The land-based technology was first deployed in Great Britain in 1995, and has since become established throughout Europe. The first satellite-based digital radio system was WorldSpace, which orbited the first of its three geostationary earth orbit (GEO) satellites, AfriStar, in 1998. Each satellite transmits three overlapping signal beams carrying more than 40 channels of programming; most of world (except mainly North America and Australia) is covered. The first satellite-based system to provide a mobile subscription digital radio service covering the United States was XM Satellite Radio, which orbited two GEO satellites in 2001. XM's ground station transmits digital signals to its satellites, which retransmit them directly to radio receivers on the ground. The receivers unscramble the signal, which contain up to 100 channels of digital audio. In metropolitan areas where tall buildings, overpasses, and other obstacles can interfere with the signals when, for example, the receiver is in a moving vehicle, a network of ground-based repeaters retransmit the signals. The receiver also buffers the signal briefly so that if it loses the satellite signal it can use one from a repeater to maintain a continuous broadcast. Sirius Satellite Radio, which launched national service to the United States in 2002, employs three satellites in inclined elliptical orbits instead of GEO satellites.
Digital Radio Mondiale is also the name of the international non-profit consortium designing and implementing the platform. Radio France Internationale, TéléDiffusion de France, BBC World Service, Deutsche Welle, Voice of America, Telefunken (now Transradio) and Thomcast (now Thomson SA) took part at the formation of the DRM consortium.
The principle of DRM is that bandwidth is the limited element, and computer processing power is cheap. So modern CPU-intensive audio compression techniques enable more efficient use of available bandwidth.
DRM can deliver FM-comparable sound quality, but on frequencies below 30 MHz (long wave, medium wave and short wave), which allow for very-long-distance signal propagation. VHF is also under consideration, under the name "DRM+". DRM has been designed especially to use portions of older AM transmitter facilities such as antennas, avoiding major new investment. DRM is robust against the fading and interference which often plagues conventional broadcasting on these frequency ranges.
The encoding and decoding can be performed with digital signal processing, so that a cheap embedded computer with a conventional transmitter and receiver can perform the rather complex encoding and decoding.
As a digital medium, DRM can transmit other data besides the audio channels (datacasting) — as well as RDS-type metadata or program-associated data as Digital Audio Broadcasting (DAB) does. Unlike most other DAB systems, DRM uses in-band on-channel technology and can operate in a hybrid mode called Single Channel Simulcast, simulcasting both analog signal and digital signal.
Current broadcasters include Vatican Radio, BBC World Service, Deutschlandradio, biteXpress, HCJB, Radio Canada International, Deutsche Welle, Radio Netherlands, Radio Telefís Éireann (RTÉ), Radio Exterior de España, Rai and Radio New Zealand International.
Until now DRM receivers have typically used a personal computer. A few manufacturers are presently producing stand alone DRM receivers (Sangean, Morphy Richards, Starwaves). Kenwood and Fraunhofer presented a prototype standalone receiver chip in September 2006. It will be produced by STMicroelectronics. Himalaya demonstrated their two models in 2006.
Morphy Richards is mass producing DRM receivers, which are being promoted by the broadcaster Deutsche Welle . The receivers cost under £169.99 in the UK (as of October 2008). At the time of writing, Morphy Richards are only distributing these sets around Germany, Austria and the UK, but Europe-wide distribution is expected shortly, with grey market importers using German sourced radios to distribute across Europe.
Broadcasters have some freedom of choice depending on the material they send. The most commonly used mode is HE-AAC (also called AAC+) that offers an acceptable audio quality somewhat comparable to FM broadcast.
The choice of transmission parameters depends on signal robustness wanted, propagation conditions. Transmission signal is affected by noise, interference, multipath wave propagation and Doppler effect.
It is possible to choose among several error coding schemes and several modulation patterns: 64-QAM, 16-QAM and 4-QAM. OFDM modulation has some parameters that must be adjusted depending on propagation conditions. This is the carrier spacing which will determine the robustness against Doppler effect (which cause frequencies offsets, spread: Doppler spread) and OFDM guard interval which determine robustness against multipath propagation (which cause delay offsets, spread: delay spread). The DRM consortium has determined 4 different profiles corresponding to typical propagation conditions:
The trade off between these profiles stands between robustness, resistance in regards to propagation conditions and useful bit rates for the service. This table presents some values depending on these profiles. The more the carrier spacing is the more the system is resistant to Doppler effect (Doppler spread). The more the guard interval is the more the system is resistant to long multipath propagation (delay spread).
The resulting low-bit rate digital information is modulated using COFDM. It can run in simulcast mode by switching between DRM and AM, and it is also prepared for linking to other alternatives (e.g. DAB or FM services).
|Mode||OFDM Carrier spacing (Hz)||Number of carriers||Symbol length (ms)||Guard interval length (ms)||Nb symbols per frame|
|9 kHz||10 kHz||18 kHz||20 kHz|
There is also a lower bandwidth two-way communication version of DRM as a replacement for SSB communications on HF - note that it is NOT compatible with the official DRM specification.
The Dream software will receive the commercial versions and also limited transmission mode using the FAAC AAC encoder.
This table shows an example of useful bitrates depending on protection classes, OFDM propagation profiles (A or B), carrier modulation (16QAM or 64QAM) and channel bandwidth (9 or 10 kHz):
|Protection class||A (9 kHz)||B (9 kHz)||B (10 kHz)||C (10 kHz)||D (10 kHz)|
Wider bandwidth channels will be used, which will allow radio stations to use higher bit rates, thus providing higher audio quality. One likely channel bandwidth is 50 kHz, which will allow DRM Plus to carry radio stations at near CD-quality. A 100 kHz DRM+ channel has sufficient capacity to carry one mobile TV channel: it would be feasible to distribute mobile TV over DRM+ rather than DMB or DVB-H.