Thursday, July 19, 2018

This Photo Sounds Terrible -- Adventures in SSTV

Slow Scan TV (SSTV) is a technique for transmitting images over a radio link. The Apollo missions used it back in the day, and radio geeks use it today. At some point, I'll have to try using my SDR dongle to pick up these transmissions.

What my caught my attention about SSTV was this video by The Modern Rogue. In it, our intrepid hosts transmit photos using a pair of cheap walkie-talkies. The main ingredient to this hack is SSTV.

SSTV is interesting because it operates at voice frequencies. That is, you can hear it. Mind you, it doesn't sound great--think fax tones or modem screeches. However, what sounds like noise to us is quite intelligible to an SSTV decoder. To see what I mean, try this: install an SSTV Decoder App on your phone, launch it and hold your phone near your computer's speaker. Then click play on the video below:

If all goes well, you should see the sound translated into an image. Here's what I received when I decoded the above audio:

That's amazing, right?

The process is admittedly fragile; here's an attempt where I held the phone at some distance from the speaker:

But still, the image is definitely there.

To complete your SSTV toolkit, you're going to want to install an encoder. Once you do, you'll be able to translate any photo on your phone into the high pitch squeals of SSTV.

I'm not entirely sure the best way to use this hack. But it's remarkably to think that if you can talk to someone, you can also exchange images.

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