Editing Digital Audio Broadcasting (section)

==Technology==
{{More citations needed section|date=June 2023}}
[[File:DAB DLS 01.jpg|thumb|A DAB radio receiver screen displaying the current station name, name of the current playing song, and other information]]

===Bands and modes===
DAB uses a wide-bandwidth broadcast technology and typically spectra have been allocated for it in [[Band III]] (174–240&nbsp;MHz) and [[L band]] (1.452–1.492&nbsp;GHz), although the scheme allows for operation between 30 and 300 [[Megahertz|MHz]]. The US military has [[Ultra high frequency#United States|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 to restrict L-Band DAB to terrestrial broadcast to avoid interference.{{Citation needed|reason=date:July 2008|date=July 2008}}

; Current mode:
* Mode I for Band III, Earth

In January 2017, an updated DAB specification (2.1.1) removed Modes II, III and IV, leaving only Mode I.
; Former modes:
* Mode II for L-Band, Earth and [[satellite]]
* Mode III for frequencies below 3&nbsp;GHz, Earth and satellite
* Mode IV for L-Band, Earth and satellite

===Protocol stack===
From an [[OSI model]] [[protocol stack]] viewpoint, the technologies used on DAB inhabit the following layers: the audio codec inhabits the [[presentation layer]]. Below that is the [[data link layer]], in charge of [[statistical time-division multiplexing]] and [[frame synchronization]]. Finally, the [[physical layer]] contains the [[error-correction coding]], [[OFDM]] [[modulation]], and dealing with the over-the-air transmission and reception of data. Some aspects of these are described below.

====Audio codec====
DAB initially only used the [[MPEG-1 Audio Layer II]] audio codec, which is often referred to as ''MP2'' because of the ubiquitous [[MP3]] (MPEG-1 Audio Layer III).

The newer DAB+ standard adopted the [[LC-AAC]] and [[HE-AAC]], including its [[parametric stereo|version 2]] audio codecs, commonly known as ''AAC'', ''AAC+'' or ''aacPlus''. AAC+ uses a [[modified discrete cosine transform]] (MDCT) algorithm,<ref name="Herre"/><ref name="Britanak"/> and is approximately three times more efficient than MP2,<ref name="autogenerated1">{{cite web |url=http://worlddab.org/pdf/DAB+brochure.pdf |title=Worlddab.org |access-date=17 November 2007 |archive-url=https://web.archive.org/web/20071128162449/http://worlddab.org/pdf/DAB+brochure.pdf |archive-date=28 November 2007 |url-status=dead}}</ref> which means that broadcasters using DAB+ are able to provide far higher audio quality or far more stations than they could with DAB, or a combination of both higher audio quality and more stations.

One of the most important decisions regarding the design of a digital radio broadcasting system is the choice of which audio codec to use because the efficiency of the audio codec determines how many radio stations can be carried on a fixed capacity multiplex at a given level of audio quality.

====Error-correction coding====
Error-correction coding (ECC) is an important technology for a digital communication system because it determines how robust the reception will be for a given signal strength – stronger ECC will provide a more robust reception than a weaker form.

The old version of DAB uses punctured [[convolutional coding]] for its ECC. The coding scheme uses unequal error protection (UEP), which means that parts of the audio bit-stream that are more susceptible to errors causing audible disturbances are provided with more protection (i.e. a lower [[code rate]]) and vice versa. However, the UEP scheme used on DAB results in a grey area in between the user experiencing good reception quality and no reception at all, as opposed to the situation with most other wireless digital communication systems that have a sharp "digital cliff", where the signal rapidly becomes unusable if the signal strength drops below a certain threshold. When DAB listeners receive a signal in this intermediate strength area they experience a "burbling" sound which interrupts the playback of the audio.

The DAB+ standard incorporates [[Reed–Solomon error correction|Reed–Solomon]] ECC as an "inner layer" of coding that is placed around the byte interleaved audio frame but inside the "outer layer" of convolutional coding used by the original DAB system, although on DAB+ the convolutional coding uses equal error protection (EEP) rather than UEP since each bit is equally important in DAB+. This combination of Reed–Solomon coding as the inner layer of coding, followed by an outer layer of convolutional coding – so-called [[concatenated error correction codes|"concatenated coding"]] – became a popular ECC scheme in the 1990s, and [[NASA]] adopted it for its deep-space missions. One slight difference between the concatenated coding used by the DAB+ system and that used on most other systems is that it uses a rectangular [[byte interleaver]] rather than [[Forney interleaving]] in order to provide a greater interleaver depth, which increases the distance over which error bursts will be spread out in the bit-stream, which in turn will allow the [[Reed–Solomon error correction|Reed–Solomon]] error decoder to correct a higher proportion of errors.

