DRM has excellent sound quality plus the ease-of-use that comes from digital transmissions. The improvement brought by DRM30 over AM is immediately noticeable, and DRM+ removes the fading that mars FM reception. 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.
DRM30 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. DRM+ is designed for the VHF bands, including the international FM band. In all VHF frequencies DRM+ occupies 100 kHz channels.
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 audio codecs to provide high quality at low data rates. Extended HE-AAC and HE-AACv2 are available.