Digital Audio Broadcasting

masonchung · 發表於 2007-1-30 15:19:11

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

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, 99% of stereo stations on DAB use a bit rate below 192 kbps MP2[1], which means that they sound worse than FM[2] for stationary reception of music. With mobile reception, FM suffers from fading caused by multipath as well as noise, and may 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 effectively 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 will support the new DAB standard will become available in spring 2007.

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.

Utilization 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 sometimes produce a "bubbling mud" sound. In an equally covered area , DAB will generally produce less hiss and crackle than FM.

Cost of ownership
DAB transmits several channels per multiplex, meaning ownership and maintenance can be outsorced and provided by one organisation instead of each radio station, lowering the maintenance cost over time.

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
In the UK, Denmark, Norway and Switzerland, which are among the leading countries with regard to implementing DAB, 99% [5] of stereo stations on DAB use a bit rate below 192 kbps MP2, which is the bit rate at which the MP2 audio codec begins to dip into the 'Annoying' audio quality band in blind listening tests [6]. This means that all these stations broadcast with a sound quality that is lower than FM sound quality[7]. In order to increase the choice of stations in each multiplex, broadcasters have utilised lower bit rates. However DAB uses the MP2 audio codec, which is meant to be used at bit rate levels of 192 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:

“ A value of 256 kbit/s has been judged to provide a high quality stereo broadcast signal [4]. However, a small reduction, to 224 kbit/s is often adequate, and in some cases it may be possible to accept a further reduction to 192 kbit/s, especially if redundancy in the stereo signal is exploited by a process of 'joint stereo' encoding (i.e. some sounds appearing at the centre of the stereo image need not be sent twice). At 192 kbit/s, it is relatively easy to hear imperfections in critical audio material. ”
— [3]

On 6 July 2006 the BBC reduced the bitrate transmission of Radio 3 from 192 kbit/s to 160 kbit/s. The resulting degradation of audio quality prompted a number of complaints to the Corporation.[8] The BBC later announced that following this testing of new equipment, it would resume the previous practice of transmitting Radio 3 at 192 kbit/s whenever there were no other demands on bandwidth.[9]

Reception quality
The reception quality on DAB is often very 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 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).

No traffic announcements
Although DAB could offer traffic announcements, there are to date no traffic announcement services available via DAB, so RDS FM remains the only way to receive localised traffic announcements when listening to the radio or CDs when driving.

Criticism of compatibillity
In 2006 tests finally begun 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 energy efficiency
DAB chipsets have never been power efficient. Revised DAB+ AAC+ chipsets are projected by manufacturers to be up to seven times more power efficient, increasing battery life for portable radios by a factor of three. [10].

Criticism of environmental issues
A national migration to the DAB standard will render all FM receivers obsolete and will result in large amounts of unnecessary waste, containing a number of environmentally unhealthy components.

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.

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Bands and modes
Eureka 147 DAB uses a wide-bandwidth broadcast technology and typically spectra have been allocated for it in Band III (174–240 MHz) and L band (1452–1492 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:

Mode I for Band III, Earth
Mode II for L-Band, Earth and satellite
Mode III for frequencies below 3 GHz, Earth and satellite
Mode IV for L-Band, Earth and satellite

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

Primary services, like main radio stations
Secondary services, like additional sports commentaries
Data services
Electronic Programme Guide (EPG)
Collections of HTML pages and digital images (Known as 'Broadcast Web Sites')
Slideshows, which may be synchronised with audio broadcasts
Video
Java Platform Applications
IP tunneling
Other raw data

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.

Single frequency networks
Main article: Single-frequency_network
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.

'DAB version 2'
WorldDMB, the organisation in charge of the DAB standards, announced in a press release in November 2006[11] , that DAB would be adopting the HE-AAC audio codec[12], which is also known as AAC+. Also being adopted are the MPEG Surround format, and stronger error correction coding called Reed-Solomon coding. The new standard has been named DAB+.

Receivers that support the new DAB standard will be released in the UK in Spring 2007, and Ofcom has said that services using the old MPEG-1 Audio Layer II audio format used on the old DAB system can be switched off once the vast majority of receivers can support the new HE-AAC audio format. The new HE-AAC audio format is around 3 - 4 times as efficient as the old MPEG-1 Audio Layer II audio format, so this will allow 3 - 4 times as many stations to transmit as is currently possible, or the audio quality can be increased, or a combination of both these things can occur.

DAB and DAB2 can't be used for mobile TV because they don't include any video codecs. DAB related standards Digital Multimedia Broadcasting (DMB) and DAB-IP are suitable for mobile radio and TV both because they have MPEG 4 AVC and WMV9 respectively as video codecs. However a DMB video subchannel can easily be added to any DAB transmission -- as DMB was designed from the outset to be carried on a DAB subchannel. DMB broadcasts in Korea carry conventional MPEG 1 Layer II DAB audio services alongside their DMB video services.

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