Online SSTV Decoder – Convert Slow Scan Television Audio to Images

Convert Slow Scan Television Audio to Images

Load an SSTV audio file (WAV, MP3, etc.) to extract the transmitted image

Try with this example file : Download test SSTV audio (MP3)

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Understand Slow Scan Television (SSTV) and
How to Decode SSTV Images

Slow Scan Television, abbreviated as SSTV, is a technique used to transmit static images over narrowband radio frequencies. It is widely used by amateur radio operators and space agencies due to its efficiency in sending visual data using minimal bandwidth. Unlike traditional video transmission, which requires high data rates, SSTV sends images line by line as modulated audio tones, making it ideal for long-distance and low-power communications.

SSTV was initially developed in the 1950s for military and scientific use. Over time, it became a popular tool among radio amateurs, especially for communicating during emergencies, engaging in hobby activities, and receiving images from space missions, including broadcasts from the International Space Station. The audio signal, once recorded or received live, can be decoded back into a visual image using this SSTV Decoder.

SSTV Signal Structure

An SSTV transmission follows a structured format that enables precise decoding. It typically includes the following components:

Leader tone: A continuous high-frequency tone, usually around 1900 Hz, that prepares the decoder for synchronization.
Break signal: A brief lower-frequency tone, around 1200 Hz, that indicates the end of the leader tone.
VIS code: A set of tones encoding the mode used, allowing the decoder to interpret the image structure correctly.
Synchronization pulses: Regular tones marking the beginning of each line or channel within the image.
Image scan lines: The core of the transmission, where each pixel's brightness or color is represented by an audio frequency.

Common SSTV Modes

SSTV supports various transmission modes, each designed to balance image resolution, transmission speed, and audio bandwidth. Some of the most widely used modes are listed below:

Mode	Resolution	Transmission Time
Robot36	320 by 240	~ 36 seconds
Scottie1	320 by 256	~ 110 seconds
Martin1	320 by 256	~ 114 seconds
Robot72	320 by 240	~ 72 seconds
Martin2	320 by 256	~ 58 seconds
Scottie2	320 by 256	~ 71 seconds
ScottieSDX	640 by 496	~ 539 seconds

Decoding Process

Decoding an SSTV signal involves converting the received audio tones into pixel data. The process generally includes the following steps:

Identify and extract the VIS header to determine the SSTV mode used for encoding.
Use the leader tone and sync pulses to align the decoding process with the signal timing.
Perform a Fast Fourier Transform, or FFT, on each time window to measure the dominant frequency.
Translate each frequency value into a luminance or color channel intensity for the corresponding pixel.
Assemble the pixels line by line to reconstruct the final image on the display.

Technical Characteristics

SSTV signals operate within the audio frequency range, typically between 1500 and 2300 Hz. Each color or brightness value is transmitted by holding a specific frequency for a short duration. In color modes like YUV, three separate channels are used: Y for brightness, U and V for color difference components.

Synchronization tones are critical for reliable decoding. These tones ensure that the decoder remains aligned with the image structure throughout the transmission. Without correct synchronization, the decoded image may appear distorted, offset, or scrambled.

Use Cases and Practical Applications

Amateur radio image transmission during regular contacts or contests
Reception of commemorative or scientific images from the International Space Station
Hands-on learning tool for signal processing and modulation techniques
Forensic audio and image recovery in security and research contexts
Remote communication in disaster scenarios where text or image data must be exchanged over voice channels

Modern SSTV Decoding Tools

SSTV decoding used to rely on desktop applications such as MMSSTV and RX-SSTV, typically available for Windows operating systems. Today, advancements in web technologies allow users to decode SSTV transmissions directly in their browser using JavaScript and the Web Audio API. This provides cross-platform access and eliminates the need for software installation, making SSTV more accessible to beginners, educators, and hobbyists.

This web-based SSTV Decoder is inspired by the open-source project sstv on GitHub, which implements image decoding in Python. For mobile users and offline decoding, the following applications are also recommended:

SSTV Decoder (Android): Decode SSTV signals using your phone’s microphone or audio input – Robot36 on Google Play
SSTV Encoder (Android): Generate and transmit SSTV signals using image files – SSTV Encoder on Google Play

These tools support various SSTV modes and are ideal for field experiments, portable ham radio setups, and educational activities.

Conclusion

SSTV remains a unique and engaging method of communication, blending audio engineering with image processing. Whether used for hobby purposes, scientific outreach, or technical education, SSTV provides a hands-on way to explore the fundamentals of analog communication. With modern digital tools, decoding SSTV transmissions has become more accessible than ever, helping preserve and expand this historic radio tradition.

Frequently Asked Questions about SSTV

What is SSTV and how does it work?

SSTV, or Slow Scan Television, is a method of sending still images over audio frequencies. It works by encoding each line of the image as a sequence of tones, which can be transmitted via radio and decoded back into a picture using software or hardware tools.

Can I decode SSTV signals directly in my browser?

Yes. With modern web technologies such as the Web Audio API and JavaScript, you can decode SSTV audio recordings in real time within your browser. This website provides a cross-platform tool for decoding common SSTV formats like Robot36, Scottie1, and Martin1.

What types of files can I use to decode an SSTV image?

You can upload most common audio file formats, such as WAV, MP3, or OGG. Ensure the recording is clear and has minimal background noise for accurate decoding.

What are the most common SSTV modes?

Popular SSTV modes include Robot36, Scottie1, Martin1 and ScottieSDX. Each mode differs in resolution, color format, and transmission time. This decoder supports a wide variety of these formats automatically by detecting the VIS code.

Do I need a license to transmit SSTV signals?

Yes. Transmitting SSTV over radio frequencies generally requires an amateur radio license, depending on your country’s regulations. Receiving and decoding SSTV transmissions, however, is legal and unrestricted in most regions.