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It is also the name of the international Non-profit Consortium designing and implementing the platform. Radio France Internationale , TéléDiffusion De France , Deutsche Welle , Voice Of America , Telefunken (now Transradio ) and Thomcast (now Thomson SA ) took part at the formation of the DRM consortium. FEATURES 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 , Simulcast ing both Analog Signal and Digital Signal . STATUS The LW/MW/SW standard has been approved by the IEC , and the ITU has approved its use in most of the world. Approval for ITU Region 2 (North and South America and the Pacific) is pending amendments to existing international agreements. The inaugural broadcast took place on June 16 , 2003 , in Geneva , Switzerland , at the ITU's annual World Radio Conference . Current broadcasters include BBC World Service , Radio Luxembourg , BiteXpress , Passion Radio , Radio Canada International , Deutsche Welle , Radio Netherlands , and Radio New Zealand International . Until now DRM receivers have typically used a . Himalaya will also demonstrate their two models in fall 2006. Morphy Richards has recently announced the start of mass production DRM receivers, which are being promoted by the broadcaster Deutsche Welle {Link without Title} . The receivers cost under €200 or £199.99 in the UK (as of June 2007), and are expected to drop further as production continues. 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. Digital Radio Mondiale is being considered by Ofcom for introduction in Britain in 2012 on the present AM medium wave band. The British Broadcasting Corporation BBC has announced that it is to undertake a trial of the digital radio mondiale (DRM) technology, which will allow it to explore digital radio using medium-wave frequencies. The trial will broadcast BBC Radio Devon using the new technology in the Plymouth area and will last for a year from the end of April 2007. International regulation On 28 September 2006 , the Australia n spectrum regulator, the Australian Communications And Media Authority , announced that it had "placed an embargo on frequency bands potentially suitable for use by broadcasting services using Digital Radio Mondiale..." being "5950–6200, 7100–7300, 9500–9900, 11650–12050, 13600–13800, 15100–15600, 17550–17900, 21450–21850 and 25670–26100 kHz {Link without Title} TECHNIQUE Audio source coding Useful bitrates with DRM range from 8 kbit/s to 20 kbit/s for a standard 10 kHz bandwidth channel. It is possible to achieve bitrates up to 72 kbit/s by using more bandwidth than 10 kHz. (For comparison, pure digital HD Radio can broadcast 40 kbit/s using 10 kHz width and 60 kbit/s using extended bandwidth.) Useful bitrate depends also on other parameters like wanted robustness to errors (error coding), power needed (modulation scheme), robustness in regard to propagation conditions (multipath, doppler). So DRM offers the possibility to use different audio coding system (source coding) depending on the bitrate:
All Codec s can optionally be combined with Spectral Band Replication . 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. Bandwidth DRM broadcasting can be done on different bandwidths:
Modulation The modulation used for DRM is COFDM (Coded Orthogonal Frequency Division Multiplexing), where every carrier is modulated with QAM (Quadrature Amplitude Modulation) with a choosable error coding. 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 tradeoff between these profiles stands between robustness, resistance in regards to propagation conditions and useful bitrates 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- Bitrate 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). DRM has been tested successfully on Shortwave , Mediumwave (with 9 as well as 10 KHz Channel Spacing ) and Longwave . There is also a lower bandwidth two-way communication version of DRM as a replacement for SSB communications on HF {Link without Title} - 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. Error coding Error coding can be chosen to be more or less robust. This table show 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): DRM PLUS While DRM currently covers the broadcasting bands below 30 MHz, the DRM consortium voted in March 2005 to begin the process of extending the system to the VHF bands up to 120 MHz. DRM Plus (DRM+) will be the name of this technology. Design, development and testing are expected to be completed by 2007-2009. 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 or DVB-H . REFERENCES
SEE ALSO
EXTERNAL LINKS DRM in general
DRM broadcast transmitter manufacturers
DRM software
DRM radio techniques
DRM's COFDM
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