Nº 5 2013 > The future of digital radio with the DRM+ broadcasting system
Contributed by P.V. Giudici, Vice-Chairman of the Radiocommunication Advisory Group and Alfredo Magenta, member of the Radio Regulations Board
Radio is of vital cultural importance throughout the world. It is consumed at home, at work and on the move. This article looks at the importance for the global information society of switching the broadcasting service from analogue to digital technology in all its assigned bands. It discusses the family of Digital Radio Mondiale (DRM) broadcasting systems and highlights the advantages and disadvantages of DRM+.
Digital radio technology, standards and equipment
The digitization of broadcasting first materialized in sound and television programme production at a time when the digital distribution of broadcasting signals to emission centres was already a reality. The aim was to improve sound and picture quality. The next steps were the digitization of satellite emission of sound and television programmes and, more recently, of terrestrial emission of television programmes.
The step that remains to be taken is to digitize the emission of sound programmes in some terrestrial broadcasting bands. This implies digitizing the terrestrial sound broadcasting services now operating in amplitude modulation (AM) and frequency modulation (FM) modes, with the aim of harmonizing the various emission systems now in operation, and their reception.
This last step will entail a complete revision of frequency allocations, a revised definition of the broadcasting services, and perhaps new frequency plans, at least for broadcasting emission. Furthermore, since the payload will always consist of a bit stream, albeit broadcast for different purposes and under different emission standards, the current distinctions between broadcast and non-broadcast applications will become obsolete.
The choice of the desired programme and the way it is presented to the user will be facilitated by appropriate measures implemented to assemble the programme files and by the use of intelligent receivers. This technological convergence will make it possible to bridge the digital divide and allow the development of the universal mobile telecommunication system (UMTS).
DRM broadcasting systems have been designed by broadcasters with the support of manufacturers of broadcast transmitters and receivers, and with the help of international organizations involved in the development of telecommunications, including many standardization bodies.
The family of DRM standards specifies different operation modes, which can be grouped into two categories: the DRM30 mode, designed for use in the AM broadcasting bands below 30 MHz; and the DRM+ mode, which uses spectrum in the VHF bands above 30 MHz and is intended as a possible replacement for analogue FM broadcasting.
The DRM broadcasting systems have been specified by ITU and by the European Telecommunications Standards Institute (ETSI). For instance, Recommendations ITU–R BS 1114, ITU–R BS 1660, ITU–R BS 1894 and Reports ITU–R BS 2214 and ITU–R BS 2280 apply to the DRM+ system.
Advantages and disadvantages of the DRM+ system
The DRM+ system, which operates in the frequency bands above 30 MHz, offers a number of advantages. It is compatible with current analogue emissions, but uses much less spectrum than current analogue stereo FM emissions, thereby improving robustness, quality, performance and ancillary services. Like the DRM30 system, it operates in orthogonal frequency-division multiplexing mode at a wide variety of bit rates and uses MPEG-4 encoding and decoding. It can thus deliver speech, mono or stereo music, or data with a high robustness suitable for mobile reception.
The DRM+ system can operate on single or multiple frequency networks, making it possible to cover the same service area with lower emitted power. Broadcasters can reuse part of their analogue equipment because the DRM+ system can use current transmitting and receiving antennas without any technical intervention. This open, non-proprietary system makes it possible to fit more than 200 channels with 100 kHz bandwidth for audio or video — or more than 800 channels with 25 kHz bandwidth for audio or data — in the 87.5–108 MHz frequency band.
All these advantages have been confirmed experimentally, for example by Vatican Radio, Italy, and the French company, TDF. For instance, joint measurements performed by Italy and the Vatican have shown that the current 10 kW analogue broadcasting service at 103.8 MHz allocated by the GE-84 Agreement to the Vatican for Rome cannot survive a specific interferer at 0.1 MHz. But good service is obtained on the same frequency, in the presence of the same interferer, when the DRM+ system is used, even if the emitted power is reduced by about 13 dB.
What are the disadvantages of the DRM+ system? First, the availability of low-cost DRM+ consumer receivers is still very limited. This would be overcome if public authorities were to foster the introduction of the DRM+ system, as was done in the past for other digital services of public interest.
Second, greater understanding is needed of the self-fading phenomenon, which is observed when the time difference between the direct signal and its reflection is short with respect to the transmitted pulse width. Depending on the robustness (bit redundancy) of the DRM+ signal, this fading may occur when the antenna of a vehicle travelling at high speed receives a reflected signal. Both these problems would be solved with a transition from analogue to digital sound broadcasting. In particular, this would open up a big market for new consumer receivers, which would prompt manufacturers to invest in the new technology, in turn reducing the cost of consumer equipment.
