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Digital Audio Broadcast or DAB is a standard for digital radio broadcast developed by EUREKA as a research project for the European Union. (Project number EU147.) The technology was mainly developed in the 1980s, although the project started in the year 1987 and ended in 2000. Over 285 million people around the world can receive more than 550 different DAB services. The United Kingdom was the first country to receive a wide range of radio stations via DAB, with over 50 commercial and BBC services available in London in 2001.
The DAB Forum represents more than 30 countries. The United States is not among them and uses IBOC to deliver digital radio.
DAB uses the Eureka 147 protocol based on orthogonal frequency division modulation for transmitting digital data over a lossy radio channel.
DAB broadcasts use the MP2 audio coding technique, a close relative of the popular MP3 format, which was also created as part of the EU147 project.
DAB has the advantage that stations do not have be re-tuned as you move from area to area, such as in a car. Also, you can receive "radiotext" (in DAB terminology, Dynamic Label Segment, or DLS) from the station giving information such as song titles or traffic updates.
Problems with the name Digital occurred with radios, as "digital" can be associated with a digital method of tuning, which is commonly found on analogue radios with LCD displays, rather than a digital signal.
One can use a radio card to hear DAB through a personal computer.
Bands and Modes
- Band III: DAB – frequency band 174–240 MHz
- L-Band: DAB – frequency band 1452–1492 MHz
- DAB-Mode I, II, III and IV: country specific transmission mode. 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
Different services are embedded in one ensemble (which is also typically called a multiplex). These can be a number of different things, including:
- Primary services, like main radio stations
- Secondary services, like additional football commentaries
- Data services
- Electronic Programme Guide (EPG)
- Collections of HTML pages
- Other raw data
An ensemble does not have a maximum bitrate that can be transmitted. Instead, it has a total "capacity unit" level of 864. Capacity Units, or CUs, depend on the amount of error correction added to the transmission: higher error correction enables the signals to be more robust, but allows less to be transmitted. In the UK, most multiplexes offer a protection level of three, which enables a total bitrate per multiplex of 1184 KBits.
As of June 2004, the BBC national multiplex contains a number of different services, including primary services like BBC Radio 4, secondary services like BBC Radio Five Live Sports Extra, and data services like BBC Vision Radio and a test EPG. The multiplex may change configuration, adding extra temporary secondary services, which requires other services on the multiplex to reduce bitrate. Typically, most music services are broadcast at 128Kbps stereo within the UK, and speech services at 80Kbps mono.
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