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Digital Radio Mondiale

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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 analog signal and digital signal.

Contents

Introduction

Advantages of DRM Technology

The main advantage of such digital broadcasting 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 Insititute for Integrated Circuits (IIS) has announced to present a System-On-Chip design at the Internationale Funkausstellung Berlin 2006 in Berlin. — [link]

DRM as 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.

Technique

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: 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 broadcasting can be done on different bandwidth:

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:

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 intervall 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.
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

There is also a lower bandwidth two-way communication version of DRM as a replacement for SSB communications on HF [link]- note that it is NOT compatible with the official DRM specification.

The Dream software ( http://drm.sourceforge.net/ ) 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):
Protection class A (9 kHz) B (9 kHz) B (10 kHz) C (10 kHz) D (10 kHz)
64-QAM 16-QAM 16-QAM 64-QAM 16-QAM 64-QAM 16-QAM 64-QAM
0 19,6 kbit/s 7,6 8,7 17,4 6,8 13,7 4,5 9,1
1 23,5 10,2 11,6 20,9 9,1 16,4 6,0 10,9
2 27,8 - - 24,7 - 19,4 - 12,9
3 30,8 - - 27,4 - 21,5 - 14,3

Conclusion

Compared to AM broadcasting DRM is very scalable and so offers many adjustments to the broadcaster depending on transmitter power, region targeted, frequency, program material. Fortunately all these parameters are transparent for the listener because they are automatically handled by the receiver.

Further developments

DRM Plus / 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.

References

See also

External links

DRM in general

DRM software

DRM radio stations

DRM radio techniques

DRM's COFDM

Index

 

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