PSK31 on the Raspberry Pi - Part 1

As I mentioned in my last post, I am a radio amateur and I'm interested in using the Raspberry Pi to operate using various digital modes, including PSK31. To do this requires an audio input to and output from the Raspberry Pi (see my previous post) and a suitable application to encode and decode the PSK31 signal, as well as a radio transceiver and antenna. The possibility of using the Raspberry Pi for operating PSK31 has intrigued me because it would offer a low cost, low power, silent and potentially mobile station setup - the only other alternative is to use a desktop or laptop computer.

If you do an internet search for "Raspberry Pi PSK31" you will get a few results, most of which bemoan the Raspberry Pi's lack of processor power for running "Fldigi" which is probably the most popular Linux application for PSK31. I have tried running "Fldigi" on the Raspberry Pi and whilst it does run, I feel that it is just too demanding on the modest resources of the Raspberry Pi.

Searching the repository for "PSK31" or "amateur radio" yields a few alternatives and I have been experimenting with some of these applications. The one I have had the most success with is called "LinPSK". To download and install LinPSK open a LXTerminal window and type the following and press Enter:

pi@raspberrypi ~ $ sudo apt-get install linpsk_

To make the USB audio device available to "LinPSK" you need to create a file called ".asoundrc" in the "/home/pi" folder containing the following text:

pcm.LinPSK_card {
       type hw
       card 1
       }
ctl.LinPSK_card {
       type hw
       card 1
       }
pcm.LinPSK_Play {
       type plug
       slave {
       pcm "LinPSK_card"
       }
}
pcm.LinPSK_Record {
       type plug
       slave {
       pcm "LinPSK_card"
       }
}


To save you having to type in all the above text, there is a file called "asoundrc" in the folder "/usr/share/doc/linpsk" which you can copy over to the "/home/pi" folder, edit and rename as ".asoundrc". If there already is a file called ".asoundrc" in the "/home/pi" folder then you should append the above text to the end of the existing file. Have a look at this post if you need help creating or editing files.

Once you have saved and closed the ".asoundrc" file the "LinPSK" application should have access to the USB audio device. The above procedure assumes that the on-board audio device is designated "card 0" and that the USB audio device is designated "card 1", but this should be the case as long as you do not have any other audio devices connected to your Raspberry Pi.

Connect the audio output from your transceiver to the input of the the USB audio device via a suitable isolating interface and tune the transceiver to a PSK31 signal. Launch the "LinPSK" application and maximize the window so that it occupies the whole screen. Click on the square button labelled "RX" located left of center. The waterfall on the bottom left of the screen should come to life. Click on one of the peaks in the waterfall and you should start to decode the PSK31 signal.

You may notice that I also have an LXTerminal window open. This is running "AlsaMixer" in the background so that I can adjust the level of the audio input to the Raspberry Pi. I found that the best setting for me was to turn the "Auto Gain Control" off and set the "Mic" capture control to about half way, but I suggest that you experiment with the settings and find what works best for you. Also note that the load on the Raspberry Pi's processor is quite acceptable (as shown by the small scrolling graph on the right hand end of the task bar) with LinPSK and AlsaMixer both running.

The LinPSK application may also be used for QPSK, RTTY and MFSK16. There is an extensive user guide for LinPSK available here, although it does relate to an older version of the application.