Digital Audio Broadcasing, Technical Description

Chris Angove's wealth of experience of plane wave radio propagation, especially at VHF, has proved invaluable in understanding the basic objectives of the DAB system. He has a good understanding of how the DAB broadcast system works, including synchronisation, and how it improves upon the Band II analog system, a summary of which is shown below.

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1          The Effective Elimination of Multipath Interference

Provided a sufficiently strong signal is available for it to operate correctly, Digital Audio Broadcasting (DAB) was designed with special features to practically eliminate multipath interference or fading, a serious affliction of most of the existing analog systems of frequency modulated (FM) broadcast services which DAB is intended to eventually replace. The standards were targeted at the largest potential audience sector for DAB: those in moving vehicles for which FM multipath fading is the most troublesome. Multipath fading also often degrades the quality of reception on portable radios which rely on an integral (telescopic rod-type) antenna when they are used in domestic environments.

2          Perception of a Service 'Threshold'

The advantages of DAB can only be experienced if sufficient signal is available at the radio receiver, and that depends on many factors such as the closeness of its antenna to the transmitting antenna, radio propagation conditions, the local geography, and the influence of hills, large buildings and metallic reflective structures such as bridges, warehouses etc. One criticism of DAB is that, if the received signal becomes marginal, with most consumer receivers there is no apparent 'threshold' warning, the audio output for the whole multiplex simply mutes until the signal strength recovers again. Sometimes it is easy to mistake this for a receiver fault. Usually the same situation occurring with an FM broadcast receiver, assuming the muting is disabled, would progressively increase the audio noise, identifying that the signal is becoming weak.

3          The Difference Between DAB and FM

The only similarity between DAB and FM is that it uses an RF medium to propagate radio waves through the atmosphere from the transmitting antenna to the receiver antenna. Whilst the RF medium alone is strictly analog, the modulation is digital and the entire DAB service incorporates various sophisticated digital features to set up a high capacity one-way data channel from the transmitter to the receiver with appreciable bandwidth. This channel is known as a multiplex or ensemble and is analogous to a data 'pipe' which carries information from the transmitter to the receiver. Each multiplex is allocated a frequency band by the radio regulatory authorities and is managed by the licence holder. Most consumer DAB receivers cannot tune to more than one multiplex simultaneously. Each multiplex has sufficient capacity to carry several audio channels, the number depending on their individual data rates or capacities. Channels with speech or talk type formats would require less bandwidth and thus be allocated less capacity than those for high fidelity music. Most multiplex operators provide a spread of both types according to their broadcasting licence. For example, in the UK the BBC DAB multiplex carries 12 audio channels with capacities (data rates) ranging from 80 kbit/s (speech format) to 192 kbit/s (quality music format), although there is only one service that uses 192 kbit/s. Debate continues on whether the other services are allocated sufficient capacity for the audio fidelities claimed. Across the RF propagation path, each DAB multiplex comprises many closely spaced, individually modulated carriers known as sub-carriers, each of which carries a small part of the overall multiplex information. For example, in the UK a DAB multiplex comprises 1536 sub-carriers, each occupying a bandwidth of about 1 kHz and each sub-carrier is modulated at a very slow rate as, in this case, it carries just 1/1536 of the full multiplex capacity. This technique allows the symbol period for each sub-carrier to be appreciable compared to the typical duration of the multipath fade that is likely to affect it.  At the receiver the demodulated information from all sub-carriers are combined to form the 'aggregate' multiplex channel which comprises traffic for all of the audio service channels plus overheads. The DAB multiplex bandwidth of about 1.536 MHz for, say 12 audio service channels, compares to a narrow frequency band of about 150 kHz, or 9%, for a traditional FM channel allocated to just one high quality analog service.

4          Coded Orthogonal Frequency Division Multiplexing

The modulation used in DAB is Coded Orthogonal Frequency Division Multiplexing (COFDM). According to this acronym the three properties of COFDM are: 'C' for coding; 'O' for orthogonal modulation and 'FDM' for frequency division multiplexing. These are described here.

4.1        Coding

Coding refers to convolutional coding and means that the original data carried over the multiplex is deliberately manipulated by splitting it into small blocks and adding some intelligently designed redundant information to each thus generating an 'overhead'. The overhead bits added to each block are determined according to rules applied to the true data content of the block. After demodulation at the receiver the digital signal processor examines both the actually received data and overhead bits and regenerates what it believes to be the original data based on a set of statistical rules known as an algorithm. The regenerated data may include a number of data bit corrections. The algorithm used in DAB is known as a Viterbi algorithm, and is an example of a maximum-likelihood algorithm. This works by maintaining a history of demodulated bit sequences, building up a view of their probabilities and then using these to finally select either a 0 or 1 for the bit under consideration. This type of coding is an example of forward error correction (FEC).

