Slow Scan Television (SSTV ) is a method of transmitting still pictures over a very narrow bandwidth, on frequencies normally used for speech by Radio Amateurs .
It was invented by Copthorne Macdonald and first used by Radio Amateurs, but now it is used bymany other services. The original idea was to find a way to transmit a television picture over a single speech channel. This meant that what was then a 3MHz wide television picture had to be reduced to around 3kHz. It was therefore decided that the scanning rates should be very slow and this decision ruled out the use of moving pictures. The choice of time base for synchronising was the readily available domestic power supply at 50 or 60 Hz .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 original SSTV systems were based on ex-government Radar screens and long persistence cathode ray tubes. Today most systems are computer based for ease of use, giving full colour with up to 16.7 million colours and transmission times of up to several minutes for each picture.
Unfortunately SSTV does not have the benefit of error checking and this can cause problems espacially when you are trying to receive that 'perfect' picture from Japan and the last quarter of the picture is spoilt by someone calling CQ CQ or tuning over the top of the picture.
So how does one get started with SSTV. Well that is easy. All you need is a PC running Windows 9X, 2000, XP or Vista. There are a number of SSTV programs available that allow you to transmit and receive SSTV using your PC sound card to encode and decode high quality color pictures. You also need an interface to connect the radio to the soundcard of your computer. (This interface also isolates the rradio electrically from yourr computer). The output of the sound card must be connected through an attenuator to the input of the transceiver. If you don't disable the microphone while transmitting SSTV pictures the microphone will pick up noise that puts black lines through your nice color picture. However many of the interfaces available today disable the microphone when transmitting a picture yet keep the microphone ready for voice communications. Interfaces normally cover both HF SSB and VHF FM transmissions.
Digital SSTV
DRM is a new digital radio standard for use by HF broadcasters and uses a bandwidth from 4.5 up to 20KHz utilising OFDM modulation. It provides near FM quality over HF and has the ability to send data. It has particular advantages over analogue SSTV in terms of the time it takes to encode/decode data. File transfer times can be up to 3 times faster than that experienced with analogue SSTV. Not surprisingly, therefore, it has been chosen to replace the RDFT system of data transfer used by analogue SSTV.
DRM allows larger files to be sent in less time with a resultant improvement in picture quality. More importantly, if pictures suffer from QRM there is a repair facility so 100% reception of data is not essential. Try it for yourself and see. For the SSTV enthusiast, the choice between analogue and digital is now yours to decide.
FSTV
Fast Scan Television (FSTV) uses considerably more bandwidth than SSTV.
FSTV is what moving pictures are called when they is sent by radio amateurs on the amateur bands and this distinguishes it from SSTV which, as you have read above, is used to transmit still pictures.
Radio amateurs cannot set up regular television stations for the general public to view, but they are allowed to use any and all means to establish two-way radio communication so long as they are not violating any licence regulations, and that their mode of communication is documented publicly. So some radio amateurs use camcorders and build some special hardware to interface it with their radios. They use their station to talk to each other, and maybe show each other what their radio shacks look like.
FSTV activity normally takes place on the micro-wave bands.
If you wish to learn more about this aspect of the hobby, specialist publications on amateur television are available from the ARRL and the RSGB
Updated 13 June, 2010 21:38:27 GMT