-{mode}-{timestamp}.png
Gallery: Previously decoded images are kept in the gallery below the canvas
Reset: Click "Reset" to clear the canvas and start a new decode

Technical Details

Signal Processing Chain

Baseband Conversion: Complex multiplication at center frequency (1900 Hz)
Baseband Lowpass Filter: Kaiser-windowed FIR filter (2ms length, 900 Hz cutoff)
FM Demodulation: Phase difference detection with scale factor (sampleRate / (bandwidth × π))
Sync Detection: Schmitt trigger detecting frequency drops to 1200 Hz
Line Decoding: Bidirectional exponential moving average filtering for horizontal resolution

Audio Parameters

Sample Rate: Auto-detected (44.1 kHz or 48 kHz, matches browser/hardware)
Center Frequency: 1900 Hz (midpoint of 1000-2800 Hz range)
Bandwidth: 800 Hz (white-black range: 2300-1500 Hz)
Sync Frequency: 1200 Hz (normalized to -1.750)
Schmitt Trigger: Low threshold = -1.563 (1275 Hz), High threshold = -1.375 (1350 Hz)

Robot36 Color Mode Specifications

Resolution: 320×240 pixels
Color Format: Interlaced YUV (even lines: Y + R-Y, odd lines: Y + B-Y)
Line Duration: ~150ms per scan line
Sync Pulse: 9ms at 1200 Hz
Sync Porch: 3ms at 1500 Hz
Luminance (Y): 88ms
Separator: 4.5ms (frequency indicates even/odd line)
Porch: 1.5ms
Chrominance (R-Y or B-Y): 44ms
Total Lines: 240 (produces 240 pixel rows, 120 even + 120 odd pairs)
Encoding: 1 row per scan line (interlaced chroma pairing)

PD120 Mode Specifications

Resolution: 640×496 pixels
Color Format: Dual-luminance YUV (Y-even + V-avg + U-avg + Y-odd)
Scan Line Duration: ~508ms per scan line
Sync Pulse: 20ms at 1200 Hz
Sync Porch: 2.08ms at 1500 Hz
Y-even Channel: 121.6ms (luminance for even row)
V-avg Channel: 121.6ms (R-Y chroma, shared)
U-avg Channel: 121.6ms (B-Y chroma, shared)
Y-odd Channel: 121.6ms (luminance for odd row)
Pixel Dwell Time: 190µs per pixel
Total Scan Lines: 248 (produces 496 pixel rows, 248 × 2)
Encoding: 2 rows per scan line (shared chroma between rows)

PD160 Mode Specifications

Resolution: 512×400 pixels
Color Format: Dual-luminance YUV (Y-even + V-avg + U-avg + Y-odd)
Scan Line Duration: ~804ms per scan line
Sync Pulse: 20ms at 1200 Hz
Sync Porch: 2.08ms at 1500 Hz
Y-even Channel: 195.584ms (luminance for even row)
V-avg Channel: 195.584ms (R-Y chroma, shared)
U-avg Channel: 195.584ms (B-Y chroma, shared)
Y-odd Channel: 195.584ms (luminance for odd row)
Pixel Dwell Time: 382µs per pixel (2× longer than PD120)
SNR Improvement: ~3.0 dB better than PD120
Total Scan Lines: 200 (produces 400 pixel rows, 200 × 2)
Encoding: 2 rows per scan line (shared chroma between rows)

PD180 Mode Specifications

Resolution: 640×496 pixels
Color Format: Dual-luminance YUV (Y-even + V-avg + U-avg + Y-odd)
Scan Line Duration: ~752ms per scan line
Sync Pulse: 20ms at 1200 Hz
Sync Porch: 2.08ms at 1500 Hz
Y-even Channel: 182.4ms (luminance for even row)
V-avg Channel: 182.4ms (R-Y chroma, shared)
U-avg Channel: 182.4ms (B-Y chroma, shared)
Y-odd Channel: 182.4ms (luminance for odd row)
Pixel Dwell Time: 286µs per pixel (50% longer than PD120)
SNR Improvement: ~1.8 dB better than PD120
Total Scan Lines: 248 (produces 496 pixel rows, 248 × 2)
Encoding: 2 rows per scan line (shared chroma between rows)

Sync Detection

9ms Pulses: Robot36/Scottie scan line sync
20ms Pulses: PD mode scan line sync
5ms Pulses: Martin mode sync / VIS calibration headers
Frequency Tolerance: ±0.125 normalized units (~50 Hz at 1900 Hz center)
All timing automatically adapts to detected sample rate (44.1 kHz or 48 kHz)
Mode-specific pulse width detection ensures correct decoder selection

Image Export

Format: PNG (lossless compression)
Resolution: Matches selected SSTV mode
Filename: Includes mode and timestamp for easy identification
Method: Canvas.toBlob() API for efficient conversion

