Technical Aspects of The On-Air System

Digital Radio Mondiale (DRM) is the universal, openly standardised, digital radio system for short-wave, medium-wave and long-wave - digital radio for the radio frequencies below 30MHz. It has been endorsed by the ITU, and is standardised as ETSI ES 201 980.

DRM has near-FM sound quality plus the ease-of-use that comes from digital transmissions. The improvement over AM is immediately noticeable. DRM can be used for a range of audio content, and has the capacity to integrate text and data. This additional content can be displayed on DRM receivers to enhance the listening experience.

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 or10 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. In addition, CELP and HVXC coders are available to provide speech-only programming at even lower data rates.

In 2005, the DRM Consortium decided to extend the DRM system under the project name DRM+ to operate in all the broadcasting bands below 120 MHz. This range includes:

• 47 MHz to 68 MHz (Band I) allocated to analogue television broadcasting;
•  65.8 MHz to 74 MHz (OIRT FM band)
• 76 MHz to 90 MHz (Japanese FM band)
• 87.5 MHz to 107.9 MHz (Band II) allocated to FM radio broadcasting.

DRM+ has a narrow bandwidth and is designed to fit in the FM broadcast band plan and a frequency grid of 100 kHz. Its small spectrum needs supports its use in crowded bands. The high commonality with the existing DRM standard allows easy and fast equipment implementation. DRM+ provides bit rates from 35 kbps to 185 kbps and, like DRM, permits up to four services. It is therefore a flexible solution allowing single or small numbers of audio services to be broadcast together. Before standardisation as a revision to the existing DRM System specification, ETSI ES 201 980, the Consortium's members will test and verify the design with both laboratory and field based tests.  These are currently underway in Germany - the authorities in Hanover/Lower Saxony and Kaiserslautern/Rhineland-Palatinate have provided test licences.