Digital Audio Broadcast Computer Science Essay

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1.0 INTRODUCTION:

Digital Audio Broadcasting (DAB) is Digital Radio Technology used for broadcasting radio stations in several countries, particularly in Europe. DAB system is the development by Eureka 147 project which was established in 1987 and now has over 40 member's countries. It comprises of manufacturers, research institutes etc. DAB system is now standardized by the European Telecommunications Standard Institute provides the means to deliver high quality radio services to the listener.

In the United Kingdom, the BBC is pioneering the introduction of the 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.

2.0 Technical Overview of DAB

Traditionally radio programmes were broadcast on different frequency. This uses a relatively large amount of spectrum for a relatively small number of stations, limiting the listening choice. To produce a digital system that operates satisfactorily under the conditions required for digital radio a large amount of work was undertaken in the development stages. Some existing digital techniques were investigated but it was realised these had significant limitations for this application. One of the major problems was that many receivers would use non-directional antennas and as a result they would pick up reflected signals. These would be delayed sufficiently for the data to become corrupted.

The technical standards for digital radio were developed under the auspices of the European Eureka Project 147. Eureka 147 DAB is a Multi-service, Digital Radio Broadcasting System and reliable, designed specifically for a robust application in Mobile and fixed receivers, using simple non-directional antennas. ETSI specification, EN 300 401(2nd Ed) [1] specifies the transmitted DAB signal.

The Eureka 147 System comprises three main elements. These are:

MUSICAM Audio Coding

Transmission Coding and Multiplexing

COFDM Modulation

2.1 MUSICAM Audio Coding

MUSICAM (Masking Pattern Universal Sub-band Integrated Coding And Multiplexing) employs the technique of psychoacoustical coding as specified for MPEG-2 Audio Layer II encoding (Also known as MP 2). This knowledge exploits the properties of the human auditory system, particularly spectral and temporal masking effects of the inner ear. The system codes only the audio signal components that the ear will hear and eliminates any audio information that according to the psychoacoustical model the ear will not perceive. Thus variable Bit-rate capacity is allocated only to coding and conveying information that to maintaining a high subjective audio quality.

The principle of audio masking shown 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 the 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 that 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 8kbit/s to 384kbit/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 will match 'CD quality' using bit-rates of 192 kbit/s or above for stereo services. The coding option selected for a given, 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 BBC is using 192 kbit/s to code its stereo national network radio services (Radio 1 - Radio 4). 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 [2].

2.2 Transmission Coding and Multiplexing

For the transmission of single Data stream for any individual date services, they may be of audio based or data/ multimedia are combined. This process is known as Multiplexing and the resulting data stream is called the Multiplex.

The frame based DAB multiplex shown in Fig. 2 contains three distinct elements:

The Synchronisation Channel which conveys reference frequency and timing information to allow receivers to synchronise and decode the received DAB signals.

The Fast Information Channel (FIC), which carries information describing the composition of the multiplex and informs receivers how to extract and decode the information for individual services.

The Main Service Channel (MSC) contains the audio frames or data packets corresponding to the different services within the multiplex. This part of the multiplex is essentially the useful payload of the DAB signal

Fig. 2 - DAB Multiplex Frame Structure.

The Forward Error Correction (FEC) channel coding, the time interleaving and frequency interleaving are applied to the multiplex date 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 [2]. 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.

The Modulation Techniques used in DAB are discussed in the next chapter.

3.0 Modulation Technique in DAB

After some extensive research into various modulation techniques, the EBU and Eureka 147 Project Team felt that coded Orthogonal Frequency Division Multiplex (COFDM) would be the modulation technique employed by the DAB system in the development. Techniques such as spread spectrum were also explored [3]. However, due to low spectrum utilization, the possibilities of using spread spectrum was abandoned. "COFDM was developed in France within the Framework of the Eureka 147(DAB) Project to meet the exacting requirements of high bit rate transmission to vehicular, portable and fixed receiver" [4].

COFDM, commonly referred to as OFDM, is a multicarrier scheme that takes the total signal and subdivides it into bit streams each with a lower bit rate. This allows having a longer duration than the expected delay spread for the transmission channel. In addition interleaving and coding procedures are used to mitigate the effects of frequency selective fading. This combined process produces a signal that is less sensitive to multipath propagation.

