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Digital Audio Broadcasting (DAB) & Digital Multimedia Broadcasting (DMB)

Digital Audio Broadcasting (DAB)

 

Digital Audio Broadcasting (DAB), also known as Eureka 147, is a technology for broadcasting of audio using digital radio transmission.

The original objectives of converting to digital transmission were to enable higher fidelity, more stations and more resistance to noise, co-channel interference and multipath than in analogue FM radio. However, in the UK, Denmark, Norway and Switzerland, which are the leading countries with regard to implementing DAB, the vast majority of stereo radio stations on DAB sound worse than on FM due to the bit rate levels they use being too low for the inefficient MPEG Layer 2 audio codec to provide good audio quality[3][4]. With mobile reception, FM can suffer from fading caused by multipath, and can sound worse than DAB.

In November 2006, WorldDMB announced that the DAB system was in the process of being upgraded, and it will adopt the AAC+ audio codec to improve the efficiency of the system and stronger error correction coding to improve the robustness of transmissions. This means there are now two different versions of the DAB system: the older one, developed in the late 1980s, and an upgraded version, which has been named "DAB+". Existing DAB receivers are incompatible with the new DAB+ standard, but receivers that support the new DAB+ standard are expected to be in the shops by summer 2007.

History

DAB has been under development since 1981 at the Institut für Rundfunktechnik (IRT).

In 1985 the first DAB demonstrations were held at the WARC-ORB in Geneva and in 1988 the first DAB transmissions were made in Germany.

Later DAB (or Eureka-147) was developed as a research project for the European Union (Eureka project number EU147), which started in 1987 on initiative by a consortium formed in 1986. The MP2 (MPEG-1 layer-2) audio coding technique was created as part of the EU147 project. DAB was the first standard based on orthogonal frequency division multiplexing (OFDM) modulation technique, which since then has become one of the most popular transmission schemes for modern wideband digital communication systems.

A choice of audio codec, modulation and error-correction coding schemes and first trial broadcasts were made in 1990. Public demonstrations were made in 1993 in the United Kingdom. The protocol specification was finalized in 1993 and adopted by the ITU-R standardization body in 1994, the European community in 1995 and by ETSI in 1997. Pilot broadcasts were launched in several countries in 1995.

The UK was the first country to receive a wide range of radio stations via DAB. Commercial DAB receivers began to be sold in 1999 and over 50 commercial and BBC services were available in London by 2001. The UK has to date been the most successful market for DAB and is being projected to be in 40% of homes by 2009.[5]

By 2006, 500 million people worldwide were in the coverage area of DAB broadcasts, although by this time sales had only taken off in the UK and Denmark. In 2006 there are approximately 1,000 DAB stations in operation world wide.[6]

The standard was coordinated by the European DAB forum, formed in 1995 and reconstituted to the World DAB Forum in 1997, which represents more than 30 countries. In 2006, World DMB Forum took over the coordination.

DAB and FM/AM compared

Traditionally radio programmes were broadcast on different frequencies via FM and AM, and the radio had to be tuned into each frequency. This used up a comparatively large amount of spectrum for a relatively small number of stations, limiting listening choice. DAB is a digital radio broadcasting system that through the application of multiplexing and compression combines multiple audio streams onto a single broadcast frequency called a DAB ensemble.

Within an overall target bit rate for the DAB ensemble, individual stations can be allocated different bit rates. The number of channels within a DAB ensemble can be increased by lowering average bit rates, but at the expense of the quality of streams. Error correction under the DAB standard makes the signal more robust but reduces the total bit rate available for streams.

DMB (Digital Multimedia Broadcasting)

Digital Multimedia Broadcasting (DMB) is a digital radio transmission system for sending multimedia (radio, TV, and datacasting) to mobile devices such as mobile phones. This technology was first developed in South Korea under the national IT project and the world's first official DMB broadcast started in South Korea in 2005, although trials were available much earlier. It can operate via satellite (S-DMB) or terrestrial (T-DMB) transmission. DMB is based on the Eureka 147 Digital Audio Broadcasting (DAB) standard, and has some similarities with the main competing mobile TV standard, DVB-H.

