Audio Modem Communication Library
Description
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, which is played to the sound card.
The receiver side records the transmitted audio, 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 48kbps bitrate = 6.0 kB/s.
A simple CRC-32 checksum is used for data integrity verification on each 250 byte data frame.
Installation
Make sure that numpy and PyAudio Python packages are installed.
$ pip install numpy
$ pip install git+http://people.csail.mit.edu/hubert/git/pyaudio.git
or, on Debian/Ubuntu:
$ sudo apt-get install python-numpy python-pyaudio
Clone and install latest version:
$ git clone https://github.com/romanz/amodem.git
$ pip install --user -e amodem
For graphs and visualization (optional), install:
$ pip install matplotlib
validation
Run:
$ export BITRATE=48 # explicitly select highest MODEM bit rate (assuming good SNR).
$ amodem-cli -h
usage: amodem-cli [-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
Calibration
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 highest MODEM bit rate (assuming good SNR).
~/sender $ amodem-cli send --calibrate
- On the receiver's side:
~/receiver $ export BITRATE=48 # explicitly select highest MODEM bit rate (assuming good SNR).
~/receiver $ amodem-cli 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:
1000 Hz: good signal
2000 Hz: good signal
3000 Hz: good signal
4000 Hz: good signal
5000 Hz: good signal
6000 Hz: good signal
7000 Hz: good signal
8000 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", the SNR is probably too low: decrease the background noise or increase the signal (without causing saturation).
Testing
- Prepare the sender (generate random binary data file to be sent):
~/sender $ dd if=/dev/urandom of=data.tx bs=16kB count=1 status=none
~/sender $ sha256sum data.tx
008df57d4f3ed6e7a25d25afd57d04fc73140e8df604685bd34fcab58f5ddc01 data.tx
- Start the receiver, which will wait for the sender to start:
~/receiver $ amodem-cli recv -vv >data.rx
- Start the sender, which will modulate the data and start the transmission:
~/sender $ amodem-cli send -vv <data.tx
- A similar log should be emitted by the sender:
2014-10-23 09:46:36,116 DEBUG MODEM settings: {'F0': 1000.0, 'Nfreq': 8, 'Fs': 32000.0, 'Npoints': 64, 'Tsym': 0.001} amodem:126
2014-10-23 09:46:36,116 DEBUG Running: ['aplay', '-', '-q', '-f', 'S16_LE', '-c', '1', '-r', '32000'] wave.py:20
2014-10-23 09:46:36,665 INFO Sending 2.150 seconds of training audio send.py:69
2014-10-23 09:46:36,665 INFO Starting modulation: <48.000 kbps, 64-QAM, 8 carriers> send.py:74
2014-10-23 09:46:37,735 DEBUG Sent 6.0 kB send.py:56
2014-10-23 09:46:38,794 DEBUG Sent 12.0 kB send.py:56
2014-10-23 09:46:39,440 INFO Sent 16.384 kB @ 2.754 seconds send.py:79
- A similar log should be emitted by the receiver:
2014-10-23 09:46:36,116 DEBUG MODEM settings: {'F0': 1000.0, 'Nfreq': 8, 'Fs': 32000.0, 'Npoints': 64, 'Tsym': 0.001} amodem:126
2014-10-23 09:46:36,238 DEBUG Running: ['arecord', '-', '-q', '-f', 'S16_LE', '-c', '1', '-r', '32000'] wave.py:20
2014-10-23 09:46:36,408 DEBUG Skipping 0.128 seconds recv.py:275
2014-10-23 09:46:36,409 INFO Waiting for carrier tone: 1.0 kHz recv.py:282
2014-10-23 09:46:37,657 INFO Carrier detected at ~886.0 ms @ 1.0 kHz: coherence=99.996%, amplitude=0.475 recv.py:40
2014-10-23 09:46:37,657 DEBUG Buffered 1000 ms of audio recv.py:64
2014-10-23 09:46:37,660 DEBUG Carrier starts at 9.531 ms recv.py:73
2014-10-23 09:46:38,119 DEBUG Prefix OK recv.py:108
2014-10-23 09:46:38,153 DEBUG Current phase on carrier: -0.497 recv.py:121
2014-10-23 09:46:38,153 DEBUG Frequency error: 0.02 ppm recv.py:123
2014-10-23 09:46:38,682 DEBUG 1.0 kHz: SNR = 34.20 dB recv.py:165
2014-10-23 09:46:38,715 DEBUG 2.0 kHz: SNR = 35.05 dB recv.py:165
2014-10-23 09:46:38,766 DEBUG 3.0 kHz: SNR = 35.52 dB recv.py:165
2014-10-23 09:46:38,803 DEBUG 4.0 kHz: SNR = 35.65 dB recv.py:165
2014-10-23 09:46:38,837 DEBUG 5.0 kHz: SNR = 35.03 dB recv.py:165
2014-10-23 09:46:38,869 DEBUG 6.0 kHz: SNR = 35.05 dB recv.py:165
2014-10-23 09:46:38,907 DEBUG 7.0 kHz: SNR = 34.80 dB recv.py:165
2014-10-23 09:46:38,943 DEBUG 8.0 kHz: SNR = 33.74 dB recv.py:165
2014-10-23 09:46:38,977 INFO Starting demodulation: <48.000 kbps, 64-QAM, 8 carriers> recv.py:197
2014-10-23 09:46:39,619 DEBUG Got 6.0 kB, realtime: 64.18%, drift: +0.02 ppm recv.py:215
2014-10-23 09:46:40,538 DEBUG Got 12.0 kB, realtime: 78.03%, drift: +0.02 ppm recv.py:215
2014-10-23 09:46:41,306 DEBUG EOF frame detected framing.py:60
2014-10-23 09:46:41,306 DEBUG Demodulated 16.520 kB @ 2.329 seconds (84.6% realtime) recv.py:244
2014-10-23 09:46:41,306 INFO Received 16.384 kB @ 2.329 seconds = 7.034 kB/s recv.py:247
- 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-cli recv --plot >data.rx
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