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Comment by alephnil | Hacker News Reader

This will alternate between two different tones, one representing 0 and another representing one, each bit lasting 50 ms, giving a bitrate of 20 bits/s. This is obviously rather slow, so more a fun weekend experiment than a practical tool.

There is a really deep rabbit hole when it comes to doing this more efficiently, getting into signal processing, modulation, encoding, error correction and more.

And of cause, things like modems, tape storage on c64 and other home computers did this a long time ago. It is just that audio has such a low bandwidth that it is not possible to get up to the bandwidth we expect these days.

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I've played around with techniques like this too.

My thought was to make a multimedia tape zine where people could submit songs and accompanying short text. One track of a stereo cassette track would store the audio mixed to mono, the other would store the text encoded something like this. You'd play the tape in a cassette player outputing to a computer that would play one track to a speaker and decode the other to a thermal printer, so the text would also appear on a "tape."

The problem is, it's really hard to find working cassette players with stereo input and output...

I'm sure there are others but you can still find a 1980's Sharp 555 double casette players. I have one and had to do minimal maintenance on it and a bit of patching up. If you're not into repairing the electronics it's not a problem. pretty much any repair shop can get these back to working state. You can also find schematics on russian/asian DC++

I've been a bit obsessed with the idea of running an 80's themed ARG, and this would be perfect.

> bitrate of 20 bits/s

I can imagine this being used as a control protocol for home/building automation, 20 bits is quite a lot if you consider each bit a device being turned on or off only.

I still don't know why would anyone do this instead of wifi, but definitely way cooler. Also, anyone visits you the first time, there's a guaranteed 20 minute talk about the setup :)

Would make for a pretty stealthy bootstrapping protocol to share things like wifi keys between devices or to do keystroke logging.

speaking of stealth, there was this (hypothetical?) malware from 2013 which spread via computer speakers: https://en.wikipedia.org/wiki/BadBIOS

Thanks. I was looking for an audio sample of this data transfer and hoping for something like the old modem dial-up sound.

You're probably thinking of the initial handshake bit rather than what the middle of the data stream sounded like. https://www.windytan.com/2012/11/the-sound-of-dialup-picture...

yes it's a rabbit hole, I know it well! When PC became fast enough to be used as DSP engines with the soundcard I spent years playing with the creation of "digital modes" as HAM radio hobbyist.

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