minimodem − general-purpose software audio FSK modem


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


Minimodem is a command-line program which decodes (or generates) audio modem tones at any specified baud rate, using various framing protocols. It acts a general-purpose software FSK modem, and includes support for various standard FSK protocols such as Bell103, Bell202, RTTY, TTY/TDD, NOAA SAME, and Caller-ID.

Minimodem can play and capture audio modem tones in real-time via the system audio device, or in batched mode via audio files.

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.


−t, −−tx, −−transmit, −−write

transmit mode: generate audio tones

−r, −−rx, −−receive, −−read

receive mode: decode audio tones


The required {baudmode} parameter may be any floating-point value to specify a baud rate, or any of the special keywords listed below. 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}

: Bell202-style at N bps −−ascii


: Bell202 1200 bps −−ascii


: Bell103 300 bps −−ascii


: RTTY 45.45 bps −−baudot −−stopbits 1.5


: TTY/TDD 45.45 bps −−baudot −−stopbits 2.0


: SAME 520.83 bps −−startbits 0 −−stopbits 0 −−sync-byte 0xAB

NOAA Specific Area Message Encoding (SAME) protocol


: Bell202 1200 bps Caller-ID (MDMF or SDMF) protocol


: UIC-751-3 600 bps train-to-ground message protocol


: UIC-751-3 600 bps ground-to-train message protocol


−a, −−auto-carrier

Automatically detect mark and space frequences from carrier.

−i, −−inverted

Invert the mark and space frequencies (applies whether the frequencies are defaults, discovered by −−auto-carrier, or specified manually).

−c, −−confidence min-confidence-threshold

Set receive confidence minimum threshold (default 1.5). The "confidence" value is a metric based primarily on the SNR (signal-to-noise ratio) of the received signal. 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 a minimum value of 1.0). (This option applies to −−rx mode only). −l, −−limit max-confidence-search-limit Set receive confidence maximum search limit (default 2.3). The "confidence" value is as described above. 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-confidence-threshold) for a sloppier analysis, with lower CPU usage. (This option applies to −−rx mode only).

−8, −−ascii


−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}
−v, −−volume {tx_amplitude or ’E’}

Sets the generated signal amplitude (default is 1.0). As a special case useful for testing, the value ’E’ sets the amplitude to the very small value FLT_EPSILON. (This option applies to −−tx mode only).

−M, −−mark {mark_freq}
−S, −−space {space_freq}
−−startbits {n}

Sets the number of start bits (default is 1 for most baudmodes).

−−stopbits {n.n}

Sets the number of stop bits (default is 1.0 for most baudmodes).

−−sync-byte {0xXX}

If this option is used, initial carrier acquisition will be suppressed until after one or more consecutive data frame(s) containing this value are received. This can be used to synchronize the stream for protocols which include a fixed preamble byte. (This option applies to −−rx mode only).

−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[={plughw:X,Y | X,Y | X }]

Use ALSA as the audio output system instead of the default PulseAudio (depending on build configuration options). The ALSA device alias "default" is used, if a specific device is not specified. For example, the following options all select ALSA device #1, sub-device #0:
−−alsa=plughw:1,0 −−alsa=1,0 −A1


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).


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).


Quit after the first carrier/no-carrier event (applies to −−rx mode only).


Print received data bits as raw binary output using characters ’0’ and ’1’. The bits are printed in the order they are received. Framing bits (start and stop bits) are omitted from the output. (This option applies to −−rx mode only).

−−binary-raw {nbits}

Print all received bits (data bits and any framing bits) as raw binary output using characters ’0’ and ’1’. Framing bits are not interpreted, but simply passed through to the output. The bits are printed in the order they are received, in lines {nbits} wide. So in order to display a standard 8-N-1 bitstream (8 databits + 1 start bit + 1 stop bit), use "--binary-raw 10"
or a multiple of 10. (This option applies to −−rx mode only).


Filter the received text output, replacing any "non-printable" bytes with a ’.’ character. (This option applies to −−rx mode only).


When transmitting from a blocking source, keep a carrier going while waiting for more data.


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

−V, −−version

print program version


minimodem --tx 100

Transmit 100 baud tones from one computer ...

minimodem --rx 100

and receive 100 baud tones on another nearby computer.

minimodem --rx -a rtty

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

minimodem --rx same

Decode NOAA SAME protocol emergency alert transmissions, e.g.

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).


minimodem does not decode AX.25 framed packets.

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


This page documents minimodem version 0.24. The latest version is available at <>.


minimodem was written by Kamal Mostafa <>.


Copyright © 2011-2016 by Kamal Mostafa <>. License GPLv3+: GNU GPL version 3 or later <>.
This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law.