I haven’t had much time for actual experimentation, but I have spent some time researching and some more time thinking about how to properly implement and test analog SSTV modulators and demodulators. I haven’t made much actual progress, but I thought I’d document some of the information that I’ve discovered and my current thoughts.
First of all, I began by trying to discover a bit of the history. Luckily, I’m an ARRL member, so I have access to all of the back issues of QST magazine. In this case, one only has to search for the author “Copthorne MacDonald” to find the 1958-1961 papers on the first amateur uses of slow scan television. I’m still perusing them a bit, but in the first paper, MacDonald suggested using a sub-carrier amplitude modulated signal which would be incompatible with virtually all SSTV modes used today, but in the 1961 he proposed the “classic” black and white, 8 second mode, consisting of 120 lines, frequency modulating between 1500 and 2300 Hz. These numbers were apparently chosen mostly for compatibility with existing telephone based fax standards of the day, but also nicely fit within the reasonably uniform region of most voice based transceivers in use for amateur radio.
This kind of “classic” black and white SSTV was apparently fairly common during the 1970s.
Here is some classic SSTV, re-rendered from a cassette recording done in 1980, documenting a QSO between KG4I in Birmingham, AL and N2BJW in Elmira, NY.
Here’s a clever way to say “Happy Trails” from AC5D:
Here’s a more modern implementation, which perhaps demonstrates a wider variety of imagery (a lot of it supermodels), still in the classic 120×120 black and white format:
Why bother with this simple mode? Well, just because it is simple. I also like the speed: the immediacy of the mode is kind of cool. I’m working on good, solid, easy to understand code to do this mode, and will post about my experiments in the presence of noise when I get some results.