Saturday, 10 April 2010
A few years back I purchased a little circuit board, which when fitted to the R1000 gives me a 12Khz output from the IF of the receiver. This output can then be connected to the input of a PC soundcard.
The reason for doing this was in order to decode something called DRM. Now this is not another amateur radio data mode, nor am talking about the digital slow scan TV which is sometimes referred to as DRM, although I think the software technology is similar. This DRM is a transmission used by International Shortwave broadcasters. This mode was supposed to revolutinise shortwave radio. it offered the potential of clear, FM- like transmission, even in stereo with a text display similar to that seen in todays DAB receivers. And when it works, it is impressive. The first time I decoded a DRM transmission I was amazed at the quality, better than the sound you get from web based streaming for example. So what happened? Why hasn't DRM taken off? Well for one thing Shortwave has been overtaken by the internet and many of the larger broadcasters have left the Shortwave spectrum. The other problem is that receivers for DRM are not and never have been widely available. Some PC based SDR type radios (like the WinRadio for example) have the facility to receive DRM but apart from that it is not possible to buy DRM receivers easily. Lastly and perhaps more importantly the mode isn't at its best on Shortwave. Like many digital mediums any interference or fading and the reception breaks up very quickly. To illustrate this, in my experience a signal strength of around S9+10db would be a minimum for reliable reception. If you want to hear a DRM signal try tuning arround 6090 khz, if you can hear a sort of "white noise" thats it.
I think Medium wave would have been the best band for DRM, allowing us to have high quality reception on the band, but for some resaon DRM on the Medium Wave just didn't happen. So for the present quite a few international broadcasters put out these DRM signals. Who is listening? Very few I suspect!
Pictured below is the "DREAM" software I use for DRM decoding.