SSTV - SLOW SCAN TELEVISION (INTRODUCTION)

One of the exciting modes of transmission used by radio amateurs, is the Slow
Scan Television or SSTV. In oppsition with the mode used at home on TVs
(Fast-Scan Television) where the maximum coverage is few hundreds of mile
because of the frequencies used, SSTV consist of exchanging color pictures over
the air, but the advantage here is the long distance as we work in HF.

Many shortwave listeners are interested in SSTV. The goal is always DX and
receiving images over thousands of miles.
Thanks to the proliferation of personal computers and sound cards, it has never
been easier to enter the world of SSTV. Today, it is possible to receive high
quality, 16-million-color SSTV pictures, store them in your computer and view
them in real time.

 Sending SSTV Pictures
Many softwares are developped to work on this mode (see Software page); you can simply take a fixed (non-moving) image in your computer, the special software will scan it line by line, convert the color and brightness variations into audio tones. Feed the
audio tones into an ordinary SSB voice tranceiver and you can send this information almost anywhere in the world.

Receiving SSTV Pictures
Receiving an SSTV image is easy and fun, you should simply receive the audio
tones sent by the sender. These audio tones are then translated back into an
image on a computer screen using an SSTV software.

SSTV Frequencies
You'll find most SSTV activity on 20 meters, especially at 14.230 and 14.233 MHz.
As the bands improve with the progress of the current solar cycle, you may see
the activity spread to other bands.

Receiving Modes
There are 27 different modes used to send/receive SSTV images, with the most
popular being Scottie One and Martin One. So to receive a picture, one must
have the software set to the proper mode, that's why the mode is usually
announced before transmitting a picture.

So, let's recapitulize; first step is to connect the audio output of you receiver to the input of your computer's sound card, or you can simply place a microphone connected to your PC, near the loudspeaker of your receiver, but pay attention, any other external sound will have an effect on your received picture!
The next step is to tune your receiver to one of the mentioned frequencies, let's take 14.230 as it is the most used.
If there is a transmitted signal, you will see the sound spectrum in the scope window (as in the picture below)

 SSTV's sound frequencies are defined as follows:
 - Synchronization pulse 1200 Hz
 - Low level of the image 1500 Hz
 - High level of the image 2300 Hz
To tune the frequency, make the signal energy be laid between the 1500Hz and 2300Hz marker lines. Alternatively, you can tune the receiver so that the synchronization pulses are on the 1200Hz marker line.
Reception mode is usually set automatically, if not try to switch between modes so you have the best picture.
The received picture's quality will depend on receiving conditions (propagation,
noise, interference...). If these conditions are good enough, the quality and
beauty will amaze you! You’ll want to save some of these to disk and build a
library of your own.

        

                                 EA2JO                                               F5JRL

 

    

Reference:
MMSSTV help
QST Septembre 1990