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 called Digital Radio Mondial (DRM). This transmission is using 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
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. 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
We can use the DRU-244A digitizer card with SRM-3000 sdr radio receiver software to receive the signals from the air. The Dream software is used 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. A jumper cable can be used to connect the two applications. However, that’s not recommended for analog conversion and back. A virtual audio cable may be used instead, which directly forwards samples from one app to the other. I’ve used the VB-Audio Virtual Cable for my test runs. See their web site for more information.
Enjoy DRM programs form world wide providers
Finally, we need to find some DRM signals on the air, or look after program guides on the net for given stations. I simply search the spectrum. It was easy to recognize some DRM signals using the SRM-3000 SDR radio receiver. One of the first transmissions I was able to find was from All India Radio, which is pretty good DX from Europe with a simple long wire antenna. We have a enough receiver station sensitivity with the DRU card and the antenna for such a DX reception.