13 March 2014

WSPR-Bone: SDR, BeagleBone and No Sound Input? 02

What you missed:

WSPR-Bone: Towards a dedicated WSPR-Computer 01

Karl-Heinz Ziegenhain asked in a comment to my former post:
3:47 PM

Looking for a small linux board with usable (stereo) sound input for similar reasons. Or better forget it and go for a smaller ARM Cortex M4 board with fast ADC? 

Still no Beaglebone - But WSPR-Bone is on its way.
That question is really important. It took me nearly half a year to answer it. My main interest was to implement sort of SDR-Receiver with waterfall and everything on a small computer. I am not a hardcore programmer, but if I find the right pieces I am able to put them together. And I found the sources for nearly everything I thought I needed: hamlib, fft, ...

So I started several months ago to glue the pieces together. But I failed when it came to hardware, better the soundcard. Although I have a reasonable laptop with a good CODEC I could not feed the stereo stream into the computer.I had only a mono microphone input and of course no line input. So the easiest way, was to connect an external sound card. I had good ones from my musical experiments, but then cam Linux and Debian. To make another long story short. I could not get things working in a way that it was reliable. To be true: I through all intentions away to work digital ham modes under Linux, revived an old Windows machine and everything went without troubles - even very good programs like MultiPSK, which run only under Windows ;-)

But still I was reading, that other hams had success with running SDR programs under Linux (for example RTL-SDR). The trick: They use an external kind of sound card.
So I looked again for embedded Linux systems with a audio stereo line in interface. I don't remember exactly: From the ~ 20 boards I was looking at only about 3 had an audio input. All had audio output even stereo, Dolby 5.1,, 7.xxx. That was the reason why they were used as Multimedia server etc. When I examined the remaining boards closely, I had to dig deep to find the relevant information. I had to look at the schematics to find out that all had no analog stereo line in interface. Often they had impressive Codecs, but they were not connected to the even mono audio input. A bummer.

Now I am sorry, I do not find the quotation again. so you have to trust me or search by yourself: In one of the discussions about a board with an impressiv Codec and Dolby ... output, a member stated: We don't need another Dolby, we need a stereo line input!

And the developer answered: He would second that, they tried but gave up. It was too demanding to engineer an audio input, that was sort of "clean". They could not manage to filter additionally input from wall chargers, hf-sources from the board, ripples from ... to get a decent clean signal as input for the Codec.

Now, at least for me it made sense and I stopped looking for this kind of solutions. They can not be produced with little money. There is a need for extensive filtering and that costs.

I still wanted to find a solution for the mono ways of digital modes like Fldigi. And invested in a new cheap USB-Mini sound card ...And now it works with the Raspberry Pi and the Beaglebone Black. The reason: Linux programmers have concentrated their efforts on new and cheap sound cards and found their ways. Have a look at this overview.

Putting it together: In the moment I have no need for an analog stereo line input interface on an embedded Linux board. Programs like WSPR, Fldigi, and some other works on my Beaglebone. But more about that later - or I wouldn't have started this series without beeing sure that I get things going ;-)

More to read:


Stay Tuned!