Like DAB, T-DMB is made for transmissions on radio frequency bands band III (VHF) and L (UHF), mainly for terrestrial and satellite, respectively. Because the United States and Canada still allocate the first band as for television broadcasting (VHF channels 7 to 13) and the United States reserves the L band for military applications, DMB is still unavailable in North America. Qualcomm's MediaFLO is a proprietary system used there instead. In Japan, 1seg is the standard, using ISDB.

T-DMB uses MPEG-4 Part 10 (H.264) for the video and MPEG-4 Part 3 BSAC or HE-AAC V2 for the audio. The audio and video is encapsulated in MPEG-2 TS. The stream is RS encoding and the parity word is 16 bytes length. There is convolutional interleaving made on this stream, then the stream is broadcast in data stream mode on DAB. In order to diminish the channel effects such as fading and shadowing, DMB modem uses OFDM-4DPSK modulation. A single-chip T-DMB receiver is also provided by an MPEG-2 transport stream demultiplexer. DMB has several applicable devices such as mobile phone, portable TV, PDA and telematics devices for automobiles.

T-DMB is an ETSI standard (TS 102 427 and TS 102 428).

Use of frequency spectrum and transmitter sites

DAB gives substantially higher spectral efficiency, measured in programmes per MHz and per transmitter site, than analogue communication. However, since there are no plans yet to cease analogue FM transmissions, and most radio channels are transmitted both over FM and digitally, this advantage is not exploited to a high degree.

Numerical example: FM requires 0.3 MHz per programme. The frequency re-use factor is approximately 15, meaning that only one out of 15 transmitters can use the same channel frequency without problems with co-channel interference. This results in 1 / 15 / (0.3 MHz) = 0.22 programmes/transmitter site and MHz. DAB with 192kbps codec requires 1.536 MHz * 192kbps / 1136 kbps = 0.26 MHz per channel. The frequency re-use factor for local programmes and multi-frequency broadcasting networks (MFS) is typically 4, resulting in 1 / 4 / (0.26 MHz) = 0.96 programmes per transmitter site and MHz. This is 4.3 times as efficient. For single frequency networks (SFN), for example of national programmes, the channel re-use factor is 1, resulting in 1/1/0.25 MHz = 3.85. 3.85. 17.3 times as efficient as FM.

Note the above capacity improvement may not always be achieved at the L-band frequencies, since these are more sensitive to obstacles than the FM band frequencies, and may cause shadow fading for hilly terrain and for indoor communication. The number of transmitter sites or the transmission power required for full coverage of a country may be rather high at these frequencies, to avoid that the system becomes noise limited rather than limited by co-channel interference.

Benefits of DAB

Current AM and FM terrestrial broadcast technology is well established, compatible, and cheap to manufacture. Benefits for DAB over and above analogue systems are as follows:

Simplicity of handling

DAB radios automatically tune to all the available stations, offering a list of all stations.

DAB can carry "radiotext" (in DAB terminology, Dynamic Label Segment, or DLS) from the station giving real-time information such as song titles, music type and news or traffic updates. Advance programme guides can also be transmitted. A similar feature also exists on FM in the form of the RDS. (However, not all FM receivers allow radio stations to be stored by name.)

Some radios offer a pause facility on live broadcasts, caching the broadcast stream on local flash memory, although this function is limited.

More stations

DAB is more bandwidth efficient than analogue for national radio stations due to the use of SFNs, enabling more stations to be placed into a smaller section of the spectrum, although it is only marginally more efficient than FM for local radio stations.

In certain areas—particularly rural areas—the introduction of DAB gives radio listeners a greater choice of radio stations. For instance, in South Norway, radio listeners overnight experienced an increase in available stations from 6 to 21 when DAB was introduced in November 2006.

Reception quality

The DAB standard integrates features to reduce the negative consequences of multipath fading and signal noise, which afflict existing analogue systems.

Also, as DAB transmits digital audio, there is no hiss with a weak signal, unlike with FM. However, DAB is not immune to reception problems, and DAB radios in a DAB fringe area can produce a "bubbling mud" sound. But in an equally covered area, DAB will generally produce far less hiss and crackle than FM.

Cost of ownership

DAB transmits several channels per multiplex, meaning ownership and maintenance can be outsourced and provided by one organisation instead of each radio station, lowering the maintenance cost over time. Norwegian national broadcaster NRK will reduce maintenance cost by two thirds with DAB.

