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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

1200 : Bell202 1200 bps --ascii

300 : Bell103 300 bps --ascii

rtty : RTTY 45.45 bps --baudot --stopbits 1.5

tdd : TTY/TDD 45.45 bps --baudot --stopbits 2.0

same : 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

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 0.23. The latest version is available at

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