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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 demonstrationin Geneva in 1988.

• In 1992, the frequencies of the L- and S-band wereallocated to DAB on a worldwide basis.

• The first consumer-type DAB-receivers were presentedin 1995 in Berlin.

• An extensive testing program has been carried out from1994 until 1997 by the American "Electronics IndustriesAssociation" (EIA).

• Totally 6 candidate systems were tested.

• Conclusions:"Of all systems tested, only the EUREKA-147 DAB

system offers the audio quality and signal robustnessperformance that listeners would expect a new DAR(Digital Audio Radio) service".

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Fig 1. Evolution of DAB Receivers

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• EUREKA-147 consortium joined the WorldDAB Forum,a forum of Digital Audio Broadcasting with worldwidemembership.

• The introduction of terrestrial DAB in Europe wasdiscussed in 1995.

• A total of 73 frequency blocks to be used in the futureand current DAB was agreed in cooperation withEuropean Broadcasting Union (EBU), EuropeanCommission (EC) and International TelecommunicationsUnion (ITU).

• Regular DAB services are already in operation in theU.K., Sweden and Germany.

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2. Overview and summary of main system features

• Can be operated at any frequency from 30 MHz to 3GHz for mobile reception.

• May be used on terrestrial, satellite, hybrid (terrestrialwith satellite) and cable broadcast networks.

• Allows flexible, general purpose digital multiplex whichcan carry a number of services (Not just audio).

• Meets all the demanding requirements drawn up by theITU (in ITU-R Recommendations 774 and 789).

• Is adopted by the European TelecommunicationsStandards Institute (ETSI) as an European Standard(ETS 300401, Mar 1997).

• The transmitted information is spread in both frequencyand time so that the effects of channel distortions andfades are eliminated in the receiver, even under severemultipath propagation conditions.

Fig 2. multipath propagation

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Summary of the Main System Features

• The DAB transmission signal carries a multiplex ofseveral digital services (audio and data) simultaneously.

• Its overall bandwidth is 1.536 MHz, providing a usefulbit-rate capacity of approximately 1.5 Mbit/s in acomplete "ensemble".

• Each service is independently error protected with acoding overhead ranging from about 25% to 300% (25%to 200% for sound), the amount of which depends on therequirements of the broadcasters (transmitter coverage,reception quality).

• A specific part of the multiplex contains information onhow the multiplex is actually configured, so that thereceiver can decode the signal correctly. It may alsocarry information about the services themselves and thelinks between different services.

• In particular, the following principal features have beenspecified:

Flexible audio bit-rate

• Varies from 8 kbit/s to 384 kbit/s

• An ensemble can provides typically 5 to 6 high-qualitystereo audio programmes or up to 20 restricted qualitymono programmes.

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Data services

• Can be a separately defined stream or can be dividedfurther by means of a packet structure.

Programme Associated Data (PAD)

• Embedded in the audio bitstream, for data transmittedtogether with the audio programme (e.g. lyrics, phone-in telephone numbers).

• The amount of PAD is adjustable (min. 667 bit/s), atthe expense of capacity for the coded audio signal.

Conditional Access (CA)

• Applicable to each individual service or packet in thecase of packet-mode data.

• The DAB ensemble transports the CA information andprovides the actual signal scrambling mechanisms.

Service Information (SI)

• Used for operation and control of receivers

• Provides information for programme selection to theuser

• Establishes links between different services in themultiplex as well as links to services in other DABensembles and even to FM/AM broadcasts.

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3. Outline of the DAB System

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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 ServiceChannel (MSC), according to a pre-determined, butadjustable, multiplex configuration.

• The multiplexer output is combined with MultiplexControl and Service Information in the Fast InformationChannel (FIC) to form the transmission frames in theTransmission Multiplexer.

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• Orthogonal Frequency Division Multiplexing (OFDM) isapplied to shape the DAB signal.

• The signal is then transposed to the appropriate radiofrequency band, amplified and transmitted.

• The bandwidth of a DAB signal is 1.536 MHz.

Fig 4. Concept of DAB Reception

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4. Details of the DAB system

Audio Services:

• Source coding:

• Low-bit-rate 32-channel sub-band coding systemenhanced by a psychoacoustic model.