Project Structure

src/
├── app/
│   ├── layout.tsx              # Root layout with metadata
│   ├── page.tsx                # Home page with mode state management
│   └── globals.css             # Global Tailwind styles
├── components/
│   ├── RTTYDecoder.tsx         # RTTY (Baudot) decoder UI
│   ├── CWDecoder.tsx           # CW (Morse) decoder UI
│   ├── SSTVDecoder.tsx         # SSTV image decoder UI
│   ├── SessionCard.tsx         # Decoded session display card
│   ├── SettingsPanel.tsx       # Mode selection settings panel
│   └── PWAInstallPrompt.tsx    # PWA install prompt
├── hooks/
│   └── useAudioProcessor.ts    # Web Audio API integration (mode-aware)
└── lib/
    ├── rtty/
    │   ├── decoder.ts          # RTTY Baudot/ITA2 decoder
    │   ├── baudot.ts           # Baudot character tables
    │   └── sessions.ts         # Session management
    ├── cw/
    │   ├── decoder.ts          # CW Morse code decoder
    │   └── morse-table.ts      # Morse code lookup table
    └── sstv/
        ├── constants.ts             # SSTV mode specifications
        ├── decoder.ts               # Main decoder orchestration (multi-mode)
        ├── sync-detector.ts         # Sync pulse detection (9ms/20ms)
        ├── robot36-line-decoder.ts  # Robot36 interlaced YUV decoder
        ├── robot72-line-decoder.ts  # Robot72 sequential YUV decoder
        ├── scottie-s1-line-decoder.ts # Scottie S1 RGB sequential decoder
        ├── scottie-s2-line-decoder.ts # Scottie S2 RGB sequential decoder
        ├── pd120-line-decoder.ts    # PD120 dual-luminance decoder
        ├── pd160-line-decoder.ts    # PD160 dual-luminance decoder
        ├── pd180-line-decoder.ts    # PD180 dual-luminance decoder
        └── fm-demodulator.ts        # DSP primitives (FM demod, filters, EMA)

doc/
├── ROBOT36.md                  # Robot36 technical specification
├── ROBOT72.md                  # Robot72 technical specification
├── SCOTTIE_S1.md               # Scottie S1 technical specification
├── SCOTTIE_S2.md               # Scottie S2 technical specification
├── PD120.md                    # PD120 technical specification
├── PD160.md                    # PD160 technical specification
├── PD180.md                    # PD180 technical specification
└── ARCHITECTURE.md             # Overall system architecture

Known Issues

Occasional false sync detections from noise/interference
Stack overflow on very long lines (>6 seconds) - indicates lost sync
Best results with clean, strong signals from radio or audio playback
Safari iOS may have slightly higher latency (~34ms) due to polling approach

Future Improvements

High Priority

VIS Code Detection: Automatic mode selection based on VIS header detection
Additional PD Modes: PD50, PD90, PD240, PD290
Audio File Upload: Decode from WAV/MP3 files for offline processing

Medium Priority

Scottie DX Mode: DX variant with 4m 34s transmission time (RGB sequential encoding)
Martin Modes: M1, M2 (GBR sequential encoding)
Wraase SC2-180: High-quality RGB mode
Improved Noise Reduction: Advanced filtering for weak signals
Signal Quality Metrics: SNR calculation and display

Low Priority

Waterfall Display: Full spectrogram history
Multi-image Gallery: Store and compare multiple decoded images
Export Metadata: Include signal quality in saved filenames

Contributing

I welcome contributions! Please see CONTRIBUTING.md for guidelines on:

Setting up your development environment
Creating feature branches
Writing and running tests
Code quality standards
Submitting pull requests
Adding new SSTV modes

License

This project is licensed under the 0BSD license (Zero-Clause BSD).

Acknowledgments

smolgroot: Upstream sstv-decoder web application this project is forked from
Ahmet Inan (xdsopl): Original Robot36 Android app DSP algorithms that informed the SSTV implementation
Amateur Radio Community: Protocol specifications and documentation for RTTY, CW, and SSTV

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doc		doc
public		public
src		src
.eslintrc.json		.eslintrc.json
.gitignore		.gitignore
CONTRIBUTING.md		CONTRIBUTING.md
LICENSE		LICENSE
README.md		README.md
jest.config.js		jest.config.js
jest.setup.js		jest.setup.js
next.config.js		next.config.js
package-lock.json		package-lock.json
package.json		package.json
postcss.config.js		postcss.config.js
tailwind.config.js		tailwind.config.js
tsconfig.json		tsconfig.json

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📡 Signal Decoder

Quick Start

Features

Supported SSTV Modes

Color Encoding Overview

RTTY Decoder

RTTY Parameters

RTTY Signal Processing

CW Decoder

CW Parameters

CW Signal Processing Pipeline

Morse Display (live visualiser)

A/B Mode (dual-channel)

CW How to Use

Technology Stack

Testing

Running Tests

Test Framework

Getting Started

Prerequisites

Installation

Building for Production

How to Use

RTTY

CW

SSTV

Technical Details

Signal Processing Chain

Audio Parameters

Robot36 Color Mode Specifications

PD120 Mode Specifications

PD160 Mode Specifications

PD180 Mode Specifications

Sync Detection

Image Export

Project Structure

Known Issues

Future Improvements

High Priority

Medium Priority

Low Priority

Contributing

License

Acknowledgments

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