Digital Radio Mondiale

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.

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. — [1]

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:
*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 broadcasting can be done on different bandwidth:
*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.

Mode	OFDM Carrier spacing (Hz)	Number of carriers				Symbol length (ms)	Guard interval length (ms)	Nb symbols per frame
Mode	OFDM Carrier spacing (Hz)	9 kHz	10 kHz	18 kHz	20 kHz	Symbol length (ms)	Guard interval length (ms)	Nb symbols per frame
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 [2]- 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)
Protection class	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

External links

DRM in general

*Digital Radio Mondiale (DRM) - official homepage
*hard-core-dx: Latest news on DRM
*(pdf) DRM - progress on the receiver front
*A Listeners' Guide to Digital AM (DRM)
*DRM Patent Licensing
*DRM - Digital Radio Mondiale - Digital AM radio below 30 MHz
*UK Digital Radio News
*KortbÃ¸lge stationsdatabase kan downloades efter registrering (tager nogle dage): The World Top Shortwave Broadcasting Database: "This ILGRadio Database is now in use by more than 23 000 listeners, living in 139 countries all over the world!"
*DRM Digital Radio - A broadcasters view of the future

DRM software

* DRM Software Radio developed by the Fraunhofer IIS
* Dream DRM receiver. An open source software radio published by the University Darmstadt (Germany)
* HamDream A modificated DREAM receiver supporting 2.5 kHz bandwidth.
* Diorama DRM receiver. An open source DRM receiver written in MATLAB by the Institute of Telecommunications of the University Kaiserslautern (Germany))
* Spark A DRM software transmitter developed by the University of Applied Sciences Kaiserslautern (Germany)

Digital Radio Mondiale: Encyclopedia BETA

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