This program can transmit a file between 2 computers, using a simple headset, allowing true air-gapped communication (via a speaker and a microphone), or an audio cable (for higher transmission speed).
The sender modulates the input data into an audio signal, which is played to the sound card.
The receiver records the audio, and demodulates it back to the original data.
The process requires a single manual calibration step: the transmitter has to find the optimal output volume for its sound card, which will not saturate the receiving microphone and provide good enough Signal-to-Noise ratio for the demodulation to succeed.
HackerNews discussion: https://news.ycombinator.com/item?id=17333257
The modem is using OFDM over an audio cable with the following parameters:
- Sampling rate: 8/16/32 kHz
- Baud rate: 1 kHz
- Symbol modulation: BPSK, 4-PSK, 16-QAM, 64-QAM, 256-QAM
- Carriers: 2-11 kHz (up to ten carriers)
This way, modem may achieve 80kbps bitrate = 10 kB/s (for best SNR).
A simple CRC-32 checksum is used for data integrity verification on each 250 byte data frame.
Make sure that all the required packages are installed (on Debian):
$ sudo apt-get install python-numpy python-pip portaudio19-dev git
Get the latest released version from PyPI:
$ pip install --user amodem
Or, try the latest (unstable) development version from GitHub:
$ git clone https://github.com/romanz/amodem.git $ cd amodem $ pip install --user -e .
For graphs and visualization (optional), install matplotlib Python package.
For validation, run:
$ export BITRATE=48 # explicitly select high MODEM bit rate (assuming good SNR).
$ amodem -h
usage: amodem [-h] {send,recv} ...
Audio OFDM MODEM: 48.0 kb/s (64-QAM x 8 carriers) Fs=32.0 kHz
positional arguments:
{send,recv}
send modulate binary data into audio signal.
recv demodulate audio signal into binary data.
optional arguments:
-h, --help show this help message and exit
On, Windows you may download the portaudio library from MinGW. Then, you should specify the DLL using the following command-line flag:
-l AUDIO_LIBRARY, --audio-library AUDIO_LIBRARY
File name of PortAudio shared library.
Connect the audio cable between the sender and the receiver, and run the following scripts:
On the sender's side:
~/sender $ export BITRATE=48 # explicitly select high MODEM bit rate (assuming good SNR). ~/sender $ amodem send --calibrate
On the receiver's side:
~/receiver $ export BITRATE=48 # explicitly select high MODEM bit rate (assuming good SNR). ~/receiver $ amodem recv --calibrate
If BITRATE is not set, the MODEM will use 1 kbps settings (single frequency with BPSK modulation).
Change the sender computer's output audio level, until all frequencies are received well:
3000 Hz: good signal 4000 Hz: good signal 5000 Hz: good signal 6000 Hz: good signal 7000 Hz: good signal 8000 Hz: good signal 9000 Hz: good signal 10000 Hz: good signal
If the signal is "too weak", increase the sender's output audio level.
If the signal is "too strong", decrease the sender's output audio level.
If the signal is "too noisy", it may be that the noise level is too high
or that the analog signal is being distorted.
Please run the following command during the calibration session,
and send me the resulting audio.raw file for debugging:
~/receiver $ arecord --format=S16_LE --channels=1 --rate=32000 audio.raw
You can see a screencast of the calibration process.
Prepare the sender (generate a random binary data file to be sent):
~/sender $ dd if=/dev/urandom of=data.tx bs=60KB count=1 status=none ~/sender $ sha256sum data.tx 008df57d4f3ed6e7a25d25afd57d04fc73140e8df604685bd34fcab58f5ddc01 data.tx
Start the receiver (will wait for the sender to start):
~/receiver $ amodem recv -vv -o data.rx
Start the sender (will modulate the data and start the transmission):
~/sender $ amodem send -vv -i data.tx
A similar log should be emitted by the sender:
