Sunday, 19 October 2008

DRM - Digital Radio Mondial (2)

I have finished boxing up the 455/12 kHz mixer (http://www/, fitting connectors etc. and I have been spending a bit of time listening to DRM broadcasts. There are about 10 European medium-wave DRM stations, that become available in the evenings and about 20 or so SW stations available here in London at various times of the day. I am also fortunate that there is a local broadcast station on 25695 kHz just up the road from here at Crystal Palace so I always have a strong station to check the set-up.

The DREAM DRM is quite easy to get it going, but it lacks help files so I had to explore to find out how it works. The key menue is 'View'. from there, go to 'Stations' menu, which will give a list of stations. Make sure you 'Update' this regularly - DRM is still experimental so stations come and go. The list automatically indicates the stations available at the current time. Once you have tuned into the station you want (on my FT-817 it best to tune about 3kHz above the stated frequency).

Next select 'Evaluation' from the View menu and select 'Waterfall ' from the 'Spectrum' submenu. With a bit of luck, you will see a screen like the one below and once all the traffic lights at the top of the screen turn green, you will hear the decoded signal from your computer speakers.

Decoding is very sensitive to getting the audio level right. Unfortunately, DREAM does not have any facility to adjust this so you will need to open your 'Sounds' screen from your computer's Control Panel (or Settings) menu. You may need to access the 'Recording' sliders. To do this, go the the 'Options' menu and select 'Properties' and then the 'Recording' button (intuitive it is not!).

Then adjust the relevant input level until the DRM signal is mostly green and yellow, with a bit of red. Please note, shortwave is subject to long-term fading so you may need to adjust the level from time to time.

Listening to DRM is very different to normal shortwave listening. When DREAM is decoding sucessfully, the signals are very good quality (almost FM as most broadcasts are Mode 'B' which is intended for the better paths). However, when it gets overwelmed by sudden changes in path or bad multi-path propagation the signal does not fade it just drops. Once the signal comes back, DREAM usually regains syncronization quite quickly so the absence of signal is minimized.

I will say more next time about the programmes I have listened to. Meanwhile please leave a comment with your own experiences of DRM.


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