Chapter 52. A 74-Line Ip Telephone

Lincoln D. Stein

Convergence! A buzzword of the dot-com era. Convergence is the magical integration of the desktop computer, the Internet, television, radio, and the telephone. In the words of the industry pundits, convergence will change everything, and the technology hailed as the forerunner of convergence is IP telephony, which allows you to make long distance calls with nothing more than an Internet connection and a high quality sound card.

I hate being left behind on the technology curve, so I decided to write my own IP telephone application in Perl. It isn’t elegant, and it lacks most of the functions of real IP telephony applications, but it works. In this article, I’ll show you the two versions of the application: a simple one which requires ISDN-speed connections to work well, and a somewhat more sophisticated version that uses the MP3 format to reduce the bitrate for slower connections.

Sound Cards and /dev/dsp

IP telephony requires three things:

  1. Reading sound data from the microphone

  2. Writing sound data to the system speaker

  3. Moving sound data across the network

We know how to do item three with Berkeley sockets, but what about items one and two?

On Unix systems, the answer is simple. Audio-capable Unix systems have a special device file for communicating with the digital signal processor (DSP) driver. Common names include /dev/dsp, /dev/audio, and /dev/sound. Just open the device like an ordinary file, read from it to capture sound data from the ...

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