Revisiting DRM

Today the amateur bands didn't seem in very good shape, but I had some radio time. I fired up my old Kenwood R1000 receiver, dug out an old usb external soundcard and did some listening to DRM shortwave broadcasts once I has loaded the DRM "Dream" software onto my main PC.

There are still quite a few DRM broadcasts to be found, I received DRM for four or five different broadcasters today. The sound obtained from DRM is at its worst similar to perhaps mono sound via the internet and at best almost as good as an FM stereo broadcast. The problem is though that you need a very strong signal S9+20Db at minimum to get a readable signal from DRM, or at least that is the case using my elderly modified receiver.

This has brought me round to thinking about SDR radios. I mentioned in an earlier entry how a local amateur friend recently purchased a flex radio transceiver. I have briefly seen it in operation and the receive side sounded very good, with its filtering and good interference rejection. Of course such a device is very expensive and personally I would rather my main equipment is capable of being manually operated, I don't want to have to always have my PC running. An SDR receiver sounds appealing, particularly as I still do quite a bit of shortwave listening but again this is a very expensive proposition. So when another local amateur told me of the Elektor SDR radio which he was buying I was very interested. The Elektor is basically a populated board with all components installed  but no casing. My amateur friend has just received his and is in the proccess of setting it up and testing it out. When it is up and running I am hoping to see it in action. if it is any good it will be very tempting at the asking price when you compare it to the other SDRs on the market.

Dreaming...

Pictured is my Trio R1000 communications receiver. I bought this from Ebay years ago, its a good little receiver but nothing really remarkable but this one is slightly different from the usual.
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.