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Selecting an Image
The transmitting operator selects a photograph, test pattern, station card or other image using SSTV software.
SSTV Basics
What Is SSTV?
Slow-Scan Television, usually abbreviated as SSTV, is a method used by radio amateurs to transmit and receive still images over radio.
Instead of sending moving video, SSTV converts a picture into a sequence of audio tones. These tones can be transmitted through a normal amateur-radio voice channel and decoded by a computer, smartphone or dedicated SSTV system.
Quick introduction
SSTV in Simple Terms
SSTV is a radio communication system that sends one still picture at a time. The image is scanned line by line, converted into audio tones, transmitted over radio and then reconstructed by receiving software.
An SSTV transmission may sound like a series of changing whistles, beeps and tones. Although these sounds may seem unusual to the human ear, SSTV software can interpret them as brightness and colour information.
Depending on the selected SSTV mode, a complete colour image may take approximately 30 seconds to several minutes to transmit.
The basic process
How Does SSTV Work?
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Scanning the Picture
The software scans the image one horizontal line at a time and analyses its brightness and colour information.
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Creating Audio Tones
The visual information is converted into continuously changing audio frequencies, normally between approximately 1200 and 2300 Hz.
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Radio Transmission
The audio signal is sent through an amateur-radio transmitter using a voice mode such as SSB or FM.
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Receiving the Signal
Another station receives the radio transmission and sends the recovered audio into SSTV decoding software.
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Rebuilding the Image
The receiving software converts the audio tones back into pixels and gradually displays the completed image.
Pictures become sound
From Image to Audio Tones
Analogue SSTV does not transmit an image as a computer file. Instead, it represents the picture using a carefully controlled range of audio frequencies.
Synchronisation
A frequency near 1200 Hz is commonly used for horizontal synchronisation pulses. These pulses help the decoder determine where each new picture line begins.
Dark Image Areas
Lower image frequencies normally represent darker brightness or colour levels in the transmitted picture.
Bright Image Areas
Higher image frequencies normally represent brighter levels in the picture.
Important:
Exact timing, synchronisation and colour sequencing depend on the
selected SSTV mode. Different modes use different methods to send
the red, green, blue or luminance information.
Narrow-band communication
How Much Bandwidth Does SSTV Use?
About 3 kHz
Designed for a Voice Channel
Most analogue SSTV modes are designed to fit inside a standard amateur-radio voice channel.
The actual picture information normally uses audio tones between approximately 1200 and 2300 Hz. A receiver or transceiver filter may be set to around 2.7, 3.0 or 3.2 kHz, depending on the equipment and operating mode.
It is therefore more accurate to say that SSTV needs about 3 kHz of radio bandwidth than to say that every SSTV signal is exactly 3.2 kHz wide.
This narrow bandwidth is one of the reasons SSTV can be used on HF. A still image can travel thousands of kilometres while using approximately the same channel width as a normal voice transmission.
One picture at a time
Why Is It Called Slow-Scan Television?
Traditional television transmits many complete images every second, producing moving video. SSTV works much more slowly.
A single SSTV picture is transmitted line by line and can take tens of seconds or several minutes to complete. This slow scanning process allows the image to fit into the limited bandwidth of an amateur-radio voice channel.
Normal Television
- Moving video
- Many frames per second
- Requires wide bandwidth
- Designed for continuous viewing
Slow-Scan Television
- Still pictures
- One image at a time
- Fits in a voice channel
- Designed for radio communication
Different transmission formats
What Is an SSTV Mode?
An SSTV mode defines how an image is encoded and how long the transmission takes. Each mode has its own resolution, timing, colour system and image quality.
Martin 1
Approximately 114 seconds
A popular colour mode commonly heard on the HF amateur bands. It offers good image quality and is widely supported by SSTV software.
Scottie 1
Approximately 110 seconds
Another popular colour mode used by radio amateurs around the world, especially for HF SSTV contacts.
Robot 36
Approximately 36 seconds
A faster colour mode often used for VHF communication and SSTV transmissions from the International Space Station.
PD120
Approximately 126 seconds
A higher-quality colour mode used during many ARISS SSTV events from the International Space Station.
Learn More About SSTV Modes
Explore the differences between Martin, Scottie, Robot, PD and other popular SSTV transmission formats.
Around the world and in space
Where Is SSTV Used?
Worldwide HF Contacts
Radio amateurs use HF propagation to exchange SSTV images over hundreds or thousands of kilometres.
Local VHF Communication
SSTV can be transmitted locally using FM on the 2-metre amateur band, repeaters or direct simplex contacts.
International Space Station
During special events, the ISS transmits commemorative SSTV images that can be received by stations on Earth.
Education and Experiments
SSTV is used to demonstrate radio propagation, audio modulation, image encoding and space communication.
Portable Radio
Operators can receive and send images from portable stations using radios, laptops, tablets or smartphones.
Station Identification
Many operators transmit personalised images containing their callsign, location, equipment and station details.
Different radio environments
HF, VHF and Space Communications
HF
Long-Distance SSTV
On HF, SSTV is normally transmitted using single-sideband voice operation. Ionospheric propagation can carry the signal between countries and continents.
- Worldwide communication is possible
- Signals may experience fading and interference
- Martin and Scottie modes are common
VHF
Local and Regional SSTV
On VHF, SSTV is often sent using FM. Reception is normally clearer than HF, but the range is usually limited by terrain, antenna height and repeaters.
