Digital Radio Mondiale is a consortium of broadcasters, manufacturers research institutes and stakeholders that have decided to work towards a world-standard for Digital broadcasting in the frequency bands below 30 MHz (i.e. the LW, MW and SW frequency bands). The base specification has been completed and was delivered to the ITU in December 1999. Field trials were conducted and revisions to the specification were completed by 2001 when roll-out of the broadcast technology started.
DRM uses the existing AM broadcast frequency bands and is designed to fit in with the existing AM broadcast band plan, based on signals of 9 kHz or 10 kHz bandwidth. It also has modes requiring only 4.5 kHz or 5 kHz bandwidth, and modes that can take advantage of wider bandwidths – 18 kHz or 20 kHz – allowing DRM to operate alongside AM transmissions in every market of the world.
The DRM system uses COFDM (Coded Orthogonal Frequency Division Multiplex). This means that all the data, produced from the digitally encoded audio and associated data signals, is shared out for transmission across a large number of closely spaced carriers. All of these carriers are contained within the allotted transmission channel. Time interleaving is applied in order to mitigate against fading. Various parameters of the OFDM and coding can be varied to allow DRM to operate successfully in many different propagation environments – the selection of the parameters allows transmissions to be planned that find the best combination of transmit power, robustness and data capacity.
The DRM system uses MPEG 4 HE AAC v2 for mixed programming providing high quality at low data rates. Due to the channel bandwidth of 10 or 9 kHz the datarate of the streamed media is limited to approx. 35 kBit/s for a single channel and 72 kBit/s when using two channels. Although the bitrate is not very high the audio quality of the streams reaches or exceeds the quality of FM mono transmissions. The advanced audio coding standard (AAC) combined with SBR and parametric stereo provide high audio quality at very low bitrates (e.g. 22 kBit/s). Beside AAC the DRM standard defines the HVXC and CELP codec to be used for transmitting speech signals at 4 kBit/s or less. Streaming raw data for image slideshows, HTML pages, etc. is also allowed by the DRM standard. Can handle up to 4 standardised AAC+ audio channels in DRM30 mode.
DRM holds great promise as it will revitalise frequency bands that have long been written off as unable to serve in the delivery of high quality audio. DRM promises the delivery of FM quality-like stereo over a good RF path.
For more information visit http://www.drm.org