• Known as MUSICAM

• Also adopted in international standards ISO/IEC11172-3 (MPEG 1 audio layer II) and ISO/IEC 13818-3 (MPEG 2 Audio layer II)

Fig 5. Psychoacoustic Masking

• The DAB Specification permits full use of the flexibilityof Layer II

• Sampling frequency: 48 or 24 kHz

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• Supports mono, stereo and dual-channel transmission(e.g. bilingual programmes).

• Encoded bit-rate options: (8, 16, 24, 32, 40, 48, 56, 64,80, 96, 112, 128, 144, 160 or 192 kbit/s) per monochannel.

• A stereophonic signal may be conveyed in the stereomode, or in the joint stereo mode.

• The latter uses the redundancy and interleaving of thetwo channels of a stereophonic programme to maximizethe overall perceived audio quality.

Data Services

Programme Associated Data

• Each audio programme contains Programme AssociatedData (PAD) with a variable capacity (667 - 65k bit/s)which is used to convey information together with thesound programme.

• Typical examples of PAD applications are• dynamic range control information,• a dynamic label to display programme titles or lyrics,• speech/music indication• text with graphic features.

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Independent Data Services

• General data may be transmitted as a separate service in1 of the 3 following forms:

• In a continuous stream segmented into 24 ms logicalframes with a data rate of n x 8 kbit/s. (n x 32 kbit/sfor some code rates)

• In packet mode, where individual packet data servicesmay have much lower capacities and are bundled in apacket sub-multiplex.

• As a part of the FIC (Fast Information Channel).

• Typical examples of Independent Data Services:• Traffic Message Channel,• correction data for Differential GPS,• paging and• electronic newspaper.

Conditional Access

• Every service can be fitted with Conditional Access ifdesired.

• The Conditional Access (CA) system includes threemain functions:

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• Scrambling/descrambling function makes the serviceincomprehensible to unauthorized users.

• Entitlement checking consists of broadcasting theconditions required to access a service, together withencrypted secret codes to enable descrambling forauthorized receivers.

• Entitlement management function distributesentitlements to receivers.

Service Information

• The following elements of Service Information (SI) canbe made available to the listener for programmeselection and for operation and control of receivers:

• basic programme-service label (i.e. the name of aprogramme service)

• programme-type label (e.g. news, sports, classicalmusic)

• dynamic text label (e.g. the programme title, lyrics,names of artistes)

• programme language

• time and date, for display or recorder control

• switching to traffic reports, news flashes orannouncements on other services

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• cross-reference to the same service being transmittedin another DAB ensemble or via AM or FM and toother services

• Essential items of SI that are used for programmeselection are carried in the FIC.

• Other Information may be carried separately as a generaldata service (in Auxiliary Information Channel).

Channel Coding and Time Interleaving

• Energy dispersal scrambling: a pseudorandom bitsequence is added to the data in order to randomize theshape of the DAB signal and thus efficiently use poweramplifiers.

• Convolutional encoding: Redundant bits are added to thedata in order to help the receiver detect and correcttransmission errors.

• Unequal Error Protection (UEP): The amount ofredundancy added to different parts of an audio framedepends on their sensitivity to transmission errors.

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Fig 6. Unequal Error Protection

• Time interleaving: The bit stream is divided into blocksof 16 bits and then delayed by a multiple of 24msaccording to a pseudo-random sequence of between 0 to15 so as to modify the burst nature of transmission errorto random nature.

Main Service Multiplex

• The encoded and interleaved data is fed to the MainService Multiplexer (MUX) where every 24 ms the datais gathered in sequences.

• The combined bit-stream output from the multiplexer

• 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 1.8Mbit/s, which depends on the convolutional code rate.

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Table 1 shows the number of audio channelspossible in a DAB ensemble at different bit-rates.

• The precise information about the contents of the MainService Multiplex

• is known as the Multiplex Configuration Information(MCI) and

• is carried by FIC

• is highly protected and repeated frequently.

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Transmission Frame

• Each transmission frame begins with a null symbol forsynchronization followed by a phase reference symbolfor differential demodulation.

• The next symbols are reserved for the FIC and theremaining symbols provide the MSC.

Fig 7. Total OFDM symbol duration

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Modulation with OFDM

• The DAB system uses a multicarrier scheme known asOrthogonal Frequency Division Multiplexing (OFDM)

• The information is divided into a large number of bit-streams with low bit-rates each.