Mobile phones produced by leading manufacturers now increasingly receive FM broadcasts. This allows capillary reception of FM broadcasts over very wide service areas — micro-diffusion over macro-areas. No costs are associated with this mode of operation because there is no radio-frequency emission from the mobile phone, which does not use telephonic algorithms and does not block a telephone channel. The mobile phone just emits an acoustic signal upon the arrival of a telephone call, and the user has the choice of switching from radio programme reception to telephone reception. This application of mobile phones consumes very little power and — with the use of earpieces — causes no acoustic pollution. The DRM+ system uses very little spectrum per channel and would be an ideal replacement for conventional analogue FM broadcasting in this application. Broadcasters would obviously remain responsible for the operation of broadcast transmitters for the application.
Economic, social and regulatory (planning) aspects
Radio-frequency services are based on international agreements on technical parameters to ensure mutual compatibility between analogue and digital transmissions, so that these transmissions can coexist without undue mutual interference. Sufficient time must be allowed for a transition from analogue to digital broadcasting to take into account the technical and economic constraints of operators and users.
A gradual transition will also simplify the planning of new emissions and reduce the digital divide, releasing spectrum that ITU Member State administrations can reassign or reuse according to national needs. The amount of spectrum that could be freed up can be computed, for instance on the basis of ITU Report BS.2208, which notes that four full-quality digital stereo programmes (or a television programme of reduced image quality) can be accommodated in a bandwidth of 100 kHz.
For the same emitted power and under ideal orographic conditions, the use of the digital DRM+ system greatly extends the service area as compared to analogue emissions. Also, the modifications required to adapt linear analogue transmitters for digital operation are quite simple and would have little impact on the budget of broadcasters currently working in the FM bands. Indeed, the cost of modifying the equipment is small compared to the cost of the equipment itself, and no modification is required to transmitting and receiving antennas. Non-linear transmitters can be operated in linear mode by reducing the emitted power, since this has no impact on the service area, which remains the same as for the FM analogue service.
Because DRM systems are open and non-proprietary, investment costs are reduced and access is facilitated. Furthermore, the use of DRM systems allows broadcasters to continue to use the channels that have been assigned to them without any need for prior agreement on the part of any entity or group, except of course the required authorization from the entity that granted the right to use those channels for analogue FM.
Who benefits from digital broadcasting?
Who would benefit from the digital broadcasting use of the current FM bands? The obvious answer is administrations that wish to resolve the overcrowding of the FM band, reduce interference, and facilitate at little cost a reliable nation-wide service. Such a service would benefit all users, particularly those who are comfortable with using a mobile phone.
Operating a digital broadcasting service with lower emitted power for the same service coverage leads to a reduction in radio interference, reduced electric power consumption, and less electromagnetic pollution, to the benefit of citizens and the environment.
Digital systems by their very nature perform in such a way that the received service is always perceived as being of top quality. This includes single-frequency network (SFN) services to car drivers and reception of broadcasting on mobile phones.
Digital technology facilitates the convergence of different services and the feasibility of a new generation of multi-service intelligent receivers for consumer use, reasonably priced and capable of receiving all broadcasting systems on all broadcasting bands, thus opening the door to a new telecommunication concept.
The larger the market for consumer receivers, the lower the cost of the receiver, thus facilitating the offer of new applications and further increasing the size of the potential market. This would eventually open the door to a UMTS, which would integrate a large number of current or new systems and services, as is happening now with mobile phones.
Among the actors in the development of new broadcasting technologies, the most important is always the end user, whose choices are oriented by market forces, conditioned by administrations and consumer equipment manufacturers.
End users can be encouraged to abandon their old analogue broadcasting devices and buy new digital devices. But when the end users are ready to buy new digital broadcasting equipment that manufacturers have put on the market, administrations should already have developed their plans for the use of the spectrum, and broadcasters should already be broadcasting interesting programmes. Coordinated actions are essential to steer the transition, consumer choices and the development of a competitive market among manufacturers.
It is important to make receiving equipment available at low cost, and to foster technological convergence. Such convergence will be fully achieved when the digital divide is closed and the millions of analogue broadcasting receivers have all been replaced by digital receivers. At that point, UMTS will be fully developed and it will be possible — through the remote control of the end-user device — to send and receive information anywhere in the world, at any time, in any language, whether from a stationary or a mobile position, ensuring the ecological protection of consumers. This will be the foundation of the global information society.