🛰️ SSTV Decoder - Cross-Platform Amateur Radio App

Decode SSTV signals in your browser! Free, open-source web application for real-time SSTV (Slow Scan Television) decoding from microphone input. Supports several modes: Robot36, Robot72, Scottie S1/S2, PD120, PD160, PD180. Perfect for ham radio operators, ISS SSTV events, and satellite image reception. Works offline as a PWA. Based on the Robot36 Android app by xdsopl.

🚀 Try the Live Demo - No Installation Required!

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

Screenshots

Quick Start

Visit → sstv-decoder.vercel.app
Allow microphone access when prompted
Play an SSTV signal near your microphone
Watch the image decode in real-time!

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

Features

Multi-Mode Support: Robot36 Color (320×240), Robot72 (320×240), Scottie S1 (320×256), Scottie S2 (320×256), PD120 (640×496), PD160 (512×400), and PD180 (640×496) with manual mode selection
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
Sync Detection: Automatic detection of sync pulses (9ms for Robot36, 20ms for PD modes)
Live Image Display: Progressive image rendering with real-time spectrum visualization
Mode Selection: Switch between SSTV modes via settings panel
Save Image: Export decoded images as PNG files
Signal Analysis: Real-time spectrum analyzer and signal strength indicator
Mobile-Responsive: Optimized for both desktop and mobile devices

Supported SSTV Modes

Current Implementation

Mode	Resolution	Sync Pulse	Line Time	Total Time	VIS Code	Status	Documentation
Robot36 Color	320×240	9ms	~150ms	~36s	8	✅ Implemented	ROBOT36.md
Robot72 Color	320×240	9ms	~300ms	~1m 12s	12	✅ Implemented	ROBOT72.md
Scottie S1	320×256	9ms	~428ms	~1m 50s	60	✅ Implemented	SCOTTIE_S1.md
Scottie S2	320×256	9ms	~278ms	~1m 11s	56	✅ Implemented	SCOTTIE_S2.md
PD120	640×496	20ms	~508ms	~2m 6s	95	✅ Implemented	PD120.md
PD160	512×400	20ms	~804ms	~2m 41s	98	✅ Implemented	PD160.md
PD180	640×496	20ms	~752ms	~3m 6s	96	✅ Implemented	PD180.md

Future Modes (Planned)

The decoder architecture supports adding these modes in future updates:

Mode	Resolution	Sync Pulse	Line Time	Total Time	VIS Code	Complexity
Scottie DX	320×256	9ms	~1069ms	~4m 34s	76	Medium (RGB sequential)
Martin M1	320×256	5ms	~446ms	~1m 54s	44	Medium (GBR sequential)
Martin M2	320×256	5ms	~226ms	~58s	40	Medium (GBR sequential)
PD50	320×256	20ms	~406ms	~1m 44s	93	Low (dual-luma like PD120)
PD90	320×256	20ms	~754ms	~3m 13s	99	Low (dual-luma like PD120)
PD240	640×496	20ms	~1018ms	~4m 13s	97	Low (dual-luma like PD120)
PD290	800×616	20ms	~954ms	~4m 54s	94	Low (dual-luma like PD120)
Wraase SC2-180	320×256	5ms	~734ms	~3m 8s	55	Medium (RGB sequential)

Mode Selection Guide

For Quick Transmissions (under 1 minute):

Robot36: 36 seconds, good quality, standard for QSOs
Martin M2: 58 seconds, RGB sequential, no interlacing
Robot72: 72 seconds, higher quality than Robot36

For High Quality (1-3 minutes):

Scottie S2: 1m 11s, RGB sequential, faster HF mode, clean colors
Scottie S1: 1m 50s, RGB sequential, original HF classic, clean colors
PD120: 2m 6s, 640×496, excellent for ISS SSTV events
PD160: 2m 41s, 512×400, balanced quality and time
PD180: 3m 9s, 640×496, extended transmission

For Maximum Quality (3+ minutes):

PD240: 4m 13s, 640×496, very high quality
PD290: 4m 54s, 800×616, maximum resolution
Scottie DX: 4m 34s, high quality RGB

Technical Comparison

Feature	Robot36/72	Scottie/Martin	PD Modes
Color Encoding	Interlaced YUV	Sequential RGB/GBR	Dual-luma YUV
Chroma Strategy	Alternating R-Y/B-Y per line	Full RGB per line	Shared U/V per 2 rows
Rows per Scan	1 row	1 row	2 rows
Vertical Subsampling	2:1 (interlaced)	None (1:1)	2:1 (shared chroma)
Complexity	Medium (interlacing)	Low (sequential)	Low (dual-luma)
ISS Usage	Rare	Occasional	Very Common (PD120)

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 <repository-url>
cd sstv-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 Mode (Optional): Click the settings icon (bottom-right) to choose between available SSTV modes
- Robot36: 320×240, fastest decode (~36 seconds)
- Robot72: 320×240, better color than Robot36 (~72 seconds)
- Scottie S1: 320×256, HF classic, RGB sequential (~1m 50s)
- Scottie S2: 320×256, faster HF mode, RGB sequential (~1m 11s)
- PD120: 640×496, high resolution (~2 minutes), used for ISS SSTV
- PD160: 512×400, balanced mode (~2m 41s), good SNR
- PD180: 640×496, highest quality (~3 minutes), best SNR
Start Decoding: Click "Start Decoding" to begin capturing audio from your microphone
Grant Microphone Permission: Allow the browser to access your microphone when prompted
Play SSTV Signal: Play an SSTV signal near your microphone (from radio, audio file, signal generator, etc.)
Watch Live Decoding: The decoded image will appear progressively on the canvas
Monitor Signal: Check the spectrum analyzer and signal strength indicator for optimal reception
Reset: Click "Reset" to clear the canvas and start a new decode
Save Image: Click "Save Image" to download the decoded image as a PNG file
- Filename format: sstv-decode-{mode}-{TIMESTAMP}.png
- Example: sstv-decode-robot36-2024-03-15-143022.png
Stop: Click "Stop" to end the decoding session (image remains visible for saving)

