Testing MMSSTV with messed-up signals

Posted on 2017-10-31 at 05:11 in Random, Videos by Robbi-985.

I applied a couple of strong vibratos to an SSTV signal (a picture encoded as one long sound) just to see what effect the unstable frequency would have when decoded using MMSSTV. Amazingly, it was still able to detect the signal and start decoding, but of course, it looks too scrambled to make out. I like how the artifacts look, though.

[Watch in HD]

I’m using Virtual Audio Cable to connect MMSSTV (encoder/decoder) with Audacity (which I used to apply the excessive vibratos), and Audio Repeater to “echo” the sound from the virtual cable to the speakers, so I can hear it live (and capture it in the video). Audio Repeater introduces about half a second of delay, though.

SSTV (slow-scan television) is a way of transmitting pictures over the air when you have very little bandwidth available (around 2.6 KHz, vs several MHz for ordinary analogue TV), sometimes used by amateur radio operators. It works by modulating the frequency of a sine wave according to the brightness of the pixels (per colour channel) row-by-row, so by applying a vibrato to the sound, the sound is pulled into and out of phase (but still stays in-phase on average). In other words, the rows are being shifted left/right (each colour channel independently). That’s why the image is rough along the vertical edges instead of being a nice straight line – sometimes, each colour channel is being pulled out of phase and dragged to the right, and sometimes it’s being pulled to the left (which causes it to wrap back to the right with inverted colours, because it’s interrupting the time slot that was dedicated to a different colour channel). Fun stuff to mess around with!

Leave a Reply

Your email address will not be published. Required fields are marked *


You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>