1.6 KiB
Audio Modem for Air-Gapped Communication
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, which will not saturate the microphone.
The modem's bitrate is currently 8kbps (constellation diagram) - so it should have no problem sending a simple transaction in O(second). 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 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:
$ virtualenv env
$ source env/bin/activate
$ pip install reedsolo numpy
$ python recv.py
I would be happy to continue developing this library, in order to be able to integrate it with popular Bitcoin wallets, to support air-gapped transaction signing.