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/
│   ├── SSTVDecoder.tsx         # Main decoder UI component
│   └── SettingsPanel.tsx       # Mode selection settings panel
├── hooks/
│   └── useAudioProcessor.ts    # Web Audio API integration (mode-aware)
└── lib/
    └── sstv/
        ├── constants.ts             # SSTV mode specifications (Robot36, Robot72, Scottie S1, Scottie S2, PD120, PD160, PD180)
        ├── 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 (better color)
        ├── scottie-s1-line-decoder.ts # Scottie S1 RGB sequential decoder (HF classic)
        ├── scottie-s2-line-decoder.ts # Scottie S2 RGB sequential decoder (faster HF)
        ├── pd120-line-decoder.ts    # PD120 dual-luminance decoder
        ├── pd160-line-decoder.ts    # PD160 dual-luminance decoder (balanced mode)
        ├── pd180-line-decoder.ts    # PD180 dual-luminance decoder (high quality)
        ├── fm-demodulator.ts        # DSP primitives (FM demod, filters, EMA)
        └── dsp.ts                   # Legacy utilities (deprecated)

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

Browser Compatibility

Desktop Browsers

Browser	Status	Notes
Chrome 90+	✅ Full Support	Uses ScriptProcessorNode for audio processing
Edge 90+	✅ Full Support	Chromium-based, same as Chrome
Firefox 88+	✅ Full Support	Automatic sample rate matching (44.1kHz or 48kHz)
Safari 14+	✅ Full Support	Uses requestAnimationFrame polling fallback
Opera 76+	✅ Full Support	Chromium-based, same as Chrome

Mobile Browsers

Browser	Status	Notes
Safari (iOS) 14.5+	✅ Full Support	Uses requestAnimationFrame polling (ScriptProcessorNode deprecated)
Chrome (Android) 90+	✅ Full Support	Full ScriptProcessorNode support
Firefox (Android) 88+	✅ Full Support	Automatic sample rate matching
Samsung Internet	✅ Full Support	Chromium-based

Requirements

HTTPS Required: Microphone access requires secure context (HTTPS)
- Exception: localhost works with HTTP for development
Microphone Permission: User must grant microphone access when prompted
Web Audio API: All supported browsers have Web Audio API enabled by default

Technical Implementation

The app uses a dual-strategy approach for maximum compatibility:

ScriptProcessorNode (deprecated but widely supported):
- Used in Chrome, Firefox, Edge, and older Safari versions
- Processes audio in 4096-sample chunks
- Efficient and low-latency
requestAnimationFrame Polling (modern Safari/iOS fallback):
- Automatically used when ScriptProcessorNode is unavailable
- Polls AnalyserNode.getFloatTimeDomainData() at 60fps
- 2048 samples per frame (~34ms latency)
- Required for Safari iOS 14+ where ScriptProcessorNode is broken
Dynamic Sample Rate:
- Automatically adapts to browser's native sample rate (44.1kHz or 48kHz)
- Required for Firefox compatibility
- All DSP calculations scale accordingly

Implementation Notes

This implementation closely follows the Robot36 Android app by Ahmet Inan (xdsopl), translating the Java implementation to TypeScript while maintaining the same DSP algorithms:

FM Demodulation: Complex baseband conversion with phase difference calculation
Lowpass Filtering: Kaiser-windowed FIR filter matching Java's ComplexConvolution
Sync Detection: Schmitt trigger logic from Robot_36_Color.java
Line Decoding: Bidirectional exponential moving average filter with proper cutoff formula
Color Conversion: ITU-R BT.601 YUV to RGB transformation with interlaced chrominance

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 (low complexity, similar to PD120/PD180)
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 mode, timestamp, signal quality in saved files

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 based on Robot36 by Ahmet Inan, which is licensed under the 0BSD license (Zero-Clause BSD).

Acknowledgments

Ahmet Inan (xdsopl): Original Robot36 Android app and DSP algorithms
Amateur Radio SSTV Community: Protocol specifications and documentation

Reference Implementation

This web implementation is based on the algorithms from the original Robot36 Android app by Ahmet Inan. If you're interested in the reference Java implementation, please visit the original repository.

Name		Name	Last commit message	Last commit date
Latest commit History 56 Commits
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

License

smolgroot/sstv-decoder

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🛰️ SSTV Decoder - Cross-Platform Amateur Radio App

Screenshots

Quick Start

Features

Supported SSTV Modes

Current Implementation

Future Modes (Planned)

Mode Selection Guide

Technical Comparison

Technology Stack

Testing

Running Tests

Test Framework

Getting Started

Prerequisites

Installation

Building for Production

How to Use

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

Browser Compatibility

Desktop Browsers

Mobile Browsers

Requirements

Technical Implementation

Implementation Notes

Known Issues

Future Improvements

High Priority

Medium Priority

Low Priority

Contributing

License

Acknowledgments

Reference Implementation

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