| About Digital Radio Mondiale (DRM) |
| Digital Radio Mondiale (DRM) is firstly an international non-profit consortium bound by a consortium agreement and committed to designing and implementing an open-source platform for digital radio broadcasting around the world, especially on shortwave and secondly the set of technologies promoted by that consortium. Unlike most other DAB systems, DRM uses IBOC technology and can operate in a hybrid mode called Single Channel Simulcast, simulcasting both analogue signal and digital signal. |
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| Advantages of DRM Technology |
| The main advantage of the DRM digital broadcasting system, is that it is capable of delivering sound quality which at its best, could be comparable to band 2 FM radio broadcasts, but over long wave, medium wave and short wave frequencies and distances. More recently (early 2006) VHF bands are also under consideration for transmitting this digital broadcast mode. DRM is robust when combatting the effects of fading and interference. As a digital medium, DRM can also transmit other digital data besides digitized music, including text, pictures, and computer programs (datacasting) — as well as RDS-type metadata or program-associated data like DAB does. DRM has been designed especially to use older transmitters designed for audio AM, so major new investments are not required for early adopters. The encoding and decoding can be performed with digital signal processing, so that small computers added to a conventional transmitter and receiver can perform the rather complex encoding and decoding. The Fraunhofer Institute for Integrated Circuits (IIS) has announced a System-On-Chip design at the Internationale Funkausstellung Berlin 2006 in Berlin. |
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| DRM an international standard |
| The organization has recently received approval for the AM standard from the IEC, and the ITU has approved its use in most of the world. Approval for the ITU region 2 is pending amendments to other existing international agreements. The inaugural broadcast took place on June 16, 2003, in Geneva, Switzerland, at the ITU's annual World Radio Conference. |
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| DRM Technical |
| Source Coding |
| Useful bitrates with DRM range from 8 kbit/s to 20 kbit/s for a standard broadcast channel (10 kHz bandwidth). It is possible to achieve bitrates up to 72 kbit/s by using more bandwidth than 10 kHz. 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: |
| MPEG-4 HE-AAC (High Efficiency - Advanced Audio Coding). AAC is a perceptual coder suited for voice and music and the High Efficiency is an optional extension for reconstruction of high frequencies (SBR: spectral bandwidth replication) and stereo image (PS: Parametric Stereo). |
| MPEG-4 CELP which is a parametric coder suited for voice only (vocoder) but that is robust to errors and needs a small bitrate. |
| MPEG-4 HVXC which is also a parametric coder for speech programs that use an even smaller bitrate than CELP. |
| All codecs 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 which is comparable, to some extent, to FM broadcast. |
| Bandwidth |
| DRM can broadcast on the following bandwidths: |
| 9 kHz or 10 kHz which are the standard bandwidth of an AM broadcasting channel so existing frequency plan can be reused. |
| 4.5 kHz or 5 kHz which are half channels. The idea is to offer a possibility for the broadcaster to do simulcast and use half a channel for AM and the other half for DRM. However the resulting bitrate and audio quality is less. |
| 18 kHz or 20 kHz which correspond to a coupling of two adjacent channels. It offers the possibility to offer a better audio quality or to multiplex audio channels in the same transmitter. |
| 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. |
| So it is possible to choose among several error coding schemes and several modulation patterns: 64-QAM, 16-QAM and 4-QAM. OFDM modulation has also got 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: |
| A: Gaussian channel with very little multipath propagation and Doppler effect. This profile is suited for local or regional broadcasting. |
| B: multipath propagation channel. This mode is suited for medium range transmission. It is nowadays frequently used. |
| C: similar to profile B but with better robustness to Doppler (more carrier spacing). This mode is suited for long distance transmission. |
| D: similar to mode B but with a resistance to large delay spread and Doppler spread. This case exists with adverse propagation conditions on very long distance transmissions. The useful bitrate for this profile is decreased. |
| 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 - 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. |
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| DRM Multiplexes |
| A Digital Radio Mondiale multiplex is assembled from one to four audio or data channels in what is termed the Main Service Channel and two descriptive channels called the Fast Access Channel and the Service Description Channel. All these channels are encoded to make the data more immune to errors and the whole multiplex is then further encoded using quadrature amplitude modulation techniques. Finally Orthogonal Frequency Division Multiplexing is used to transmit the data packets via analogue carriers! |
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| Error encoding |
| 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): |
| 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 |
| A |
41,66 |
204 |
228 |
412 |
460 |
26,66 |
2,66 |
15 |
| B |
46,88 |
182 |
206 |
366 |
410 |
26,66 |
5,33 |
15 |
| C |
68,18 |
* |
138 |
* |
280 |
20,00 |
5,33 |
20 |
| D |
107,14 |
* |
88 |
* |
178 |
16,66 |
7,33 |
24 |
|
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| Future development - DRM+ |
| 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 broadcasting bands up to 120 MHz. DRM Plus will be the name of this technology and wider bandwidth channels will be used, which will allow radio stations to use higher bit rate, thus providing higher audio quality. One of the new channel bandwidths that is likely to be specified is 50 kHz, which will allow DRM+ to carry radio stations at near CD-quality. The design, development and testing phases of DRM’s extension, which are being conducted by the DRM consortium are expected to be completed by 2007-2009. A 100 kHz DRM+ channel has sufficient capacity to carry one mobile TV channel: it would be feasible to distribute mobile TV too over DRM+ than via either DMB or DVB-H. |
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| Digital Radio Mondiale (DRM) Links |
| DRM General |
| Digital Radio Mondiale (DRM) Official DRM site, with information and news from around the globe. |
| A Listeners' Guide to Digital AM (DRM) |
| DRM Patent Licensing Information on DRM licensing. |
| ILGRadio (A06) The Advanced World Radio Database SystemShort Wave Radio Software and Databases for Amateurs, Short Wave Listeners, DXers. |
| DRM Digital Radio A broadcasters view of the future. |
| DRM Software |
| DRM Software Radio The DRM Software Radio is based on the proven Fraunhofer DRM engine, which is considered to be the most advanced DRM engine today, built around fully licensed proprietary DRM encoding technology. |
| Diorama An open source DRM receiver written in MATLAB by the Institute of Telecommunications of the University Kaiserslautern in Germany. |
| Dream Open-Source Software Implementation of a DRM Receiver under the GNU General Public License (GPL). |
| HamDream A modificated DREAM receiver supporting 2.5 kHz bandwidth. |
| Spark A DRM software transmitter developed by the University of Applied Sciences Kaiserslautern, Germany. |
| DRM Radio Stations |
| biteXpress A German experimental radio station broadcasting in DRM, the station is also available also as live stream. |
| Deutsche Welle Germany's International Broadcaster, a pioneer in DRM. |
| Passion Radio, Radio station based in Sussex, UK trialling a DRM Broadcast, also available also as live stream. |
| Radio Engineering - DRM Information on DRM. |
| Radio Luxembourg The new RTL Group is using the new DR technology to re-launch Radio Luxembourg in English on several short wave frequencies, notably 7145 KHz. |