The SSTV amateur mode easily explained
It is not my intention to spend a million words on SSTV, as tons of technical pages have already been written and today they are available on the Internet. As too many websites have been built around this topic, should you need additional details you may easily refer to them, while the purpose of *this* website instead is to introduce you to the pragmatic aspects of this branch of ham radio communications.
In old analog fast scan television broadcasts, multiple complete frames per second were transmitted and they were the result of continuous interlaced scans consisting of 525 or 625 lines each, depending on the standard adopted. To keep it short (and simple), as this kind of video transmission will take about 5 MHz bandwidth or more, it is not suitable for amateur radio communications across the HF spectrum where we must contain our emissions within kHz, not MHz. SSTV was conceived just to send images over a narrow bandwith, despite of some technical limitations that won't allow transmission of moving pictures like commercial TV broadcasters.
SSTV is the acronym for Slow Scan Television. [aka Narrow Band Television] - It's about a way of transmitting still pictures by analog frequency modulation, so it's *not* a digital transmission method actually. Typical bandwidth is roughly 2.7kHz, the same as ssb voice operation. When transmission occurs information ranges from 1500Hz (black) to 2300Hz (white color) while a sync pulse is sent at 1200Hz. The concept is based on audio shift keying to transmit image scanlines. A 30ms vertical sync pulse is sent at the end of a frame (picture) while horizontal sync pulses will be issued between lines and last 5ms each. For every single line scanned, any variations in pixel values such as grayscale/brighness and color will produce a frequency shift in an audio signal that can be fed into an SSB transceiver. Every time a transmission is started a calibration header and a VIS code are sent first. A VIS code is an identifier and it's the only digital thing in modern SSTV; it was conceived to allow remote stations to automatically set reception to the correct video mode (e.g., Martin 1). Then the actual transmission follows and a single picture will be aired line by line; you will hear its well known chirpy sound due to continuous shifts occurring to the transmitted signal.
Today SSTV is simple and not so much expensive if you already own a good stable SSB transceiver and a personal computer equipped with a medium-quality soundcard. All you will need to start is a small I/O audio device to interface your rig to your PC and obviously, your sstv application software. Your audio interface must be capable of PTT operations if you are going to transmit. On the internet you'll find plenty of free sstv applications running fine even on low speed computers, so that's it. Have fun with Slow Scan Television and please remember: sstv transmitting operation is FULL DUTY CYCLE. Reduce RF to half power (or even less) simply using computer or interface volume control to protect your rig from overheating.
One last thing I have to tell you. SSTV was born on 27 MHz and its early pioneer, Copthorne "Coppie" Macdonald began on 11 meters first sstv experiments in 1958. One more reason for SSTV to maintain a place of honour on what today we are calling "the freeband".
All the best to you and I hope we can meet each other very soon, over the airwaves!
73 de 1SFØ72