To some extent the types of errors most likely to be present with DAB can be mathematically predicted and therefore corrected for. The addition of FEC requires extra information to be transmitted at the same time as the original traffic data and therefore requires an increased channel capacity needing extra bandwidth compared to if it had been uncoded. DAB carries different 'strengths' of FEC, a stronger one being used for the control of critical features in the receiver.

4.2        Orthogonal

Orthogonal is the mathematical term applied to two RF sinusoidal signals when their phase relationship is precisely 90 degrees. Alternatively they may be said to be in ‘quadrature'. In DAB the sub-carrier frequency spacing is chosen to be the reciprocal of the active symbol period. Under this condition the DAB modulation results in successive sub-carriers having a quadrature relationship with each other. The frequency spectra components of one modulated sub-carrier will therefore integrate to zero at the corresponding components from both of the adjacent sub-carriers. This has two advantages: (a) the modulated sub-carrier spectra will efficiently occupy the allocated bandwidth with a degree of controlled overlapping and (b) simple I-Q demodulation to zero intermediate frequency (zero-IF) can be used in the receiver without needing the costly overhead of many bandpass filters to extract the sub-carriers.  

4.3        Frequency Division Multiplexing

Frequency division multiplexing (FDM) is the process where two or more basic information channel bandwidths or basebands are shifted in absolute frequency and are added to others to form an aggregate wider bandwidth containing the information from all of the constituent basebands. To avoid mutual interference, their bandwidths would normally require shifting (translating) in frequency and no two translated basebands would occupy any part of the same frequency spectrum. In the context of DAB, FDM refers to the manner in which the modulated sub-carriers are assembled across the allocated frequency range.

5          Modulation Type

DAB uses a digital modulation type known as differential quadrature phase shift keying (DQPSK), which is an incoherent modulation scheme. DQPSK differs from the more common quadrature phase shift keying (QPSK) in that the modulated carrier phase for the current symbol being detected depends on its phase relative to that for the previous one. In QPSK it is just the absolute phase of the modulated carrier that determines the associated symbol. A differential modulation scheme can be more resilient to the typical fading scenarios of DAB. The modulation scheme also incorporates a form of Gray coding in that only one bit changes on moving from one symbol state to an adjacent one. The consecutive set of symbols are represented by the bit pairs 00, 01, 11 and 10.  

6          Time Interleaving

DAB uses data buffering which enables the data symbols to be transmitted over the RF path in a different time-order from which they were generated by the audio source (studio). At the receiver they are re-assembled and returned to the original time-order before conversion back to analog signals at the receiver audio output. This process is called time interleaving. Typical multipath interference experienced in a moving vehicle is regular over time so an intelligent choice of time interleaving to some degree 'averages' out the resulting error bursts over time. This data buffering and other processing contributes to a delay, typically of a few seconds, between the studio source and the receiver. This is much longer than the equivalent delay for am FM broadcast channel which would typically be a fraction of a second. For most broadcasts such a delay would be unimportant but it does mean that, for example, real-time reference signals for setting clocks such as those re-broadcast by the BBC on DAB from their national FM service are actually quite inaccurate.

7          Frequency Interleaving

DAB also uses frequency interleaving, a similar technique to time interleaving but applied to the sub-carriers centre frequencies in the RF spectrum instead. The data stream from the studio is deliberately not modulated serially onto sub-carriers across the frequency range, but in a more random way. Multipath and other forms of selective fading generally affect a relatively narrow part of the RF multiplex bandwidth at any one time so frequency interleaving would tend to average out 'bursts' of errors resulting from these.

 

8          Single Frequency Networks

A major advantage of DAB over FM is the provision of single frequency networks (SFNs). Provided the transmitters are synchronised, the multiplex licence holder may operate several in a relatively small geographic area at the same multiplex frequency without any destructive interference occurring at the receiver. SFNs allow substantial service areas to be built up steadily and efficiently as the network develops, funding allows and frequency spectra becomes available. Compared to FM where service areas operating at the same carrier frequency cannot overlap, a typical DAB network will comprise several relatively low powered closely spaced transmitters operating at the same multiplex frequency. This saves frequency spectrum, reduces the complexity and cost of the transmitter hardware and avoids the need for frequent re-tuning of mobile receivers as they move about within the network. It also means that each transmitter has a smaller audience, thus mitigating the service loss should a transmitter fail. Because of this synchronisation, receivers which are located in places where the service areas of two or more transmitters overlap will interpret one of the signals as a slightly delayed version of the other, effectively an apparent ''deliberate multipath interference''. The actual delays will depend on the radio path geometry and any extra delays that may be added artificially when the network is commissioned. Within the receiver then a relatively simple form of delay filtering may be applied to extract the desired data.

 

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