Fabrice Bellard's linmodem made to build on modern Linux
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Here is the generic Linux Modem. This modem is totally software, it means that all the DSP stuff is done by the main CPU, as in some so called "winmodems". The project was originally written by Fabrice Bellard. The latest available source code version was imported into the Osmocom Retronetworking project. The current project page is available here. Linmodem is also a research project. It contains all the necessary stuff to test new digital communication algorithms. The line simulator and the X11 interface can be used to simulate a complete communication chain.

Linmodem is the first modem to integrate a graphical user interface which show the data transmitted graphically (QAM constellation, real time FFT of the received signal, etc...) and which will allow you to monitor the line quality.


  • V34 modulator (sampling rate of 8000 Hz)
  • V34 demodulator (sampling rate of 8000 Hz), but no echo cancellor.
  • Algebraic part of V90.
  • DTMF dialing/receive.
  • V8 protocol.
  • V21 modulation & demodulation
  • V23 modulation & demodulation
  • sample code to test the protocol.
  • sample code to test V21, V22, V23, V34 and V90 independently from the modem.
  • a basic phone line simulator (with echos & typical line amplitude/phase distortion).
  • an X11 interface (see README.x11)
  • soundcard interface.
  • AT command parser & sample tty simulator.
  • asynchronous protocol.

Read the file ARCHITECTURE.md to know the details of the implementation. Next versions will contain the first draft on the algorithms which are implemented.



  • sudo dnf install make gcc libXext-devel libX11-devel


  • sudo apt install build-essential libxext-dev libx11-dev




Yes, you can already hear the modem!

compile everything, then type:

lm -sv

You will see a lot of debug stuff. Then you can press Control C to stop the call. If you play the files 'ans.sw' and 'cal.sw' on your soundcard (16 bit, signed 8kHz), you will hear the DTMF pulses, the V8 negociation, and a sample V21 connection. The X11 interface allows you to see the signals exchanged.

Data pumps

With the option '-m modulation_name', you can test the V21, V22, V23, V34 and V90 data pumps. You can understand what's going on in this test only if you have a basic knowledge of the modulations.

The X11 interface can be used to monitor all the main data pump parameters, except for V90 which is not yet completely integrated in the tests (see README.x11).

Real modems

Linmodem won't contain any hardware modem to support modems directly, but it will use kernel drivers which give a unified API to every driver (see README.arch to have an idea of the API)

Some test code is included in lmreal.c to work with the LTModem stuff available at http://www.close.u-net.com/ltmodem.html. However, it was not tested so don't expect it to work in this version.

Standard tests

If you have two PCs connected by soundcards (connect line in -> line out), you may try the soundcard support of linmodem (not tested now, but should work):

'lm -tv' launches linmodem on your soundcard (device '/dev/dsp'). Then you can type 'ATDTxxx' to compose a number and launch a connection. On the other PC, type 'ATA' to receive the connection. It should work in V23 (or V21 if you change the defaults modulations in lm.c).

With 'lm -t', linmodem simulates a serial line on '/dev/ttyz0'. You can use minicom or any other terminal emulator to send AT commands.


  • Fabrice Bellard (original author)
  • Harald Welte (compilation patch)
  • Manawyrm (updating X11 bitsize)
  • Eloy (update docs)

Notes on X11

Linmodem contains an X11 interface which can be used to monitor the modem parameters. It is used for example in v34 testing (lm -m v34). The X11 simulation works only in 16 bit mode. If you are interested by other screen depths, please implement them :-)

The function keys are used to change the parameters which are dumped on the screen:

  • for V34:

F1 : dump the brut samples which enter into the modem (at 8000Hz). A hamming windowed FFT is done to output the spectral power.

F2 : dump the echo cancellor parameters (not implemented yet).

F3 : dump the samples after frequency normalization & symbol timing recovery. You have 3 samples per baud for V34. A hamming windowed FFT is done to output the spectral power.

F4 : dump the equalizer filter in both time & frequency domains.

F5 : dump the QAM points just after the equalizer and phase recovery. This graphic can be used to monitor the overall modem quality.

  • for V21/V23 tests

F1: same as V34

F3: decision samples