| Digital Radio Mondiale |
Article Index for Digital |
Shopping Mondiale |
Website Links For Digital Radio |
Information AboutDigital Radio Mondiale |
| CATEGORIES ABOUT DIGITAL RADIO MONDIALE | |
| digital radio | |
| international broadcasting | |
| open standards | |
| radio hobbies | |
|
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 Transmitter s designed for audio AM , so major new Investment s 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. 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 Americas ( 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 kbits/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 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 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 . There is also a lower bandwidth two-way communication version of DRM as a replacement for SSB communications on HF. This 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): 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 radio stations
DRM radio techniques
DRM's COFDM
Index
|
|
|