A J Bower (BBC) INTRODUCTION Radio is entering the digital era, where services are delivered from the studio to the receiver, entirely in the digital domain. The Digital Audio Broadcasting (DAB) System developed within the Eureka 147 Project and now standardised by the European Telecommunications Standards Institute provides the means to deliver high-quality digital radio services to the listener. In the United Kingdom, the BBC is pioneering the introduction of digital radio services by establishing a national transmission network for DAB, and exploring the potential of DAB to deliver exciting new programming that would have previously been impossible using existing analogue AM/FM radio systems. THE UNIQUE BENEFITS OF DAB The Eureka 147 Digital Audio Broadcasting system is the most significant advance in radio technology since the introduction of FM stereo. It offers both listeners and broadcasters a unique combination of benefits and opportunities. These are:
TECHNICAL OVERVIEW OF THE EUREKA 147 SYSTEM Eureka 147 DAB is a reliable, multi-service, digital radio broadcasting system, designed specifically for robust reception by mobile, portable, and fixed receivers, using simple non-directional antennas. ETSI specification, EN 300 401 (Second Edition) specifies the transmitted DAB signal. The Eureka 147 System comprises three main elements. These are:
MUSICAM AUDIO CODING MUSICAM is an acronym for: Masking Pattern Universal Sub-band Integrated Coding And Multiplexing. MUSICAM employs the technique of psychoacoustical coding as specified for MPEG-2 Audio Layer II encoding. This exploits knowledge of the properties of the human auditory system; in particular, the spectral and temporal masking effects of the inner ear. Essentially, the system codes only audio signal components that the ear will hear, and discards any audio information that, according to the psychoacoustical model, the ear will not perceive. Thus valuable bit-rate capacity is allocated only to coding and conveying information that is important to maintaining a high subjective audio quality. The principle of audio masking is illustrated in Fig. 1. The signal component at 1 kHz, distorts and raises the masking threshold which defines the level that other signal components must exceed in order to be audible. If a second audio component is present at the same time and close in frequency to the first, then for the second component to be perceived by the ear, it must be at a higher level than it would otherwise need to be if present only on its own; otherwise it is masked by the first signal.
Fig. 1 - Psychoacoustical Masking. MUSICAM is a highly efficient audio coding process which can compress digital audio signals to one of a number of possible encoding options in the range 8 kbit/s to 384 kbit/s, at a sampling rate of 48 kHz or, when a service can tolerate lower audio quality, 24 kHz. It is capable of delivering high-quality audio signals, that are subjectively perceived to be of 'CD quality' using bit-rates of 192 kbit/s or above for stereo services. The coding option selected for a given service in a multiplex will depend on the audio-quality required for that service; the type of service, e.g. music or speech; and the number of services that the broadcaster wants to accommodate within the fixed capacity of the DAB multiplex. Five or more high-quality services within a single multiplex is typical. Currently, the BBC is using 192 kbit/s to code its stereo national network radio services (Radio 1 - Radio 4) and 96 kbit/s to code Radio 5 Live which is mono. Other mainly speech based services, such as additional Radio 5 sports commentaries and BBC Parliament, are encoded at lower rates, typically less than 96 kbit/s. TRANSMISSION CODING & MULTIPLEXING Data for individual services, be they audio-based or data / multimedia, must be combined into a single data stream ready for transmission. This process is known as multiplexing, and the resulting data stream is called the multiplex. The frame-based DAB multiplex (shown in Fig. 2) comprises three distinct elements:
Fig. 2 - DAB Multiplex Frame Structure. Convolutional forward-error-correction (FEC) channel coding, time interleaving and frequency interleaving are applied to the multiplex data to provide strong protection against bit-errors, so that the services decoded at the receiver are quasi-error-free even when the received signal is impaired. The gross data capacity for the entire DAB signal is approximately 3 Mbit/s, of which the Main Service Channel occupies approximately 2.3 Mbit/s. After allowing for the redundancy provided by the channel encoding, a net useful payload in the range of 0.6 - 1.7 Mbit/s is available. For the BBC's national multiplex, the useful payload of the MSC is approximately 1.2 Mbit/s. COFDM MODULATION DAB uses the spectrally-efficient multi-carrier digital modulation scheme, Coded Orthogonal Frequency Division Multiplexing (COFDM). Instead of a having a single digitally modulated carrier with a very high symbol rate, COFDM uses many carriers -- up to 1536, spaced at 1 kHz separation for DAB -- with each carrier independently modulated using differential quadrature phase shift keying. The multiplex data is distributed amongst all the carriers, occupying approximately 1.54 MHz of spectrum, see Fig. 3. Consequently, the symbol-rate on any individual carrier is much lower and results in a longer symbol period which affords some protection against multipath echoes where the receiver sees a signal direct from the transmitter, plus a number of delayed signals due to reflections from terrain and buildings. By deliberately repeating part of each symbol during the so-called guard-interval, COFDM provides enhanced tolerance against multipath. As long as the delay of the echo signals is less than the guard-interval, there will be a constructive benefit to reception. In fact, it allows the broadcaster to introduce the concept of the single frequency network (SFN) of transmitters, where "man-made multipath" is deliberately created by having all the transmitters in the network transmitting the same signal on the same frequency -- something that would have been impossible with existing analogue radio systems. This allows efficient use of the scare RF spectrum available -- a key advantage of the Eureka 147 DAB system.
Fig. 3 - RF Spectrum of the DAB Signal. ROLL OUT OF DAB IN THE UNITED KINGDOM In the United Kingdom, 12.5 MHz of Band III spectrum from 217.5 - 230 MHz has been allocated to DAB. This will accommodate seven multiplexes. The BBC has been allocated one of these channels for its national DAB multiplex to operate on a single frequency network of transmitters throughout the UK. The remaining six multiplexes are to be regulated by the Radio Authority and will be licensed to commercial broadcasters for national, regional and local independent radio services, starting in the Spring of 1998. The BBC's local & regional services will also accommodated within these multiplexes. The BBC is committed to the introduction of digital radio in the UK and is establishing a single frequency network of 27 transmitting stations, that is on target for completion by Spring 1998 (Fig. 4). This SFN will initially serve at least 60% of the UK population and providing coverage to the major motorway routes. CONCLUSIONS The Eureka 147 DAB system provides both the listener and the broadcaster with new opportunities and significant benefits compared with existing analogue radio systems. It will provide reliable and rugged reception of high-quality digital radio services, including multimedia services, to mobile, portable and fixed receivers. Broadcasting is about to experience a revolution as it enters the digital era. The Eureka 147 DAB system ensures that radio is well prepared for this revolution, and the commitment of the BBC to implementing DAB will ensure that the British public will be well served by digital radio, at the earliest possible opportunity.
For further information, please visit: BBC R&D DAB project page. |
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