- Good audio quality
- Suitable for local contacts
- Can be used through SSTV repeaters
SPACE
Receiving the ISS
The International Space Station occasionally transmits SSTV images on the amateur-radio downlink frequency of 145.800 MHz FM.
- No amateur-radio licence is needed to receive
- A simple VHF receiver may be sufficient
- Signals are available during visible passes
Receive SSTV Images from Space
Read our complete introduction to receiving SSTV transmissions from the International Space Station.
Getting started
What Equipment Do You Need?
Receiving SSTV can be surprisingly simple. You do not always need an expensive amateur-radio station to decode your first image.
Radio or SDR Receiver
You need a receiver that can tune to an SSTV frequency and provide clear audio. This may be an amateur transceiver, scanner, shortwave receiver or software-defined radio.
Suitable Antenna
The antenna must be suitable for the frequency being received. Better antennas usually improve signal strength and image quality.
Computer or Smartphone
SSTV decoding software runs on computers, smartphones, tablets and some dedicated radio systems.
Audio Connection
Radio audio must reach the decoding device. This can be done through an audio cable, USB sound interface or simply by placing a smartphone near the radio speaker.
Beginner tip
To receive your first SSTV picture, try holding a smartphone running an SSTV decoder near the speaker of a radio. A direct audio connection usually gives better results, but acoustic decoding is an easy way to start.
Decoding software
Popular SSTV Software
MMSSTV
WindowsOne of the best-known SSTV programs for transmitting and receiving analogue SSTV images.
RX-SSTV
WindowsA simple receiving program that automatically detects and decodes many common SSTV modes.
QSSTV
LinuxA feature-rich application for receiving and transmitting analogue and digital SSTV.
Robot36
AndroidA popular mobile application that can decode SSTV audio using the smartphone microphone.
Software availability and compatibility can change. Always obtain
applications from a trusted source and check whether they support
your operating system and preferred SSTV modes.
Licence requirements
Receiving Versus Transmitting
Receiving SSTV
In many countries, anyone may receive amateur-radio SSTV transmissions using suitable receiving equipment.
Receiving is an excellent way to explore SSTV before obtaining an amateur-radio licence.
Transmitting SSTV
Transmitting on amateur-radio frequencies normally requires a valid amateur-radio licence and an authorised callsign.
Operators must follow their national licence conditions, band plans, power limits and identification requirements.
Always check local regulations.
Amateur-radio rules and frequency allocations vary between
countries. Never transmit without the required authorisation.
Understanding received pictures
Why Do SSTV Images Sometimes Look Distorted?
SSTV pictures are transmitted as live audio. Any disturbance to that audio can become visible in the decoded image.
Radio Noise
Electrical noise and weak signals may create coloured dots, lines or missing sections.
Signal Fading
HF signals can change strength during transmission, producing horizontal bands or altered colours.
Incorrect Tuning
If an SSB receiver is not tuned correctly, the audio pitch shifts and the image may become slanted or incorrectly coloured.
Audio Problems
Audio that is too loud, too quiet or distorted can reduce decoding accuracy.
Interference
Other stations, local electronics and nearby signals may appear as patterns or disruptions in the picture.
Clock Differences
Small timing differences between the transmitter and receiver can cause an image to lean sideways.
Did you know?
Interesting SSTV Facts
SSTV images can be received from other continents using ordinary HF amateur-radio propagation.
The ISS has transmitted many series of commemorative SSTV images for radio listeners around the world.
A smartphone microphone can sometimes decode SSTV directly from a radio speaker.
SSTV signals are often recorded as ordinary audio and decoded again later.
Different SSTV modes provide different balances between transmission speed, resolution and image quality.
You can receive SSTV without transmitting, making it accessible to shortwave listeners and beginners.
Frequently asked questions
SSTV FAQ
Is SSTV a digital mode?
Traditional SSTV modes such as Martin, Scottie and Robot are analogue image modes. The picture is represented by continuously changing audio frequencies.
Digital image systems also exist, but they work differently and should not be confused with classic analogue SSTV.
Is SSTV real television?
SSTV is a television system in the sense that it sends visual information, but it normally transmits still pictures rather than moving video.
What does an SSTV signal sound like?
It sounds like a sequence of beeps, whistles and continuously changing tones. The sound changes as each line of the picture is transmitted.
How wide is an SSTV signal?
Analogue SSTV is designed to fit inside a standard voice channel of about 3 kHz. The image tones normally occupy approximately 1200 to 2300 Hz in the recovered audio.
A receiver filter may be set to around 2.7, 3.0 or 3.2 kHz depending on the radio and operating mode.
Do I need an amateur-radio licence?
A licence is normally not required simply to receive SSTV. A valid licence is generally required to transmit on amateur-radio frequencies.
Regulations vary by country, so always check the rules that apply in your location.
Can I decode SSTV with a smartphone?
Yes. Smartphone applications can decode SSTV using the built-in microphone or an audio connection from a receiver.
Can SSTV images be recorded and decoded later?
Yes. Because an SSTV transmission is an audio signal, it can be recorded as a WAV file or another high-quality audio format and decoded later.
Can the ISS transmit SSTV?
Yes. During special ARISS events, the International Space Station transmits commemorative images over amateur radio.
These transmissions are not continuous and only take place during announced events.
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