Thursday, June 21, 2012

The SDR Chronicles (#4: Getting Down to Business)

Several interesting things:

1. This location is prone to overload from several nearby AM broadcast stations.  Obviously any SDR with the typical barn door front end is going to have issues with this.  The Excalibur Pro has a hardware MW filter that rolls off the RF input starting around 2 MHz, and it works.  Without it, the noise floor below 4-5 MHz is mostly broadcast intermod and general crud.  With it, the noise is the usual HF static.

2. There is another hardware RF filter which can do passbands.  This can be cranked in to also roll off above your tuned frequency. This is not needed here, yet anyway, but if you have a 27 MHz ratchet jaw or local ham DX ace with "maximum legal power," I can see where it would come in handy.

3. I had no interest in DRM until I got this radio. Now, I have Radio New Zealand International going in beautiful, noise-free audio while I write this.  The Excalibur Pro actually comes with a DRM demodulator, but one needs to pony up a non-trivial sum of money (around $70 US at current Euro exchange) for a license to use it. Once this is done, the DRM works just fine on strong signals.

As with any DRM receiver, audio dropouts start occurring when the s/n ratio displayed by the demodulator drops below about 15 dB. This setup here is actually good down to about 12.5 dB.  Below that, you will hear a brief reverb effect sounding like an old Fender amp, followed by silence.

4. "Dream," a free DRM demodulator program from Source Forge, works very nicely with the Excalibur Pro. I don't know if you can get it to work just using "stereo mix" or the "virtual audio cable." I'm using WinRadio's "Virtual Sound Card," aka "Digital Bridge," another paid add-on.  Clicking "IF" sends out a real IF output, user adjustable to the necessary 48000 bps, and centered on zero with stereo I and Q.  After this, Dream needs to be started after the Excalibur software, and using the following command line:

C:\[path]Dream.exe -c 5

where [path] is the usual full path to wherever Dream.exe lives on your box.  This puts it into the proper config to handle this I/Q mode.  The audio, at least on this computer, has a bit more midrange punch than on WinRadio's decoder, but it is also more prone to dropouts and affected a LOT more by fading.

5. One last thing about DRM: it will NOT work with the noise blanker on.  I found this out the hard way.  You'll never get above a 9.5 dB s/n with it on, but you'll rarely get that low with it off.

6. The manuals make a considerable fuss over something called the XRS Variable.    This is a Windows environment variable which tells the software where to install and look for plugins.  If it doesn't match the path shown in the registry, plugins won't work.

No matter what I did, this variable showed up as null, even though the path in the registry was correct.  The "G3 Diagnostic Tool" ( a free download from WinRadio) would pick this problem up, but the "XRS Repair" would not fix it.

DO NOT re-install the driver and application software, because this is a pain in the butt and unnecessary. The real fix is actually well explained in a document buried deeply on the WinRadio web site. I might as well save you the time, however.  Here goes.

You need to find out how to set "environment variables" on your particular version of Windows.  On mine, it's right click on "computer," then take the "Advanced" tab and click the box labelled "Environment Variables."  Then create one called XRS_PLUGIN_PATH, in both user and system variables, and set its value to whatever the diag tells you it says in the registry.  Here, it's

C:\Program Files (x86)\WINRADIO\PLUGINS

and case does not seem to matter.  I copied and pasted the text right out of the G3 diag.  Voila.  Now plugins install to the right folder, and the software finds them, and they work as well as they're ever going to.  "Presets" is a useful little plugin.

There's more, but this is getting pretty geeky if you don't have one of these radios.  And the fun continues......