MINIMODEM

NAME

minimodem − software audio Bell-type or RTTY FSK modem

SYNOPSIS

minimodem --tx [options] {baudmode}
minimodem --rx [options] {baudmode}

DESCRIPTION

minimodem is a command-line program which generates (or decodes) audio modem tones at any specified baud rate, emulating an old Bell-type or radio-teletype FSK modem. The tones can be played to (or recorded from) the system audio (PulseAudio or ALSA) or to an audio file.

minimodem can be used to transfer data between nearby computers using an audio cable (or just via sound waves), or between remote computers using radio, telephone, or another audio communications medium.

TX/RX MODE

−t, −−tx, −−transmit, −−write

transmit mode: generate audio tones

−r, −−rx, −−receive, −−read

receive mode: decode audio tones

OPTIONS

−a, −−auto-carrier

automatically detect mark and space frequences from carrier

−c, −−confidence min-snr-threshold

Set receive confidence minimum SNR threshold (default 2.0). This value acts as an FSK decoder "squelch" control. Increase to accept only very clean signals (up to INFINITY, but a value around 5.0 is more practical). Decrease to accept partial decoding of noisy signals (down to 1.0).

−l, −−limit max-snr-search-limit

Set receive confidence maximum SNR search limit (default 2.3). This value acts as a performance vs. analysis quality control. Increase (up to INFINITY) for a more pedantic analysis and higher CPU usage. Decrease (down to the min-snr-threshold) for a sloppier analysis, with lower CPU usage.

−8, −−ascii

ASCII 8−N−1

−5, −−baudot

Baudot 5−N−1.5

−f, −−file filename.wav

encode or decode an audio file (extension sets audio format)

−b, −−bandwidth {rx_bandwidth}
−M, −−mark {mark_freq}
−S, −−space {space_freq}
−T, −−txstopbits {m.n}
−q, −−quiet

Do not report CARRIER / NOCARRIER or signal analysis metrics.

−R, −−samplerate {rate}

Set the audio sample rate (default rate is 48000 Hz).

−A, −−alsa

Use ALSA as the audio output system instead of the default PulseAudio (depending on build configuration options).

−−lut={tx_sin_table_len}

Minimodem uses a precomputed sine wave lookup table of 1024 elements, or the size specified here. Use −−lut=0 to disable the use of the sine wave lookup table. (This option applies to −−tx mode only).

−−float-samples

Generate 32-bit floating-point format audio samples, instead of the default 16-bit signed integer format (applies to −−tx mode only; −−rx mode always uses 32-bit floating-point).

−−benchmarks

Run and report internal performance tests (all other flags are ignored).

−V, −−version

print program version

{baudmode}

The required {baudmode} parameter may be any floating-point value to specify a baud rate, or the string "rtty". The {baudmode} also implies certain other parameter defaults depending on the rate, including standard (or at least reasonable) default mark and space tone frequencies.

{any floating point value N}

: Bell-like N bps −−ascii

EXAMPLES

minimodem --tx 100

Transmit 100 baud tones from one computer ...

minimodem --rx 100

and receive 100 baud tones on another nearby computer.

minimodem --rx --auto-carrier rtty

Decode amateur radio RTTY signals (listen near 14.085 MHz).

minimodem --tx 0.5

Experiment with very low baud rates (works in noisy conditions).

minimodem --tx 12000

Experiment with very high baud rates (works with audio files).

NOTES

minimodem does not decode AX.25 framed packets.

minimodem does not support modem control ("AT") commands, nor does it produce DTMF telephone dialing tones.

VERSION

This page documents minimodem version 0.10.1. The latest version is available at <http://www.whence.com/minimodem>.

AUTHOR

minimodem was written by Kamal Mostafa <kamal@whence.com>.

COPYRIGHT

Copyright © 2011-2012 by Kamal Mostafa <kamal@whence.com>. License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law.