SSTv was originally invented by Copthorne Macdonald and first used by Radio Amateurs, but is now used by large groups of professionals such as Police & Security services. The original idea was to find a method of transmitting a television picture over a single speech channel. This meant that a typical (at that time) 3MHz wide television picture had to be reduced to around 3kHz (1000:1 reduction). It was decided at the outset that the scanning rates must be very slow, which precludes the use of moving pictures. The choice of time base for synchronising was the readily available domestic power supply at 50 or 60 Hz (depending on the country of origin).This gave a line speed of 16.6Hz and 120 or 128 lines per frame (against the then UK standard of 405 lines (now 625) per frame), giving a new picture frame every 7.2 or 8 seconds.

The composition of a single SSTV line to the original specification of 8 sec is as follows: -

The maximum bandwidth is 3kHz, therefore the SSTv signals bandwidth is restricted to 2.3kHz, Black is represented by a 1500Hz tone and white by a 2300 Hz tone together with a sync pulse at 1200Hz well below the black level so it would be invisible. The Sync pulses are sent at the end of each line are 5ms in length and at the end of each frame 30ms

The original SSTV systems were based on ex-government Radar screens and long persistence cathode ray tubes. Now most systems are computer based for ease of use, giving full 'living' colour with up to 16 million colours and transmission times of up to several minutes for each picture.

The difference between SSTV and Fax is blurring, but both systems are still only one way, without the benefit of error checking we are so use to on the Internet. This sometimes causes problems particularly when you are receiving that 'perfect' picture from New Zealand and the last 40 lines are spoilt by someone calling CQ CQ over the top of the picture.

Information supplied from ‘Slow Scan Television Explained’ by Mike Wooding

The 'Pasokon' System

The Pasokon is a PC based, Slow Scan Television system, designed by John Langner (WB2OSZ) , with a purpose made interface card that fits in one of the 16bit expansion slots, a cable then connects to the AF output and Microphone (or similar) Input of your Tranceiver. The Card converts the analogue signals from your radio into digital pulses that the software decodes into the various colours displayed in the centre of the screen.
The system is capable of resolving most SSTV modes presently in use today, with Martin Mode M1, being the most popular in Europe, Various Filters are also available to aid in refining the picture, and an automatic tuning device is incorporated to help with the fine tuning or when working in a net.
Further information on all SSTV systems and modes can be seen on WB4EJC's Home Page.