• Each is used to modulate individual orthogonal carriers(Differential QPSK).

Fig 8. Orthogonal Frequency Division Multiplexing (OFDM)

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• Techniques used in OFDM to eliminate transmissionerror and multipath effect:

• Long symbol duration

• Symbol duration is larger than the delay spread ofthe transmission channels so as to solve multipatheffect.

• It is designed to suffer neither from Doppler spreadnor from delay spread, both inherent in mobilereception with multipath echoes.

• Guard interval

• A guard interval is inserted between successivesymbols such that channel selectivity and multipathpropagation will not cause inter-symbol interference.

• Frequency interleaving

• With multipath propagation, some of the carriersexperience constructive/destructive interference.

• The OFDM system provides frequency interleavingby a re-arrangement of the digital bit-stream amongthe carriers.

• The noise degradation at the highest frequency is equalto 1 dB at 100km/h under the most critical multipathconditions.

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• The large number of orthogonal carriers is knowncollectively as a "DAB block" and occupies a bandwidthof 1.536 MHz.

Transmission modes

• There are 4 transmission modes.

Table 2. DAB transmission parameters for eachtransmission mode.

• Mode I

• is most suitable for a terrestrial Single-FrequencyNetwork (SFN) in the VHF range

• allows the greatest transmitter separations (96km).

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• Mode II

• is for medium-scale SFN in L-band and for local radioapplications that require one terrestrial transmitter.

• Mode III

• is most suitable for cable, satellite and complementaryterrestrial transmission, since

• can be operated at all frequencies up to 3 GHz formobile reception and

• has the greatest phase-noise tolerance.

• Mode IV

• is used in L-band and allows a greater transmitterspacing in SFNs.

• is less resistant to degradation at higher vehicle speeds.

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5. Implementation of DAB networks

• The specification of the DAB signal gives the details ofthe characteristics of a signal.

• A terrestrial DAB distribution network can beimplemented as follows.

Fig 9. Conceptual DAB distribution network.

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• A Service Provider creates and manages the data.

• An Ensemble Provider assembles information/datafrom many different services Providers into a set ofdata representing the complete DAB ensemble.

• Transmitter Stations generate and radiate an ensemble.

• DAB system is suitable for satellite as well as forhybrid/mixed terrestrial/satellite broadcasting, using asimple Omni-directional receiving antenna.

• Complementary terrestrial transmitters may benecessary.

• The DAB satellite system will have the samemodulation/coding system parameters as the terrestrialsystem such that the same receiver and antenna can beused both for terrestrial and satellite DAB.

Fig 10. Conceptual DAB satellite transmissionCYH/MMT/DAB/p.24

6. DAB-base multimedia and data services

• DAB also opens up opportunities for completely newservices.

• In the future, radio programmes will be supplemented bypictures, texts and graphics (Multimedia Radio).

• A transmission protocol for multimedia applications anda standard digital interface are both essential forMultimedia radio.

• Multimedia applications generally rely on filescontaining relevant data for the selected service (e.g.text, picture, sound or video transmission) together withadditional information to allow for data presentation andclassification.

• Each item consisting of a file and plus the additionalinformation is referred to as a "Multimedia Object".

• It is managed with a Multimedia Object Transferprotocol (MOT).

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• Application examples:

Data management:• Traffic and travel information:• Traffic messages• Traffic navigation• Travel information

Text transmission

• Dynamic label: intended for short messages (<128characters) to be shown on a simple receiver display.

• Interactive text transmission system: allows for menu-driven operation.

Electronic newspaper

• Provide on-line multimedia information.

• Benefits• Real-time coverage of large geographic zones.• Immediate delivery• Low cost per reader• No consumption of paper• Wireless reception• Access on demand• Mobility• Easy access• Easy re-use of information• Novel use of the data (e.g. voice technology)• Low network installation costs

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Picture transmission

• Compressed image files can be transmitted separatelyas PAD.

• Files containing texts and pictures can be transmittedin an HTML format such that Internet services can beaccessible via DAB.

Differential GPS

• GPS provides a positional accuracy of between 300 to100 meters.

• It is possible to improve the position accuracy to lessthan 20 meters by placing an additional GPS receiverat a precisely known location.

TV transmission to mobiles

Fax