Variable bandwidth

Mono talk radio, news and weather channels and other non-music programs need significantly less bandwidth than a typical music radio station, which allows DAB to carry these programs at lower bit rates, leaving more bandwidth to be used for other programs.

Criticisms of DAB

Criticism of Sound Quality

The original objectives of converting to digital transmission were to enable higher fidelity, more stations and more resistance to noise, co-channel interference and multipath than in analogue FM radio. However, in the UK, Denmark, Norway and Switzerland, which are the leading countries with regard to implementing DAB, the majority of stereo radio stations on DAB have a lower sound quality than FM[7] due to the bit rate levels they use being too low for the inefficient MPEG Layer 2 audio codec to provide good audio quality [8].However DAB uses the MP2 audio codec, which should be used at bit rate levels of 160 kbit/s or higher to provide good audio quality.

The following paragraph about bit rate levels to be used on DAB was written by an engineer in the BBC Research & Development department and highlights why bit rates as low as 128 kbit/s should not be used on DAB:

Reception quality

The reception quality on DAB can be poor even for people that live well within the coverage area. The reason for this is that the old version of DAB uses weak error correction coding so that when there are a lot of errors with the received data not enough of the errors can be corrected and a "bubbling mud" sound occurs. This situation will be improved upon in the new DAB standard (DAB+, discussed below) that uses stronger error correction coding and as signal powers are increased.

Coverage

As DAB is either at the early stages of deployment, DAB coverage is poor in nearly all countries in comparison to the high population coverage provided by FM.

Criticism of transmissions cost

Transmission on DAB is far more expensive than on FM, and measures taken by broadcasters to limit their costs have resulted in some DAB ensembles having to carry too many channels, forcing bit rates to be reduced to levels that deliver sound quality inferior to traditional FM (Digital audio broadcasting, Criticisms of DAB in the UK).

Criticism of compatibility

In 2006 tests finally began using the much improved HE-AAC codec for DAB. However, the new DAB standard is not backwards compatible, leaving early DAB adopters with incompatible hardware.

Criticism of 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. This translates into needing more power to effect this conversion than compared to an analogue FM to audio conversion, meaning that portable receiving equipment will tend to have a shorter battery life, or require higher power (and hence more bulk).

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 DAB, this is often between one-sixth and one-twelfth of the time they can play when in FM mode.

Other criticism

If the signal reception becomes marginal the audio will first start to burble or cut out rapidly and if the signal continues to degrade the audio will cut out more often. There is also less chance of long distance reception that hobbyists enjoy because each frequency/multiplex is used more often.

Technology

Bands and modes

Eureka 147 DAB uses a wide-bandwidth broadcast technology and typically spectra have been allocated for it in Band III (174240 MHz) and L band (14521492 MHz), although the scheme allows for operation almost anywhere above 30 MHz. The US military has reserved L-Band in the USA only, blocking its use for other purposes in America, and the United States has reached an agreement with Canada that the latter will restrict L-Band DAB to terrestrial broadcast to avoid interference.

DAB has a number of country specific transmission modes (I, II, III and IV). For worldwide operation a receiver must support all 4 modes:

Services and ensembles

Various different services are embedded into one ensemble (which is also typically called a multiplex). These services can include:

Bitrates

An ensemble has a maximum bitrate that can be carried, but this depends on which error protection level is used. However, all DAB multiplexes can carry a total of 864 "capacity units". The number of capacity units, or CU, that a certain bit rate level requires depends on the amount of error correction added to the transmission: the stronger the error protection (which requires higher levels of redundant information to be added) the more robust the transmission will be, but this reduces the overall bit rate that can be transmitted. In the UK, most services transmit using 'protection level three', being an FEC of 0.5 which equates to a maximum bit rate per multiplex of 1152 kbit/s.

Transmission

Immunity to fading and inter-symbol interference (caused by multipath propagation) is achieved without equalization by means of the OFDM and DQPSK modulation techniques.

OFDM also features Single frequency network (SFN) communication, meaning that a network of transmitters over a whole a country or a region sends the same radio programmes over the same frequency channel without interference problems. 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.

Further Information

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