{| class="wikitable sortable"
|+ {{nowrap|Equal Error Protection}}<ref name="ETSI EN 300 401 V2.1.1" />{{rp|p=43}}<!-- notation e.g. 1-A used here, while in other similar standards e.g. EEP-1A seems to be more common -->
! class="unsortable" | Profile
! [[Code rate]]
|- <!-- data-sort-value max chosen 2520=2³·3²·5·7 for hypothetical 1/1 in order to get smallest integer exact values -->
| EEP-1A || data-sort-value=630  | 2/8 (1/4)
|-
| EEP-2A || data-sort-value=945  | 3/8
|-
| EEP-3A || data-sort-value=1260 | 4/8 (1/2)
|-
| EEP-4A || data-sort-value=1890 | 6/8 (3/4)
|-
| EEP-1B || data-sort-value=1120 | 4/9
|-
| EEP-2B || data-sort-value=1440 | 4/7
|-
| EEP-3B || data-sort-value=1680 | 4/6 (2/3)
|-
| EEP-4B || data-sort-value=2016 | 4/5
|}

The ECC used on DAB+ is far stronger than is used on DAB, which, with all else being equal (i.e., if the transmission powers remained the same), would translate into people who currently experience reception difficulties on DAB receiving a much more robust signal with DAB+ transmissions. It also has a far steeper "digital cliff", and listening tests have shown that people prefer this when the signal strength is low compared to the shallower digital cliff on DAB.<ref name="autogenerated1" />

====Modulation====
Immunity to fading and inter-symbol interference (caused by multipath propagation) is achieved without equalization by means of the [[OFDM]] and [[DQPSK]] modulation techniques. For details, see the [[OFDM system comparison table]].

Using values for Transmission Mode I (TM I), the [[OFDM]] modulation consists of 1,536 subcarriers that are transmitted in parallel. The useful part of the OFDM symbol period is 1.0&nbsp;ms, which results in the OFDM subcarriers each having a bandwidth of 1&nbsp;kHz due to the inverse relationship between these two parameters, and the overall OFDM channel bandwidth is 1.537&nbsp;MHz. The OFDM guard interval for TM&nbsp;I is 0.246&nbsp;ms, which means that the overall OFDM symbol duration is 1.246&nbsp;ms. The guard interval duration also determines the maximum separation between transmitters that are part of the same single-frequency network (SFN), which is approximately 74&nbsp;km for TM&nbsp;I.

====Single-frequency networks====
[[OFDM]] allows the use of single-frequency networks ([[single-frequency network|SFN]]), which means that a network of transmitters can provide coverage to a large area – up to the size of a country – where all transmitters use the same transmission frequency block. Transmitters that are part of an SFN need to be very accurately synchronised with other transmitters in the network, which requires the transmitters to use very accurate clocks.

When a receiver receives a signal that has been transmitted from the different transmitters that are part of an SFN, the signals from the different transmitters will typically have different delays, but to OFDM they will appear to simply be different multipaths of the same signal. 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.

Low power ''gap-filler'' transmitters can be added to an SFN as and when desired in order to improve reception quality, although the way SFNs have been implemented in the UK up to now they have tended to consist of higher power transmitters being installed at main transmitter sites in order to keep costs down.

====Bit rates====
An ensemble has a maximum [[bit rate]] 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, as described above. In the UK, most services transmit using 'protection level three', which provides an average [[ECC code rate]] of approximately {{sfrac|1|2}}, equating to a maximum bit rate per multiplex of 1,184&nbsp;kbit/s.

===Services and ensembles===
Various different services are embedded into one [[DAB ensemble|ensemble]] (which is also typically called a [[multiplexing|multiplex]]). These services can include:
* Primary services, like main radio stations
* Secondary services, like additional sports commentaries
* Data services
** [[Electronic program guide|Electronic Programme Guide]] (EPG)
** Collections of [[HTML]] pages and [[digital image]]s (known as 'broadcast [[website]]s')
** [[Slideshow]]s, which may be synchronised with audio broadcasts. For example, a police appeal could be broadcast with the [[e-fit]] of a suspect or [[closed-circuit television|CCTV]] footage.
** [[Video]]
** [[Java (software platform)|Java platform]] applications<ref>{{Cite web|url=https://www.etsi.org/deliver/etsi_ts/101900_101999/101993/01.01.01_60/ts_101993v010101p.pdf|archiveurl=https://web.archive.org/web/20220625082756/https://www.etsi.org/deliver/etsi_ts/101900_101999/101993/01.01.01_60/ts_101993v010101p.pdf|url-status=dead|title=Archived copy|archivedate=25 June 2022}}</ref>
** IP [[Tunneling protocol|tunnelling]]
** Other raw data