No Sound from my SDR

Posted on Friday, April 15th, 2016 at 8:39 am in



And in other news, I’m trying to play with this cute little SDR receiver (the cuteness which I am sharing with you in another post), but I’m having mega problems.

Don’t get me wrong: The issue is not SDR or this cute little Rx. I own and operate a Flex-1500 which has quietly become my almost all-time favourite desktop radio. Software Defined Radios are just awesome. I can install Virtual Audio Cables and serial port replicators and connectors in all the configurations I’m using. I have also played extensively with other variations of hardware kits over time and I would like to think that I’m not an expert but that I can make successful contacts with a SDR.

Thus, when I needed to squeeze an experimental (not top-of-the-line) and not very good (yet) but definitely tiny (as in physically small) receiver into my Ultimate 3S beacon from Hans Summers over at QRP-Labs, I bought a PSDR module from my friend Anton Janovsky over at Giga. Measuring a fractional 30mm x 40mm the size was exactly right, and it requires a 12v input which I already feed to the Ultimate 3S so that wasn’t going to be a problem. The receiver itself wasn’t going to win any awards for noise or quality, the circuit is a simple Johnson counter with 7474 circuit group and Tayloe detector circuits – more info over at Pandatron. No filters or pre-selection or anything, but good enough for a proof of concept prototype. I might even use the pre-existing LPF filters with a bit of cutting and soldering to the boards.

The really exciting thing is that in the latest Ultimate 3S from QRP-Labs with the Si5351A synthesizer, it is a piece of cake to tap another output from the frequency generator and use that as the LO (Local Oscillator) input to the PSDR module. A very elegant solution emerges! Forge ahead and spend the money!

Coming home and temporally connecting it to an AD9851 DDS Function Signal Generator (also from Anton at Giga) and using my trusty Lenovo ThinkPad in the shack, I can’t decipher anything. Really? Nothing.

To cut a long story short the issue is that the modern laptops don’t have stereo (yes, two channel) inputs. Google it and sift through the millions of hits. The best summary I found was at temporaryland dated 2008 over at his website of rm42 called “Consumer alert – Sound-Mix intentionally crippled laptops” in which he explains that for reasons that makes no sense, modern laptops can only record in MONO from their built-in sound cards. And in the case of Lenovo, maybe they are scared of the DRM police, but they made it a HARDWARE limitation.

This is important to me, as the PSDR gives out I/Q signals similar to just about any other experimental SDR kit, it needs TWO signals to the processor. Ultimately (pun intended) in the Ultimate 3S I was going to consider using the Analogue inputs’ A/D converter to pick up the sounds for me although I’m not there yet in the design.

But to hit the wall so early with such a silly limitation is simply stupid. There is a work-around – buy an external USB sound card like the STLab M330 (only 16 bits). But that is an unnecessary expense and extra wires to plug in and extra things that can go wrong. Needless to say, I’m no longer a Lenovo fan.

So the point of this message is: For the moment, if you want to play with I/Q signals from a SDR receiver, make sure you have a stereo line input. From there, SDRSharp will make your life easy. But without the underlying hardware you are screwed.

Let me know what you did as a work-around.