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Comment by m0ther | Hacker News Reader

Can you get it to transfer data ultrasonically?

Computer sound devices tend to have a roll-off filter just beneath the Nyquist limit of the DAC. So, higher-end audio devices that can sample at 96 KHz or higher could presumably do so.

But, then, you'd need speakers that can reproduce it and a microphone that can pick it up, too. Also, reflections are surely a problem at higher rates. The examples shown are well within the range of human hearing (but, old modems had to work within the bandpass range allowed by phone networks, which was rolled off at something like 8 KHz).

Lots of devices which sample at 96kHz still have a 20kHz roll off filter :-(

I read that some recording studios were filtering to ~40 kHz, on the premise that the ultrasonics would generate audible intermods in the ear. Maybe it’s audiophile nonsense.

In any case, the 96 kHz sampling would yield an additional 3 dB dynamic range vs the 44 kHz sample rate.

I happen to have a small recording studio. I just looked up specs, my speakers will do 40khz, but my mics cut out past 20khz. I'd imagine even if I had a mic that could pick it up, audio drivers might also cut it out.

It depends on what you mean by "ultrasonically". You can quite easily send and recieve 19 khz tone using ordinary 44.1khz equipment, and virtually nobody is going to hear it.

Young folks and dogs would probably be annoyed. I could hear up to about 19k into my 20s. So, depending on your use case, it might not be workable. I also used to get headaches and my ears would feel fatigued from a CRT monitor that squealed at a frequency I couldn't "hear" (but could detect by other means), so I suspect those frequencies do interact with our ears and brains in ways that we can't "hear" in the usual sense. (Though I also know that there's a lot of woo around this issue in audiophile circles...I think anything beyond about 22k can be considered safely out of range of human hearing. Our ears just don't have receivers for those frequencies.)

I'm trying to think of use cases for ultrasonic data transmission, and they're mostly nefarious. Traversing an air gap, for instance. Higher frequencies are difficult to transmit over distances and through walls, so you'd need to get your receiver into the same room or make it a very high SPL.

I have a wifi camera that uses bleeps and whistles and warbles from a phone or tablet to configure itself before it gets on the network, which is pretty neat but seems weird in a modern world. But, I guess that's something that could be done ultrasonically to make it seem more like magic. But, since it is already pretty error-prone, I would bet pushing tablets and phones (including crappy ones) to accurately produce very high frequencies at a volume sufficient to program a nearby device wouldn't be worth the added magicalness.

Could the reflections be used to your advantage, like with MIMO on wifi?

Around the turn of the century when PSK31 data modulation was new to ham radio operators, it was a very standard demo at club meetings or whatever to fire up two laptops using mic/speakers instead of radios and talk a couple feet which was considered impressive because most other modulation methods are wider band and will not work well.

The relevance to your question, is hams being hams, they gotta experiment, turn of the century laptop hardware often had codecs that operated up to 96 KHz or whatever, but speakers usually cut off somewhere around the high level of human hearing, so operating at 10 KHz worked and might be silent to old people with partial hearing loss, but you could forget about 25 KHz even if the signal looked great on an oscilloscope because the speakers and mics were not broadband enough. Very few customers care about audio quality in frequency bands they can't hear.

The same hardware hooked up to a simple I/Q software defined radio works fine up to the limits imposed by Nyquist and the claimed hardware sample rate, so its obviously limited by mic/speaker not the A/D converters.

This topic comes up repeatedly on HN. If HN had a wiki this meme would be a strange attractor for the HN community.

I could see "there's a message being transferred, and it's just outside of our ability to sense it" being catnip to the kid in all of us. Like invisible ink handwritten notes for adults. I looked up specs, my mic doesn't go beyond 20KHz. Do clubs like this still exist? I get the sense that I wasn't alive when they were most popular.

Probably not, but what you could try to do is to transfer data in such a way that people would ignore the sounds because they thought the sounds were not data being transmitted.

Make it sound like birdsong in the presence of other birds, make it sound like fragments of human speech when there are people present, make it sound like fan noise in a DC and so on.

That is a sexy idea. I feel bad that my second thought was "but likely has no commercial application". Now I wonder when I got old... I'll be writing an audio engine in the next few months; I'll try hacking together something like this then :D.

Very cool :) Please let me know if you get it working.

And then you record outside and find out that the birds are gossiping about us humans all along!

Sounds like you're discussing the hypotheticals of https://en.wikipedia.org/wiki/BadBIOS

If you want to try ultrasonic data transfer from your browser, check out https://quiet.github.io/quiet-js

quietnet does that (near-ultrasonic) (https://github.com/Katee/quietnet)

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