May 2026 Club Talk: SSTV

This month's Club talk, by David Vickers (G7BEH), covered an introduction to (and demonstration of) Slow Scan Television, or SSTV.

The term SSTV is a little bit of a misnomer really, as it doesn't cover moving images, but that shouldn't detract from what can be a fun way of communicating with others! Typically used on HF, SSTV is also used via satellites, most notably the ISS, to send images back down to Earth.

But before we delve too far in, let's hand over the website to David for more information.

A CQ QRP call from G7BEH as received in Italy

Introduction

As mentioned above, SSTV doesn't really transfer television (moving picture) images, but static ones. This is primarily because the bandwidth required for black and white TV images is around 3MHz, and around 6MHz for colour. Given that SSTV is normally found on the 20m HF band, using USB, we are somewhat restricted as to what we can use. About 3kHz... a 1000:1 reduction in bandwidth!

SSTV then, is a communications mode that allows amateurs to transfer static images. These are usually overlaid with text to give basic information that one would normally pass during a QSO - CQ calls, callsigns, signal strengths, names, etc.

There is an excellent background on the history of SSTV is Martin Buchanov's book Image Communication on Short Waves, which is available online at https://www.sstv-handbook.com/download/sstv-handbook.pdf. It is definitely recommended reading for anyone with a passing (or greater) interest in SSTV.

Suffice to say that back in the late 1950's, when Copthorne Macdonald was developing SSTV, he didn't have the software, computers and displays that we do now. But understanding where SSTV came from is important to understanding why things evolved the way they did, and although we now have digital SSTV (which is more of a file transfer system), SSTV set out to transfer an image by encoding it into a series of audio tones.

He also used CRT tubes with long persistent phosphor as a display. The timing of the frames used to create the images was such that as the last line was received, the first was still visible on the tube. Although it soon began to fade away. The duration to send an image so that it could be displayed was around 8 seconds - and you'll find that even now, you can select a B&W 8 second mode to use.

The signal band used to transmit the video data is between 1500Hz and 2300Hz, for black and white, respectively. Sync pulses have a frequency of 1200Hz and are therefore rendered as black (or "blacker than black"!) and are not visible on the screen.

Even though that original mode was B&W, 8sec, new modes have become available. Lots of them! With improvements in technology, transmission times could be increased to provide greater resolution. This gives the amateur an option... faster transmissions times but lower resolution, or longer transmission times and better image quality. However, with longer transmissions comes any potential issues with fading signals and noise...

The 'standard' modes used today are:

• Martin M1
• Martin M2
• Scottie S2
• Robot 36C
• PD120

The mode used is automatically selected by software (MMSSTV for Windows, QSSTV for Linux based machines, SSTV by Black Cat Systems for Apple iOS). This is achieved by a VIS (Vertical Interval Signal) signal which is detected and read by the software. This means that cross-mode image communication is perfectly possible!

Vertical Information Signal

The VIS contains a binary code consisting of 10 bits:

• 1x start bit
• 7x 'mode information' bits
• 1x parity bit (even parity)
• 1x stop bit

The VIS structure is shown below (image from page 24, Image Communication on Short Waves).

The VIS structure used with SSTV to decode mode used

The 7 'mode information' bits tell the decoding software the horizontal and vertical resolution, as well as the mode in use.

Timings & Resolution

For the common modes, the time taken to transmit an image and the resolution, is shown below:

Mode	Time to send	Resolution
Martin M1	114 seconds	320x256
Martin M2	58 seconds	160x256
Scottie S1	110 seconds	320x256
Scottie S2	71 seconds	160x256
PD120	126 seconds	640x480
Robot 24C	24 seconds	320x120
Robot 36C	36 seconds	320x240

As there are some advantages to using the Robot modes for VHF FM contacts, we will be using Robot 36C for the demonstration. The SSTV calling frequency on 2m FM, as shown on the RSGB band plans, 144.500MHz.

A Simple Portable Setup

SSTV, particularly on the VHF bands, can be very simple and quite a cheap way to have some fun with image communications. For the demo, David used a Quansheng K5 connected to his iPhone. The iPhone was running the Black Cat Systems SSTV app, which although basic, does the job.

SSTV running on an iPhone

The Baofeng UV-R5 works just as well. Remember that you will need to set VOX to on and also set the volume on your phone to ensure reliable operation.