Definition and Invention of Digital audio broadcasting (DAB)
Digital audio broadcasting (DAB) is a system or process of broadcasting radio services. It is a digital radio standard for broadcasting digital audio radio services. Digital transmission of FM radio is the application of the DAB systems and technologies. DAB receivers have been available in many countries in the end of the 1990.The DAB standard was initiated as a European research project in the 1980. The Norwegian Broadcasting Corporation launched the first DAB channel in the world on 1 June 1995 and the BBC and Swedish Radio launched their first DAB digital radio broadcasts in September 1995. Now a day it is used in several countries across Europe and Asia Pacific.
Vast use of DAB technology around the world
At present more than thirty countries provide DAB radio transmissions. Several other countries such as Norway, UK, Australia, Italy, Malta, Switzerland, Netherlands and Germany, are transmitting DAB+ stations. It is a fact that DAB will gradually replace the FM radio. An upgraded version of the DAB system was released in February 2007, which is called DAB+. DAB is not forward compatible with DAB+, which means that DAB only receivers are not able to receive DAB+ broadcasts. Though, broadcasters can mix DAB and DAB+ programs inside the same transmission. DAB+ is approximately twice as efficient as DAB, and more robust.
Common aspects of DAB technology
DAB can offer more radio programs over a specific spectrum than analogue FM radio. DAB is more robust in respect to noise and multipath fading for mobile listening. DAB reception quality first degrades rapidly when the signal strength falls below a critical threshold, whereas FM reception quality degrades slowly with the decreasing signal. Audio quality varies depending on the bitrate used and audio material.
Bands and modes
DAB uses a wide-bandwidth broadcast technology. Typically spectra have been allocated for it in Band III (174–240 MHz) and L band (1,452–1,492 MHz). 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. The United States has also reached an agreement with Canada to 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 as mentioned bellow;
- 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
Differences between DAB and AM/FM
Traditionally radio programs were broadcast on different frequencies via AM and FM. In that case the radio had to be tuned into each frequency, as needed. This used up a comparatively large amount of spectrum for a relatively small number of stations. It often limits listening choice. DAB is a digital radio broadcasting system that through the application of multiplexing and compression combines multiple audio streams onto a relatively narrow band centered on 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.
Services and ensembles of DAB
Various different services are embedded into one ensemble which is also called a multiplex. These services can include the following;
- Primary services- main radio stations
- Secondary services- additional sports commentaries
Data services also offer various other services as follows.
- Electronic Program Guide (EPG)
- Collections of HTMLpages and digital images also known as Broadcast Web Sites
- Slideshows, which may be synchronized with audio broadcasts.
- Java PlatformApplications
- IP tunneling
- Other raw data
Benefits of DAB
Current AM and FM terrestrial broadcast technology is well established. They are also compatible, and cheap to manufacture. Benefits of DAB over analogue systems are explained below.
Improved features for users
DAB radios automatically tune to all the available stations and offer a list for the user to select. DAB receivers can display time of day as encoded into transmissions, so is automatically corrected when travelling between time zones and when changing to or from Daylight Saving. Some radios offer a pause facility on live broadcasts although kthis function is limited.
DAB is not more bandwidth efficient than analogue measured in programs per MHz of a specific transmitter. It is less susceptible to co-channel interference, which makes it possible to reduce the reuse distance. The system spectral efficiency is a factor three more efficient than analogue FM for local radio stations. In certain areas such as particularly rural areas, the introduction of DAB gives radio listeners a greater choice of radio stations.
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, which can happen on FM.
Mono talk radio, news and weather channels and other non-music programs need significantly less bandwidth than a typical music radio station. It allows DAB to carry these programs at lower bit rates, leaving more bandwidth to be used for other programs.
It is common belief that DAB is more expensive to transmit than FM. It is true that DAB uses higher frequencies than FM and therefore there is a need to compensate with more transmitters, higher radiated powers, or a combination, to achieve the same coverage. However, the last couple of years have seen significant improvement in power efficiency for DAB-transmitters. It is also possible that hopefully DAB will take the place of traditional AM/FM in the nearest future.