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I have a WebAudio modem that works reasonably well, and I went a different direction with it. Mine uses a C DSP/SDR library with good error correction, run thro

I have a WebAudio modem that works reasonably well, and I went a different direction with it. Mine uses a C DSP/SDR library with good error correction, run through emscripten to generate the samples. You can try it here https://quiet.github.io/quiet-js https://quiet.github.io/quiet-js

ravenstine 9y ago
Actually, I think it was your project that gave me the idea to attempt near-ultrasonic transmission. I could only get to ~15000hz before it would just not work and I never figured out why that was. Maybe there's a flaw in how I used the Goertzel algorithm. (reason I went with Goertzel is because I built it for a previous project and wanted to see how far I could take it) Is that example page new? That's brilliant. Your ultrasonic transmission seems to work pretty consistently.
- tonmoy 9y ago
  A lot of microphones and speakers have built in hardware low pass filters that kill anything above 15kHz
- Arve 9y ago
  Someone pointed out low-pass filters to you, but that's not the entire story. Built-in speakers in phones and laptops generally don't have a naturally extended frequency response. A while ago, I made audio measurements of the built-in speakers on the iPhone 7 Plus - https://imgur.com/gallery/DRbu5 https://imgur.com/gallery/DRbu5 The iPhone's extension is not entirely atypical of such devices, and leaves you with a reliable passband of 500 Hz to 10 kHz - above and below those frequencies, you're getting too much noise. If you're airgapping with audio cables, you'll have a reliable passband of 20 - 20 000 Hz or thereabouts, with some caveats for built-in audio on devices such as a Raspberry Pi that fizzle out at 15 kHz. As noted by brian-armstrong's comment, you'll only get reliable near-ultrasonic airgapping over a wired connection.
  - brian-armstrong 9y ago
    This isn't correct, by the way. Quiet's near-ultrasonic mode (~19kHz) works fine from several feet away using phone hardware. As long as you're using relatively narrowband communication, it works fine.

ravenstine 9y ago

Actually, I think it was your project that gave me the idea to attempt near-ultrasonic transmission. I could only get to ~15000hz before it would just not work and I never figured out why that was. Maybe there's a flaw in how I used the Goertzel algorithm. (reason I went with Goertzel is because I built it for a previous project and wanted to see how far I could take it) Is that example page new? That's brilliant. Your ultrasonic transmission seems to work pretty consistently.

tonmoy 9y ago
A lot of microphones and speakers have built in hardware low pass filters that kill anything above 15kHz
Arve 9y ago
Someone pointed out low-pass filters to you, but that's not the entire story. Built-in speakers in phones and laptops generally don't have a naturally extended frequency response. A while ago, I made audio measurements of the built-in speakers on the iPhone 7 Plus - https://imgur.com/gallery/DRbu5 https://imgur.com/gallery/DRbu5 The iPhone's extension is not entirely atypical of such devices, and leaves you with a reliable passband of 500 Hz to 10 kHz - above and below those frequencies, you're getting too much noise. If you're airgapping with audio cables, you'll have a reliable passband of 20 - 20 000 Hz or thereabouts, with some caveats for built-in audio on devices such as a Raspberry Pi that fizzle out at 15 kHz. As noted by brian-armstrong's comment, you'll only get reliable near-ultrasonic airgapping over a wired connection.
- brian-armstrong 9y ago
  This isn't correct, by the way. Quiet's near-ultrasonic mode (~19kHz) works fine from several feet away using phone hardware. As long as you're using relatively narrowband communication, it works fine.