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, TV towers used to transmit rasterized pictures (frames) at a given frame rate as television cameras would run continuous and interlaced scans of the scene consisting of 525 or 625 lines per second, depending on the standard adopted. To keep it short (and simple), as this kind of transmission would take about 5 MHz bandwidth or more it's not suitable for amateur radio communications across the HF spectrum where we must contain our emissions within kHz, not MHz. SSTV was conceived to send images over a narrow bandwith, despite 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 and *not* a digital mode. Don't be fooled by the fact that today's SSTV is completely managed by software, the only thing that truly defines SSTV is *modulation*. Let me be frank with you, in my opinion 'Digital SSTV' strictly speaking does not even exist. EasyPal and other DRM-based software are actually computer applications designed to operate a data tranfer over radio using QAM-OFDM multiplexing. In addition, EasyPal is also capable of taking advantage of the Internet when utilizing the so-called "hybrid mode". Kinda interesting but SSTV has nothing to do with this thing! No slow scan, no fast scan either... there's no image scan at all. 'D-SSTV' is a mere file transfer, nothing more, nothing less. Period! - Getting back on topic, typical bandwidth for an analog SSTV signal is roughly 2.7kHz, the same as ssb voice operation. Whenever a station transmits an SSTV signal a vertical sync pulse establishes the start of a new frame. Vertical sync pulses have a duration of 30ms and are transmitted as bursts of 1200Hz tones at the beginning of every image. Picture information ranges from 1500Hz (black) to 2300Hz (white color). The concept is based on audio shift keying to transmit image scanlines. Before each line another burst signal is transmitted for line synchronization. Horizontal sync pulses are issued between lines and will last 5ms each. For every single line scanned any variation in pixels such as grayscale, brightness, and today even 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 particular code called Vertical Interval Signaling are sent along with the vertical sync pulse. A VIS code is an identifier conceived to allow remote stations to automatically set reception to the correct video mode (e.g., Scottie 1) and it's the only digital thing in today's SSTV. Hence actual transmission follows and a single picture will be aired line by line; that chirpy sound we all know about is 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 honor and survive in this chunk of HF today known as the "freeband".
All the best to you and I hope we can meet each other very soon, over the airwaves!
73 de 1SFØ72