31 December 2013

SDR with QRP Computing running Linux

After my last experiments using the Raspberry Pi as a CAT controller for my FT-817 I got carefree. It was in fact so easy - after catching up with some problems - that I thought everything is possible. And if it is possible I will find a solution. But things turned out the other way. I was digging for 2 weeks and came out with nothing except a lot of knowledge, which is good to know but didn't help to fulfill my dream of a little portable QRP SDR. During this research I was meandering through a lot of software and hardware, but I will try to present it in an organized way.

Campers große Stunde  © Joachim Lehrer  2013

Raspberry Pi and PMSDR
I have since several years a PMSDR lying on my table which ran without a glitch on a Windows XP system using HDSDR, the control software of  Martin Pernter, IW3AUT. I used it mainly to see what my antenna was really worth by running the Cw-Skimmer software. But when I changed to Linux the PMSDR was just a cold piece of electronic.

With the Raspberry Pi running on Wheezy (Debian) I looked around and found an installation guide written by Patrick (DH2SPK). With this paper you can install DttSP.
 DttSP implements the basic modulation, demodulation, signal conditioning, and synchronization processes required to operate a high performance transceiver using DSP as the detection and synthesis stages. 
It was no problem to compile this software on the Raspi when you istalled the necessary libraries and the revision system. I still was happy. So next came the sdr-shell for the pmsdr. Still no problems and the gui looked really nice. (Although when I later tried to install this on another system I ran into trouble for the library "qt3-devel" seems to be outdated and produced errors when it was install to other versions).
Now came the last step: Controlling the PMSDR via the USB interface

So still no problems ... until I plugged in the PMSDR and the external soundcard.

I had three external soundcards. They were all recognized by the system, but I could not address them. And with the PMSDR, I got problems in the udev part for Martin IW3AUT had choosen another USB controller on my PMSDR. I took some time to solve that problem.

Embedded Linux System Boards and SDR

In the meantime it was clear, that the Raspi lacks of computer power. And needed a faster CPU and if possible a better codec on the board. It took me several days to find out, that these little devices are nice and run fast but they are not intended to help me creating my project. Here you find two lists as a starter
From the Beaglebone Black, Pandaboard, ... to the UDOO Board. Some of these boards were represented with impressive figures. They used quad-core processors and had codecs on board which could solve up to 192 kHz. But when I was looking for a stereo line in interface: NADA. Some of them had stereo output with Dolby Surround, but no input. On one board there was a stereo mic jack. But when looking at the circuit, I found a short cut from one channel to the other and the codec was fed with only one channel. While digging through the discussion boards I found the reason:
The developers had problems to filter incoming interferences from the powersupplys. These were mixed with the audio from the line-in and could hardly be eliminated.

One or two boards seemed to be capable to do the task, but when you added all the things you needed you could buy a refurbished Thinkpad. So in the moment I could save the money for an embedded fast Linux-system and take my old laptops.

Laptops, Debian and SDR
It is another chapter but perhaps you already guess the outcome. Most of my laptops had no line-in. Some had  a mic-in and these were all mono. So I had to use again the USB-Soundcards. I could configure it, saw them in jackctl but were unable to run them. The programs wanted to work with the internal soundcard. So I had the option to disable the internal soundcard. But that was not an option to me. Later - or for me in the moment too late, I found out that Debian based Linux system had somewhere a switch which made the internal soundcard as the only option. My dear ...

But roaming through the net I found some solutions. One main developer succeeded to run an SDR on 4 Raspberry Pis. And this system was the point were all my wanderings suddenly turned my trouble into success: HPSDR High Performance  Software Defined Radio.

This is really an amazing system. The components, boards can be bought by TAPR.

The general idea, well I am still studying the ghpsdr3-alex project.

So folks, thats all for today and for this year. I wish you a wonderful and happy New Year 2014.

This is a dedication to my friend Vytas, LY2PU. Another one gone. So sad.

But the rest of you: Stay Tuned!