Chat on the Air with LinPsk

Long before the Internet was established, an international group of people was chatting on the air—the radio amateurs. In the beginning, they used Morse code, a special form of digital communication. Over the years, more digital modes have been introduced to ham radio. Today, you can use satellites for communication or the moon to reflect radio waves. In the past, you could communicate with the Mir Space Station; now, it's with the International Space Station. To learn more about the fascinating world of ham radio, take a look at the on-line Resources section for this article.

One of the most popular new communication modes in use today is PSK31. Peter Martinez (G3PLX) developed PSK31 based on an idea by Pawel Jalocha (SP9VRC). PSK31 was designed for enabling keyboard-to-keyboard chats. PSK31 is a phase shift keying modulation technique, requiring a bandwidth of about 31Hz. By comparison, a ham FM signal is almost 500 times as large, about 10kHz.

In the early days, hams built their equipment themselves and tried to improve it. Today, it is hard to compete against industrial transceivers, but under certain conditions, digital modes offer a new chance to compete. All you need is your transceiver, a computer with a sound card and a program that supports the desired mode. If the program is open source, you also have an opportunity to do your own experiments.

Radio operators have developed a kind of language that they use for communication. Some ham-related abbreviations used in this article are listed in Table 1.

My first contact with PSK31 was in 1999. Even though I was a fan of Linux, I had to use a Microsoft Windows program for my first PSK31 QSOs. Later, I tried to develop a PSK31 program for Linux. Luckily, I found WinPsk 1.0, by Moe Wheatley (AE4JY), which had been released in source code form. I used this code as the basis for my initial release of LinPsk 0.2. The version 0.7 release of LinPsk included the expanded functionality of RTTY, and around this time, LinPsk also was made available for Mac OS X as DarwinPsk.

The primary source for getting LinPsk is the LinPsk home page, where you can find the current prerequisites. At the time of this writing, they are GCC 3.3, Qt 3.3.x, fftw 3.0.1 and portaudio v18. See the on-line Resources section for LinPsk sources. A Debian package of the LinPsk is available. Currently, it requires either OSS sound drivers or the OSS emulation under ALSA.

For those of you who are new to Linux, an ISO image is available that contains a complete Linux system with many ham-related programs. This image contains LinPsk. You can download it and burn it onto a CD, then boot your PC from this CD without installing any programs.

If you decide to install LinPsk yourself, the installation is straightforward:

tar xzf linpsk-0.8.0.3.tar.gz
cd linpsk-0.8.0.3
./configure

Or, you can type:

./configure --prefix="installation directory"

for an installation directory other than the standard /usr/local/bin.

Finally, do:

make
su
make install

That's all. You should find a Linpsk executable in /usr/local/bin.

Now, it's time to connect the sound card to the transceiver. There are different ways of doing so, and a few proposals can be found at WM2U's PSK31 page.

No other program should use the sound card while Linpsk is starting. KDE's artsd, for example, should be stopped before running LinPsk, because it allocates the sound device.