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Digital Radio Mondiale has used established technology and processes to create an extremely efficient digital radio system. The DRM system can carry up to four audio or data channels in a multiplex or bouquet. Data channels are basically file transfers or multimedia slideshows rather than the scrolling text that is associated with an audio signal. Just how many of these can be carried effectively in the multiplex depends on the data capacity available.
The Digital Radio Mondiale system has been designed to be compatible with the existing broadcasting bands so much effort has gone into containing DRM signals into half-channels, channels and double-channels based on the ‘standard’ AM channel sizes of 9 kHz and 10 kHz. Consequently there are DRM channels of 4.5 kHz, 5 kHz, 9 kHz, 10 kHz, 18 kHz and 20 kHz specified. Each of these permutations has a theoretical data capacity as well as a practical one when all the overheads of the encoding processes are added. Broadcasts use 4-QAM, 16-QAM or 64-QAM for different parts of the multiplex with 4-QAM offering most protection against errors.
Typically broadcasters use 11.6, 14.6 , 17.4 and 20.9 kilobits per second (kbps) on Short-wave and the handful of Medium Wave broadcasts have used bit rates about 18.4 kbps in Mode B and 19.6 kbps in Mode A. Some tests have used bit rates as high as 23.5 or even 27.5 kbps but these high coding rates make reliable reception difficult.
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