Hi all,

I have been working during the last weeks on an audio modem library in Python: https://bitbucket.org/romanz/amodem
EDIT: moved to GitHub: https://github.com/romanz/amodem.

I am using a simple headset, whose speaker is connected to the transmitting PC and the microphone is connected to the receiving PC.
Then, I bring the speaker and the microphone close together and use the PC's sound cards to perform the communication.

The sender is modulating data.send binary file using send.py script into 32kHz audio file (tx.int16), which is played using aplay Linux utility.
The receiver is using arecord Linux utility to record the audio file into rx.int16 32kHz audio file, which is demodulated by recv.py script into data.recv binary file.
The process requires a single manual calibration step in order to find the maximal volume for the speaker, so that it won't saturate the microphone.

The modem's bitrate is currently 8kbps - so it should not have a problem sending a simple transaction in O(seconds).
Moreover, I am sure it can be optimized by using better modulation, error correction and better audio equipment.

Currently, the documentation is quite lacking, but today was the first time I successfully transmitted 1KB of random binary data between 2 PCs, so I am quite excited
The recorded audio file is currently stored at rx.int16 - and can be demodulated by running:

I would be happy to continue developing this library, in order to integrate it with popular Bitcoin wallets, aiming to support air-gapped transaction signing.

Comments are welcome