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minimodem

General-purpose audio FSK modem — Bell 103, RTTY, AX.25 and anything else

What it does

Modulates and demodulates audio FSK at a baud rate and mark and space frequency you specify. It knows the classic presets -- Bell 103 and 202, RTTY at 45.45 baud, AX.25 at 1200, several caller-ID and paging formats -- and it will also take arbitrary parameters for a mode it has never heard of. Input and output are a sound card or a WAV file.

Why you would want it

It is a modem as a Unix filter. Anything that produces or consumes bytes on a pipe can go over the air with it, which makes it the right tool for experiments, for one-off decoding of an unfamiliar FSK signal, and for scripts. When you have a recording of something that is clearly FSK and you do not know what it is, this is the tool you guess parameters into.

Before it will work

Audio in or out, or a WAV file. For over-the-air use, whatever couples the sound card to the radio, plus PTT arranged separately -- minimodem does not key anything.

How it installs

  • apt: minimodem

Known problems

It does not detect the mode for you: baud rate, mark and space must be right or the output is noise, and there is no indication which of the three is wrong. Precision matters -- 45.45 baud RTTY is not 45 baud. For an established mode with an established program, that program will do better; this is the general instrument, not the specialist one.

Keeping it current

  • probe: apt policy
  • strategy: apt_upgrade

Where to get help with the software itself

Kamal Mostafa. Source and documentation at the URL above.

Source: catalog/packages/minimodem.yaml