SSTV (slow scan TV) is one of the few ways to send and receive images from all over the world via radio. Normal analog TV is an example of fast scan TV. SSTV works similarly to a fax machine where the image is displayed line by line from top to bottom and left to right until the entire image appears.
There is no audio associated with SSTV. Black and white as well as color images can be sent and received. Images take approximately one minute to receive on average but this also differs with which mode is being used and whether the image is black and white or in color.
How SSTV Works
SSTV is pretty easy to set up. All it requires is a stereo cable or USB cable coming from the audio output of your HAM radio going into the audio input of your computer and some free software. The software will receive the audio and process it to be able to display the image.
SSTV works by using frequency modulation where information is encoded using a carrier wave. As the data being sent changes, the frequency of the wave changes to correspond. For example, in a black and white image, a white pixel is represented as 2300 Hz and a black pixel is represented as 1500 Hz. When the SSTV software receives a 2300 Hz tone, it knows to display a black pixel.
For color, most SSTV modes use an RGB color scheme where different frequencies will all be given for how much red, green and blue are in that particular pixel. After the RGB frequencies are sent, another frequency is sent to determine brightness.
The sending and receiving units have to be in sync otherwise the image will appear slanted. To make sure the image does not appear slanted, each image is sent with synchronization tones. Where the synchronization tone occurs depends on the mode of SSTV.
The most popular SSTV mode for the United States is Scottie S1. In Europe the popular mode is Martin 1. Most of the modes share the same properties with the only difference being image quality.
Back in the day, hardware was a little bit more of a concern as computers weren’t as fast and robust as they are today. Basically any computer running an updated OS will have no problem running software designed for SSTV. Also any amateur radio you might have is going to get the job done.
The tricky part will be connecting an audio output from the radio into the audio input of a computer. If you’re relying on a cable going from the radio to the computer, you might also struggle with interference and noise which will come through on your image. You will want to be sure to have a shielded audio cable that is as short as possible.
A popular device used to interface a HAM radio to a computer is called a SignaLink USB sound card interface. A USB cable connects the SignaLink to the computer. The SignaLink USB can be connected to your radio’s Mic, data or accessory port.
There are several different free software available for SSTV. For Windows, a popular software is called MMSSTV. Most software are able to transmit, receive and store images.
For Linux and MAC operating systems, the most popular software is called QSSTV. This software works almost identically to the MMSSTV software. Getting started with SSTV would be a great Raspberry Pi project.
Finding SSTV Frequencies
If you are having a hard time finding out which frequencies will have the most traffic for SSTV, a list of popular frequencies have been included below. SSTV is typically transmitted on HF bands although that does not mean you won’t be able to find some good images on the VHF/UHF bands as well.
Most activity will happen on 20m and more specifically 14.230 MHz. Also you will probably have the most luck on the weekends. You could even potentially receive images from the ISS in space! How cool is that?
Popular SSTV Frequencies:
- 3.845 MHz
- 7.171 MHz
- 14.230 MHz
- 14.233 MHz
- 21.340 MHz
- 28.680 MHz