2015-02-06 18:12:46,222 DEBUG Audio OFDM MODEM: 48.0 kb/s (64-QAM x 8 carriers) Fs=32.0 kHz 2015-02-06 18:12:46,222 INFO PortAudio V19-devel (built Feb 25 2014 21:09:53) loaded 2015-02-06 18:12:48,297 INFO Sending 2.150 seconds of training audio 2015-02-06 18:12:48,297 INFO Starting modulation 2015-02-06 18:12:49,303 DEBUG Sent 6.000 kB 2015-02-06 18:12:50,296 DEBUG Sent 12.000 kB 2015-02-06 18:12:51,312 DEBUG Sent 18.000 kB 2015-02-06 18:12:52,290 DEBUG Sent 24.000 kB 2015-02-06 18:12:53,299 DEBUG Sent 30.000 kB 2015-02-06 18:12:54,299 DEBUG Sent 36.000 kB 2015-02-06 18:12:55,306 DEBUG Sent 42.000 kB 2015-02-06 18:12:56,296 DEBUG Sent 48.000 kB 2015-02-06 18:12:57,311 DEBUG Sent 54.000 kB 2015-02-06 18:12:58,293 DEBUG Sent 60.000 kB 2015-02-06 18:12:58,514 INFO Sent 60.000 kB @ 10.201 seconds 2015-02-06 18:12:59,506 DEBUG Closing input and output
A similar log should be emitted by the receiver:
2015-02-06 18:12:44,848 DEBUG Audio OFDM MODEM: 48.0 kb/s (64-QAM x 8 carriers) Fs=32.0 kHz 2015-02-06 18:12:44,849 INFO PortAudio V19-devel (built Feb 25 2014 21:09:53) loaded 2015-02-06 18:12:44,929 DEBUG AsyncReader thread started 2015-02-06 18:12:44,930 DEBUG Skipping 0.100 seconds 2015-02-06 18:12:45,141 INFO Waiting for carrier tone: 3.0 kHz 2015-02-06 18:12:47,846 INFO Carrier detected at ~2265.0 ms @ 3.0 kHz 2015-02-06 18:12:47,846 DEBUG Buffered 1000 ms of audio 2015-02-06 18:12:48,025 DEBUG Carrier starts at 2264.000 ms 2015-02-06 18:12:48,029 DEBUG Carrier symbols amplitude : 0.573 2015-02-06 18:12:48,030 DEBUG Current phase on carrier: 0.061 2015-02-06 18:12:48,030 DEBUG Frequency error: -0.009 ppm 2015-02-06 18:12:48,030 DEBUG Frequency correction: 0.009 ppm 2015-02-06 18:12:48,030 DEBUG Gain correction: 1.746 2015-02-06 18:12:48,198 DEBUG Prefix OK 2015-02-06 18:12:48,866 DEBUG 3.0 kHz: SNR = 34.82 dB 2015-02-06 18:12:48,866 DEBUG 4.0 kHz: SNR = 36.39 dB 2015-02-06 18:12:48,867 DEBUG 5.0 kHz: SNR = 37.88 dB 2015-02-06 18:12:48,867 DEBUG 6.0 kHz: SNR = 38.58 dB 2015-02-06 18:12:48,867 DEBUG 7.0 kHz: SNR = 38.86 dB 2015-02-06 18:12:48,867 DEBUG 8.0 kHz: SNR = 38.63 dB 2015-02-06 18:12:48,867 DEBUG 9.0 kHz: SNR = 38.07 dB 2015-02-06 18:12:48,868 DEBUG 10.0 kHz: SNR = 37.22 dB 2015-02-06 18:12:48,869 INFO Starting demodulation 2015-02-06 18:12:49,689 DEBUG Got 6.000 kB, SNR: 41.19 dB, drift: -0.01 ppm 2015-02-06 18:12:50,659 DEBUG Got 12.000 kB, SNR: 41.05 dB, drift: -0.00 ppm 2015-02-06 18:12:51,639 DEBUG Got 18.000 kB, SNR: 40.96 dB, drift: -0.00 ppm 2015-02-06 18:12:52,610 DEBUG Got 24.000 kB, SNR: 41.47 dB, drift: -0.01 ppm 2015-02-06 18:12:53,610 DEBUG Got 30.000 kB, SNR: 41.06 dB, drift: -0.00 ppm 2015-02-06 18:12:54,589 DEBUG Got 36.000 kB, SNR: 41.37 dB, drift: -0.00 ppm 2015-02-06 18:12:55,679 DEBUG Got 42.000 kB, SNR: 41.13 dB, drift: -0.00 ppm 2015-02-06 18:12:56,650 DEBUG Got 48.000 kB, SNR: 41.31 dB, drift: -0.00 ppm 2015-02-06 18:12:57,631 DEBUG Got 54.000 kB, SNR: 41.23 dB, drift: +0.00 ppm 2015-02-06 18:12:58,605 DEBUG Got 60.000 kB, SNR: 41.31 dB, drift: +0.00 ppm 2015-02-06 18:12:58,857 DEBUG EOF frame detected 2015-02-06 18:12:58,857 DEBUG Demodulated 61.205 kB @ 9.988 seconds (97.9% realtime) 2015-02-06 18:12:58,858 INFO Received 60.000 kB @ 9.988 seconds = 6.007 kB/s 2015-02-06 18:12:58,876 DEBUG Closing input and output 2015-02-06 18:12:58,951 DEBUG AsyncReader thread stopped (read 896000 bytes)
After the receiver has finished, verify the received file's hash:
~/receiver $ sha256sum data.rx 008df57d4f3ed6e7a25d25afd57d04fc73140e8df604685bd34fcab58f5ddc01 data.rx
You can see a screencast of the data transfer process.
The audio can be written/read to an intermediate PCM file (instead of the speaker/microphone) using:
$ echo 123 | amodem send -o /tmp/file.pcm Sending 0.800 seconds of training audio Starting modulation Sent 0.004 kB @ 0.113 seconds $ amodem recv -i /tmp/file.pcm Waiting for carrier tone: 2.0 kHz Carrier detected at ~150.0 ms @ 2.0 kHz Carrier coherence: 100.000% Carrier symbols amplitude : 1.000 Frequency error: 0.000 ppm Starting demodulation 123 Received 0.004 kB @ 0.011 seconds = 0.376 kB/s
Make sure that matplotlib package is installed, and run (at the receiver side):
~/receiver $ amodem recv --plot -o data.rx
