Digital Audio Broadcasting
[11][12][13] In recent years, DAB has become the most popular radio listening platform in Norway, Switzerland and the UK,[14] and has become a requirement for all new cars sold in the EU since 2021.
DAB is more robust with regard to noise and multipath fading for mobile listening,[18] although DAB reception quality degrades rapidly when the signal strength falls below a critical threshold (as is normal for digital broadcasts), whereas FM reception quality degrades slowly with the decreasing signal, providing more effective coverage over a larger area.
[citation needed] DAB+ is a "green" platform and can bring up to 85 percent energy consumption savings[19] compared to FM broadcasting (but analog tuners are more efficient than digital ones,[20] and DRM+ has been recommended for small scale transmissions).
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.
[28] For various reasons such as high receiver costs and limited reception, adoption of DAB had initially been slow, with the exception of the United Kingdom and Denmark.
[33] DAB adoption in automobiles became increasingly common during this time, and by 2016 it was standard in most cars sold in the UK, Norway and Switzerland.
[34] In October 2005, the World DMB Forum instructed its Technical Committee to carry out the work needed to adopt the AAC+ audio codec and stronger error correction coding.
[38] By 2006, 500 million people worldwide were in the coverage area of DAB broadcasts, although by this time sales of receivers had only taken off in the United Kingdom (UK) and Denmark.
DAB+ broadcasts have launched in several countries like Australia, Czech Republic, Denmark, Germany, Hong Kong (now terminated), Italy, Malta, Norway, Poland, Switzerland, Belgium (October 2017),[42] the United Kingdom and the Netherlands.
[48][49] Digital multimedia broadcasting (DMB) and DAB-IP are both suitable for mobile radio and TV because they support MPEG 4 AVC and WMV9 respectively as video codecs.
[56][57][58][59][11][60] Whilst many countries have expected a shift to digital audio broadcasting, a few have moved in the opposite direction following unsuccessful trials.
Finally, the physical layer contains the error-correction coding, OFDM modulation, and dealing with the over-the-air transmission and reception of data.
The newer DAB+ standard adopted the LC-AAC and HE-AAC, including its version 2 audio codecs, commonly known as AAC, AAC+ or aacPlus.
When DAB listeners receive a signal in this intermediate strength area they experience a "burbling" sound which interrupts the playback of the audio.
[101] Immunity to fading and inter-symbol interference (caused by multipath propagation) is achieved without equalization by means of the OFDM and DQPSK modulation techniques.
Reception difficulties can arise, however, when the relative delay of multipaths exceeds the OFDM guard interval duration, and there are frequent reports of reception difficulties due to this issue when propagation conditions change, such as when there's high pressure, as signals travel farther than usual, and thus the signals are likely to arrive with a relative delay that is greater than the OFDM guard interval.
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, as described above.
Error correction under the DAB standard makes the signal more robust but reduces the total bit rate available for streams.
DAB can give substantially higher spectral efficiency, measured in programmes per MHz and per transmitter site, than analogue systems.
The frequency reuse factor for local programmes and multi-frequency broadcasting networks (MFN) is typically 4 or 5, resulting in 1 / 4 / (0.26 MHz) = 0.96 programmes/transmitter/MHz.
The original objectives of converting to digital transmission were to enable higher audio fidelity, more stations and more resistance to noise, co-channel interference and multipath than in analogue FM radio.
[105] However, many countries in implementing DAB on stereo radio stations use compression to such a degree that it produces lower sound quality than that received from FM broadcasts.
[108] 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.
[110] DAB devices perform band-scans over the entire frequency range, presenting all stations from a single list for the user to select from.
Radio stations can provide the metadata to augment the listening experience, particularly on car receivers which have large display panels.
For instance, in Southern Norway, radio listeners experienced an increase in available stations from 6 to 21 when DAB was introduced in November 2006.
The DAB standard integrates features to reduce the negative consequences of multipath fading and signal noise, which afflict existing analogue systems.
Due to sensitivity to Doppler shift in combination with multipath propagation, DAB reception range (but not audio quality) is reduced when travelling speeds of more than 120 to 200 km/h, depending on carrier frequency.
It is also argued that the power consumption will be lower for stations transmitted on a single DAB multiplex compared with individual analog transmitters.
This is done by applying a delay to the incoming signal at the transmitter based on a timestamp generated at the multiplexer, created taking into account the maximum likely propagation time, with a generous added margin for safety.
