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Digital Audio Broadcasting 1. Evolution of DAB • The EUREKA-147 consortium was founded in 1987. • First equipment was assembled for mobile demonstration in Geneva in 1988. • In 1992, the frequencies of the L- and S-band were allocated to DAB on a worldwide basis. • The first consumer-type DAB-receivers were presented in 1995 in Berlin. • An extensive testing program has been carried out from 1994 until 1997 by the American "Electronics Industries Association" (EIA). • Totally 6 candidate systems were tested. Fig 1. Evolution of DAB Receivers • Conclusions: "Of all systems tested, only the EUREKA-147 DAB system offers the audio quality and signal robustness performance that listeners would expect a new DAR (Digital Audio Radio) service". CYH/MMT/DAB/p.1 CYH/MMT/DAB/p.2 2. Overview and summary of main system features • EUREKA-147 consortium joined the WorldDAB Forum, a forum of Digital Audio Broadcasting with worldwide • Can be operated at any frequency from 30 MHz to 3 membership. GHz for mobile reception. • The introduction of terrestrial DAB in Europe was • May be used on terrestrial, satellite, hybrid (terrestrial discussed in 1995. with satellite) and cable broadcast networks. • A total of 73 frequency blocks to be used in the future • Allows flexible, general purpose digital multiplex which and current DAB was agreed in cooperation with can carry a number of services (Not just audio). European Broadcasting Union (EBU), European Commission (EC) and International Telecommunications • Meets all the demanding requirements drawn up by the Union (ITU). ITU (in ITU-R Recommendations 774 and 789). • Regular DAB services are already in operation in the • Is adopted by the European Telecommunications U.K., Sweden and Germany. Standards Institute (ETSI) as an European Standard (ETS 300401, Mar 1997). • The transmitted information is spread in both frequency and time so that the effects of channel distortions and fades are eliminated in the receiver, even under severe multipath propagation conditions. Fig 2. multipath propagation CYH/MMT/DAB/p.3 CYH/MMT/DAB/p.4 Summary of the Main System Features Data services • The DAB transmission signal carries a multiplex of • Can be a separately defined stream or can be divided several digital services (audio and data) simultaneously. further by means of a packet structure. • Its overall bandwidth is 1.536 MHz, providing a useful bit-rate capacity of approximately 1.5 Mbit/s in a Programme Associated Data (PAD) complete "ensemble". • Embedded in the audio bitstream, for data transmitted together with the audio programme (e.g. lyrics, phone- • Each service is independently error protected with a in telephone numbers). coding overhead ranging from about 25% to 300% (25% • The amount of PAD is adjustable (min. 667 bit/s), at to 200% for sound), the amount of which depends on the the expense of capacity for the coded audio signal. requirements of the broadcasters (transmitter coverage, reception quality). Conditional Access (CA) • A specific part of the multiplex contains information on • Applicable to each individual service or packet in the how the multiplex is actually configured, so that the case of packet-mode data. receiver can decode the signal correctly. It may also • The DAB ensemble transports the CA information and carry information about the services themselves and the provides the actual signal scrambling mechanisms. links between different services. Service Information (SI) • In particular, the following principal features have been specified: • Used for operation and control of receivers • Provides information for programme selection to the Flexible audio bit-rate user • Varies from 8 kbit/s to 384 kbit/s • Establishes links between different services in the • An ensemble can provides typically 5 to 6 high-quality multiplex as well as links to services in other DAB stereo audio programmes or up to 20 restricted quality ensembles and even to FM/AM broadcasts. mono programmes. CYH/MMT/DAB/p.5 CYH/MMT/DAB/p.6 3. Outline of the DAB System Generation & Reception of a DAB Signal Fig 3. Concept of DAB Signal Generation • Each service signal is coded individually at source level, error protected and time interleaved in the channel coder. • The services are multiplexed in the Main Service Channel (MSC), according to a pre-determined, but adjustable, multiplex configuration. • The multiplexer output is combined with Multiplex Control and Service Information in the Fast Information Channel (FIC) to form the transmission frames in the Transmission Multiplexer. CYH/MMT/DAB/p.7 CYH/MMT/DAB/p.8 • Orthogonal Frequency Division Multiplexing (OFDM) is 4. Details of the DAB system applied to shape the DAB signal. Audio Services: • The signal is then transposed to the appropriate radio frequency band, amplified and transmitted. • Source coding: • The bandwidth of a DAB signal is 1.536 MHz. • Low-bit-rate 32-channel sub-band coding system enhanced by a psychoacoustic model. • Known as MUSICAM • Also adopted in international standards ISO/IEC 