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Comment by brian-armstrong | Hacker News Reader

FSK across the room? Does that have noise restrictions?

I’ve found all the phase modulation schemes work poorly across the room. DSSS works ok but is very slow. I would be curious how reliable FSK is in that situation.

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With 1270Hz/1070Hz you've got over 3 complete cycles to identify the tone at 300 baud, at higher frequency pairs it gets better. The person who wrote it was Tom Lyon at Sun but I bet I could reproduce it with a gnuradio-companion flow graph. That would make it pretty easy for anyone to play with it.

Interesting, I will have to give it a try then. Thanks!

I don't have first hand experience but I believe a big problem with sound channels is multipath propagation/challenging frequency response. FSK naturally deals quite well with those; it's similar to OFDM but more robust due to single carried symbols. You can extend the symbol period as long as you like while using an adequate number. Finally in challenging environments it'd a good idea to use error correction, not just error detection as done here -- you can (in theory) achieve reliable transmission over arbitrarily noisy channels by adding redundancy.

I have experimented with FSK modulation myself and was able to achieve quite reliable transmission across the room [1]. Although the bandwidth is quite low (8-16 B/s), it works most of the time.

[1] https://ggerganov.github.io/jekyll/update/2018/05/31/data-ov...

Neat library!

Your post mentions not finding any reliable modem libraries. I'm curious what issues you had with mine, if you tried it?

https://quiet.github.io/quiet-js

Hi, I did try it and it was working quite good when the devices (phone and laptop) were within ~30 cm. After increasing the distance and putting on the TV in the background it wasn't working that well any longer. I tried only the default protocol with audible frequencies (the ultrasound option didn't work, but my phone does not support the ultrasound option with my application too. guess it's hardware limitation).

I am willing to sacrifice bandwidth to achieve better reliability of the communication and I believe I was able to achieve it. Although, I haven't done tests with many other devices/conditions than my own, so my result might be biased.

Interesting. What modulation does yours use?

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