In this post you can read about the interception of worldwide DRM broadcasting signals with Sagax SRM-3000 SDR radio receiver system. For additional content visit our resources page or subscribe to our newsletter here!
Digital broadcast on HF bands
Due to significant progress in Digital Signal Processing (DSP) technology, it is now possible to use spectrum efficient digital transmissions for high quality audio broadcast in HF bands. The current and already stable standard is the Digital Radio Mondial (DRM). This transmission uses the same 10 kHz bandwidth as a traditional AM broadcast station, although with a multi-carrier modulation format, and delivers content as source coded streams. The spectrum of such transmissions is typically noise-like and rectangular, as you can see on the screen of an SRM-3000 SDR radio receiver.
Demodulation and decoding of DRM signals
Firstly, we need a special DSP tool to demodulate such waveforms with several sub-carriers and QAM modulation of each carrier. Fortunately, we have an open source community project delivering such a software, called Dream DRM receiver. Besides you can download the latest version of their software from its SourceForge repository. The digital stream is provided by the demodulator. However, we need to use the decoder to generate an audio sample and other meta info associated with the transmission, i.e., station name, program characteristics, etc. The DRM stations use AAC+ codec. It is built in the Dream software stack as a dynamic library.
Connecting the SDR radio receiver station components
Secondly, we can use the DRU-244A digitizer card with SRM-3000 SDR radio receiver software to receive the signals from the air. Further we can use the Dream software to demodulate the stream and to decode the audio signal. Both of them are able to feed signals to and consume signals from a sound device. You can use a jumper cable to connect the two applications. However, we don’t recommend it for analog conversion and back. You can use a virtual audio cable instead, which directly forwards samples from one app to the other. We used the VB-Audio Virtual Cable for our test runs. See their web site for more information.
Enjoy DRM programs from world wide providers with SDR radio receiver
Finally, we need to find some DRM signals on the air, or look after program guides on the net for given stations. We simply search the spectrum. It was easy to recognize some DRM signals using the SRM-3000 SDR radio receiver. One of the first transmissions you able to find is from All India Radio, which is pretty good DX from Europe with a simple long wire antenna. We have enough receiver station sensitivity with the DRU card and the antenna for such a DX reception.
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