I was trying to determine the exact timings for the classic “8 second” black and white SSTV mode. Copthorne MacDonald suggests 15 lines per second, to make 120 lines in 8 seconds. The vertical sync pulse has a duration of 30ms, and the horizontal sync pulse duration is just 5ms. The sync frequency is 1200Hz, and black and white interpolate between 1500Hz and 2300Hz. The aspect ratio is 1:1 (to match the usual oscilloscope tubes of the day).
So, I wrote a program to generate my best guess from this basic description, and here’s an example sound file:
Here’s a spectrogram of the first 0.5 seconds or so, showing the vertical sync pulse, followed by a few scanlines:
I decided that the horizontal sync pulse should go in the final 5ms of each scanline (somewhat arbitrarilly).
So, the question of the hour is: can any modern software decode this basic image? I’ll be working on my own implementation, but I’m curious. I’ll leave it as a bit of a puzzle: if someone emails me the decoded image, you’ll be immortalized in these pages and have my thanks. Otherwise, you’ll just have to wait to see what the picture actually looks like in a day or so.