This OFDM concept lies in the two dimensional distribution of modulation signal. In time, the spacing allows for guard interval that protects against delay spread. In frequency, that signals are overlapping one another such that each signal's peak amplitude occurs at a zero crossing for all other symbols [5]. Hence, the system characteristically has high spectrum efficiency because the OFDM signals only requires guard interval in the time domain.

The DAB signal is transmitted in a frame structure. Each frame is divided into a integral number of modulated carriers dependent on the transmission mode. These carriers are modulated using Differential Quadrature Phase shift Keying (D-QPSK). the differential modulation is applied after the frequency interleaving. It is applied to facilitate but recovery at the receiver.

Each OFDM carrier contains two bits of gray-coded QPSK data [4]. Details of the convolution encoding have been provided later in this chapter. Therefore, determining the parameters for the convolution coding, modulation technique, frequency interleaving, time interleaving, guard interval, and bandwidth consequently defines a COFDM signalling procedure.

3.1 Generation of the DAB Signal

You will see in Figure 3 how each service signal is coded individually at source level, error protected and time interleaved in the channel coder. Then 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, which travel in the fast Information Channel (FIC), to form the transmission frames in the Transmission Multiplexer. Finally, Orthogonal Frequency Division Multiplexing (OFDM) is applied to shape the DAB signal, which consists of a large number of carriers. The signal is then transposed to the appropriate radio frequency band, amplified and transmitted.

dab tx bd.png

Fig.3- Block Diagram of DAB transmitter.

3.2 Reception of a DAB signal

Figure 4 demonstrates a conceptual DAB receiver. The DAB ensemble is selected in the analogue tuner, the digitised output of which is fed to the OFDM demodulator and channel decoder to eliminate transmission errors. The information contained in the FIC is passed to the user interface for service selection and is used to set the receiver appropriately. The MSC data is further processed in an audio decoder to produce the left and right audio signals or in a data decoder (Packet Demux) as appropriate.

dab rx bd.png

Fig.4- Block Diagram of DAB Receiver.

4.0 Advantages and Disadvantages

What does DAB have to offer?

More choice - A range of extra digital radio channels not available on FM, including BBC7, BBC 5 Live Extra, TalkSport, Planet Rock, Absolute Radio and Premier Radio

Digital quality - Signals that are less prone to interference and hiss than FM

Improved station selection - Tune in to a station by station name or format, not frequency

Extra features - Most DAB radios support scrolling radio text, but some have extras like MP3 playback and the ability to pause or rewind live radio [6].

Advantages of DAB

It has a consistent, high quality reception even in adverse propagation condition thereby overcome the problem of multipath propagation, which greatly upsets FM reception in vehicles.

DAB also has a unique ability to serve the mobile audience, thus providing high-quality coverage wherever and whenever required like that of CD, which is effectively free from interference.

Very efficient use of available VHF or UHF radio spectrum.

Good coverage for moderate transmitter power.

Push-button controlled receivers which are easy to use.

It has additional facilities which is not possible using analog FM.

Disadvantages of DAB

Extra radio channels are always a good thing and with DAB, user can listen to stations that are not available on FM or AM. DAB is not perfect though. Here are some of the things to consider with DAB:

Signal: Although the DAB signals are less prone to interference, they are currently weaker than FM signals, which can cause problems if in a fringe reception area.

Quality: DAB radio stations are "compressed", so that lots of stations can be accommodated into the available bandwidth. Because of this compression, some stations on DAB are not in as good a quality as on FM. In particular, the BBC stations are transmitted on a low bit rate to squeeze their range of stations into the available space [3].

Formats: There is a new format on the run, DAB+, which will replace the existing DAB format in use in the UK. At the time of buying a DAB radio, the user has to consider one that supports DAB+ or can be upgraded via a software update.

Cost: More expensive transmitters and receivers when compared to AM/FM systems.

Power requirements: As DAB requires digital signal processing techniques to convert from the received digitally encoded signal to the analogue audio content, the complexity of the electronic circuitry required to do this is high. Thus, needing more power to effect this conversion than compared to an analogue FM to audio conversion. So, portable receiving equipment will have a shorter battery life or require higher power (and hence more bulk). This effectively means that they are less energy efficient than an analog Band II VHF receiver. As an indicator of this increased power consumption, dual FM/DAB radios quote the length of time they can play on a single charge. For a commonly used FM/DAB-receiver from manufacturer PURE, this is stated as: DAB 10 hours, FM 22 hours [6].

DAB+ and DMB

Due to some of its (DAB) disadvantages. There has been further research conducted as a result of it the new improvised broad casting techniques are possible. Following are the two of them.

