I had a Timex Data Watch over 30 years ago.

https://en.wikipedia.org/wiki/Timex_Datalink

It'd transfer phone numbers and calendar entries to the watch by blinking the screen of your computer. Kinda a 1 pixel version of this.

(I wonder where that ended up? From memory I threw it in a drawer when I finished the Windows 95 project I'd been working on, and switched to Mac. There was no Mac software to talk to it.)

> between devices that doesn't rely on online services -- no clue why we didn't solve that ages ago

Airdrop is a thing, but remember that Android phones are controlled by the online services company that wants you to use online services.

Indeed encryption should not be hard yet which situation do you have in mind where simply pulling away the emitting phone would not be enough? AFAIK the normal scenario is two persons in the same physical space so I am not sure what encryption would add.

As someone who has implemented acoustic modems (underwater comms)... yes, yes very annoying. It's like a demonic dolphin.

Bringing it into the ultra sonic band would help.

Have a look at minimodem. In short: yes, but it's slow and annoying.

Encryption is a toilet paper roll.

You can do PSK31 across a room with just audio, but it is a kind of annoying noise after a while.

It's interesting that you learn to recognise patterns in the burbling tones after a while. "CQ CQ CQ" is very distinctive and after a while you can hear your own callsign in it. I guess it's not that different to very fast Morse.

Oooooooooooo.

I bet that's how satellite radios get their firmware updates and stuff.

Actually I bet it's how they get almost everything aside from the audio streams themselves. Metadata. Channel art. Subscription info. Traffic data. Weather maps. Receiver firmware... The satellite is a textbook-perfect fountain, and the receiver may tune in at any time during the stream, and may miss any random number of packets, but fairly quickly it has what it needs.

I believe the various categories of metadata are sent at different rates according to how promptly a newly-awakened receiver should be able to update itself, so what's actually on the air is an interleaved collection of fountain streams, yeah? Hmm.

> The receiver can start receiving blocks from any point, receive blocks in any order, with any erasure probability – fountain codes will work as soon as you received K+ different blocks.

Thats just cool.

It is. Though there's almost always some uncompressed header so you know how to decompress it.