On the other day I was listening SSTV (yes, this is about SSTV 🙂 – again) transmissions and heard familiar sound that should not be there, a Windows USB disconnect-connect-disconnect-connect … … …
My analysis is that the user sending the SSTV picture has some RF leaking to the computer system he/she uses causing Windows to connect and disconnect some USB connected device.
This is a very good reminder about the importance of shielding computer systems from the RF noice generated by the HAM RIG, especially when using high power systems.
I do not know what is the real reason for this, could also be intentional injection of audio to the transmission with low enough frequency that it would not cause problems with the actual SSTV signalling, then again that is not allowed in the SSTV channel so I stick with my analysis about the problems with the sending computer.