11172-3 (MPEG 1 audio layer II) and ISO/IEC 13818- 3 (MPEG 2 Audio layer II) Fig 4. Concept of DAB Reception Fig 5. Psychoacoustic Masking • The DAB Specification permits full use of the flexibility of Layer II • Sampling frequency: 48 or 24 kHz CYH/MMT/DAB/p.9 CYH/MMT/DAB/p.10 • Supports mono, stereo and dual-channel transmission (e.g. bilingual programmes). Independent Data Services • Encoded bit-rate options: (8, 16, 24, 32, 40, 48, 56, 64, • General data may be transmitted as a separate service in 80, 96, 112, 128, 144, 160 or 192 kbit/s) per mono 1 of the 3 following forms: channel. • In a continuous stream segmented into 24 ms logical • A stereophonic signal may be conveyed in the stereo frames with a data rate of n x 8 kbit/s. (n x 32 kbit/s mode, or in the joint stereo mode. for some code rates) • In packet mode, where individual packet data services • The latter uses the redundancy and interleaving of the may have much lower capacities and are bundled in a two channels of a stereophonic programme to maximize packet sub-multiplex. the overall perceived audio quality. • As a part of the FIC (Fast Information Channel). • Data Services Typical examples of Independent Data Services: • Traffic Message Channel, • Programme Associated Data correction data for Differential GPS, • paging and • Each audio programme contains Programme Associated • electronic newspaper. Data (PAD) with a variable capacity (667 - 65k bit/s) which is used to convey information together with the Conditional Access sound programme. • Every service can be fitted with Conditional Access if • Typical examples of PAD applications are desired. • dynamic range control information, • a dynamic label to display programme titles or lyrics, • The Conditional Access (CA) system includes three • speech/music indication main functions: • text with graphic features. CYH/MMT/DAB/p.11 CYH/MMT/DAB/p.12 • Scrambling/descrambling function makes the service • cross-reference to the same service being transmitted incomprehensible to unauthorized users. in another DAB ensemble or via AM or FM and to other services • Entitlement checking consists of broadcasting the conditions required to access a service, together with • Essential items of SI that are used for programme encrypted secret codes to enable descrambling for selection are carried in the FIC. authorized receivers. • Other Information may be carried separately as a general • Entitlement management function distributes data service (in Auxiliary Information Channel). entitlements to receivers. Service Information Channel Coding and Time Interleaving • The following elements of Service Information (SI) can • Energy dispersal scrambling: a pseudorandom bit be made available to the listener for programme sequence is added to the data in order to randomize the selection and for operation and control of receivers: shape of the DAB signal and thus efficiently use power • basic programme-service label (i.e. the name of a amplifiers. programme service) • Convolutional encoding: Redundant bits are added to the • programme-type label (e.g. news, sports, classical data in order to help the receiver detect and correct music) transmission errors. • dynamic text label (e.g. the programme title, lyrics, names of artistes) • Unequal Error Protection (UEP): The amount of redundancy added to different parts of an audio frame • programme language depends on their sensitivity to transmission errors. • time and date, for display or recorder control • switching to traffic reports, news flashes or announcements on other services CYH/MMT/DAB/p.13 CYH/MMT/DAB/p.14 Fig 6. Unequal Error Protection • Time interleaving: The bit stream is divided into blocks of 16 bits and then delayed by a multiple of 24ms according to a pseudo-random sequence of between 0 to 15 so as to modify the burst nature of transmission error Table 1 shows the number of audio channels to random nature. possible in a DAB ensemble at different bit- rates. Main Service Multiplex • The precise information about the contents of the Main Service Multiplex • The encoded and interleaved data is fed to the Main • is known as the Multiplex Configuration Information Service Multiplexer (MUX) where every 24 ms the data (MCI) and is gathered in sequences. • is carried by FIC • The combined bit-stream output from the multiplexer • is highly protected and repeated frequently. • Is known as the Main Service Channel (MSC) • Has a gross capacity of 2.3 Mbit/s. • The net bit-rate ranges from approximately 0.6 to 1.8 Mbit/s, which depends on the convolutional code rate. CYH/MMT/DAB/p.15 CYH/MMT/DAB/p.16 Transmission Frame Modulation with OFDM • Each transmission frame begins with a null symbol for • The DAB system uses a multicarrier scheme known as synchronization followed by a phase reference symbol Orthogonal Frequency Division Multiplexing (OFDM) for differential demodulation. • The information is divided into a large number of bit- • The next symbols are reserved for the FIC and the streams with low bit-rates each.
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