DAB+

DAB+ is based on the original DAB standard but uses a more efficient audio codec. DAB uses MPEG Audio Layer II, where as in DAB+ its HE-AAC v2 (better known as MP4 or AAC+). This allows better audio quality to be broadcast at lower bit rates [7].

Benefits of DAB+

Latest MPEG-4 audio delivers exceptional performance efficiency.

More stations can be broadcast on a multiplex.

Lower transmission costs for Digital stations.

DAB receivers are compatible with existing scrolling text and multimedia services.

Current MPEG Audio Layer II services and consumers unaffected.

Fast re-tuning response time (low zapping delay)

Many countries around the world have acknowledged the benefits of DAB+ for their individual markets. Manufacturers are also ensuring DAB+'s quick roll out with an increasing number of DAB+ receivers in the market.

DMB

Digital Multimedia Broadcasting is a video and multimedia technology based on DAB. It offers broad range of new innovative services, such as Mobile TV, traffic and safety information and many other applications. DMB is currently the most popular Mobile TV standard in Europe and Asia for commercial services.

Since DMB is a based globally used DAB core standard, DMB devices are always backward compatible and can not only receive DMB services but also DAB services. This means that DAB transmission system can be used for DMB transmission by simply adding a DMB video encoder to existing DAB system [7].

Benefits of DMB

Existing DAB transmitter networks to be adapted to carry these new services.

Both DAB and DMB services to be accessed on the same receiver.

A wide range of TV and interactive services to be broadcast simultaneously on the same multiplex (video services, DAB and DAB+ radio services etc.)

Multimedia content to be delivered without the risk of network congestion.

DMB demands less spectrum commitment than other mobile TV standards which ranges in 6-8 MHz. DMB can offer both TV and Radio services within a multiplex of just 1.5 MHz. DMB has the further benefits of being broadcast in Band III or L-Band, where higher powers give rise to broader and more comprehensive coverage. DMB technology is proven popular on a worldwide scale.

6.0 Conclusion

The Eureka 147 DAB system has given both the listener and the broadcaster with new opportunities and significant benefits compared with existing analogue radio systems. It has reliable and rugged reception of high-quality digital radio services, including multimedia services, to mobile, portable and fixed receivers. DAB provides an effective and highly flexible means of delivering digital radio services. COFDM provides a rugged radio frequency signal suitable for reception in adverse conditions. Time and frequency interleaving improves the performance of the COFDM system and the use of guard interval allows a networks of transmitters to operate on a single frequency over a wide area resulting in very efficient use of spectrum. The flexible multiplexing structure of DAB means that it is only suitable for the delivery of the digital sound broadcasting, but also a wide variety of data services offering the possibility of a genuine multimedia broadcast system.

DAB is not only technological revolution but also an economical revolution for the Broadcaster.

7.0 Referencing

[1] ETS 300 401: Radio Broadcasting Systems; Digital Audio Broadcasting (DAB) to mobile, portable and fixed receivers. ETSI, May 1997. [Available at]

http://www.rthk.org.hk/about/digitalbroadcasting/DSBS/DABETS300401.PDF [Accessed on 8 Jan 2011]

[2] Bower, A. J., 1998. Digital Radio - The Eureka 147 DAB system.[online] BBC. Pp. 55-56. [Available at] http://www.bbc.co.uk/rd/pubs/papers/paper_21/paper_21.shtml [accessed on 8 Jan 2011]

[3] Gupta, N., 1996. The Eureka 147 Digital Audio Broadcasting System Adapted to the U.S. M.S. Massachusetts Institute of Technology.

[4] EBU contribution to CCIR Report 1203: Digital Sound Broadcasting to Mobile Portable and Fixed Receivers Using Terrestrial Transmitters. First International Symposium on DAB June 1992.

[5] Eureka Project 147. Digital Audio Broadcasting System: Guidelines for Implementation and Operation, Joint Eureka-147 DAB WGI/EBU Task Force. Issue 2.2. July 1995. Vol. I

[6] BBC R & D, A Technical Overview of Digital Radio. [Available at] http://downloads.bbc.co.uk/rd/pubs/papers/pdffiles/mwrf-all.pdf [Access on: 8 Jan 2011]

[7] WorldDMB, 2011. Introduction to Digital broadcasting. [Available at] http://www.worlddab.org/introduction_to_digital_broadcasting/dab_digital_radio/how_dab_works [Accessed on 11 Jan 2011]

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