Set up a soft modem – minimodem will help

We can say that modems have moved into the category of retrotechnologies. But to work with them today, it is not necessary to purchase a physical device. To establish an audio connection enough sound card and open source minimodem utilities.

Photo: Akshar Dave.  Source:
Photo: Akshar Dave. Source:

What is a minimodem

Older computers lacked the computing power to simultaneously perform the tasks of digital data processing, modulation and demodulation, and coding – they were solved by the modem’s processor. With the development of technology, the so-called soft modems (or winmodems), which were responsible only for the DSP, modulation and demodulation were transferred to the operating system driver. For modern machines, a sound card and special software are enough to establish this kind of connection.

An example of such software is minimodem. It generates and decodes standard FSK sequences, including Bell103, Caller ID, NOAA SAME, RTTY and even TTY/TDD. The latter protocol was used in telephone line messaging devices – their used people with an ear or speech disorder.

Minimodem allows you to set up a data transmission channel between two computer systems. It can work both with pre-recorded audio signals and in direct connection format through the audio port. See how it works you can on youtube – the author posted a demo. But if you are wearing headphones, we recommend turning down the volume, as the tones in the video are quite noticeable.

By the way, if you want to take a closer look at the sound of old modems, for this purpose one enthusiast has compiled a “library” of handshakes. There are modems in his collection from T.I. and Motorola with the shortest DIL sequence.

As for setting up minimodem, all instructions can be found at official website. Interestingly, minimodem can be run under Windows via Cygwin – the list of required dependencies is published in repositories on GitHub.

Who else uses sound to transmit data

There are other programs that allow you to transfer data between computers using sound. For example, a small home project by a British engineer − AudioTransmit. The utility encodes binary data into an audio recording and broadcasts it to another computer through a speaker, in turn, the receiver reads it using a microphone and decodes it.

Photo: Akshar Dave.  Source:
Photo: Akshar Dave. Source:

So far, the project only supports one-way transmission, which means that two different programs are responsible for sending and receiving data. But as an experiment, they can be run on the same computer.

Another similar project is called wave-share. The utility allows you to establish communication between two browsers via the WebRTC protocol using sound. The operation of the utility can be evaluated in practice in this demo.

Of course, the applicability of such technologies is quite limited – they are at least affected by interference and the quality of the transmitted signal. But first of all, they are interesting from an educational point of view, as they allow you to better understand the principles of operation of technologies of past years.

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