This program can be used to transmit a specified file between 2 computers, using a simple audio cable (for better SNR and higher speeds) or a simple headset, allowing true air-gapped communication (via a speaker and a microphone).

The sender modulates an input binary data file into an 32kHz audio file, which is played to the sound card, using aplay Linux utility.

The receiver side uses arecord Linux utility to record the transmitted audio to an audio file, which is demodulated concurrently into an output binary data file.

The process requires a single manual calibration step: the transmitter has to find maximal output volume for its sound card, which will not saturate the receiving microphone.

The modem is using OFDM over an audio cable with the following parameters:

Sampling rate: 32 kHz
BAUD rate: 1 kHz
Symbol modulation: 64-QAM
Carriers: (1,2,3,4,5,6,7,8) kHz

This way, modem achieves 48kpbs bitrate = 6.0 kB/s.

A simple CRC-32 checksum is used for data integrity verification on each 1KB data frame.

Installation

Run the following command (will also download and install numpy and bitarray packages):

$ sudo pip install amodem

For graphs and visualization (optional), install:

$ sudo pip install matplotlib

Calibration

Connect the audio cable between the sender and the receiver, and run the following scripts:

On the sender's side:

~/sender $ amodem send --calibrate

On the receiver's side:

~/receiver $ amodem recv --calibrate

Increase the sender computer's output audio level, until the received amplitude and peak values are not higher than 0.5, while the coherence is 1.0 (to avoid saturation).

Testing

Prepare the sender (generate random binary data file to be sent):

~/sender $ dd if=/dev/urandom of=data.tx bs=125kB count=1 status=none
~/sender $ sha256sum data.tx
008df57d4f3ed6e7a25d25afd57d04fc73140e8df604685bd34fcab58f5ddc01  data.tx

Start the receiver:

~/receiver $ amodem recv >data.rx

Start the sender:

~/sender $ amodem send <data.tx

After the receiver has finished, verify that the file's hash is the same:

~/receiver $ sha256sum data.rx
008df57d4f3ed6e7a25d25afd57d04fc73140e8df604685bd34fcab58f5ddc01  data.rx

Visualization

Make sure that matplotlib package is installed, and run (at the receiver side):

 ~/receiver $ amodem recv --plot >data.rx