Digital audio broadcasting
Digital Audio Broadcasting ( DAB ) is a digital transmission standard for terrestrial reception of digital radio . It is suitable for the frequency range from 30 MHz to 3 GHz and therefore also includes the distribution of radio programs via cable and satellite. DAB was developed in the EU's Eureka 147 project between 1987 and 2000. The DAB standard is available online from the European standardization organization ETSI under the code EN 300 401 .
The lists of DAB stations are linked - if available - in the respective DAB country articles.
DAB transmitters are in operation in 35 countries around the world, which can reach over 400 million people. Over 50 million receivers have been sold, including twelve million car radios (as of October 2016).
In most European countries such as Germany , Switzerland , Belgium, the Netherlands, Denmark and the United Kingdom, DAB is available almost everywhere. In France, it is currently only available in individual metropolitan regions such as Paris , Marseille or Nice . In Italy , the private broadcasters in particular are driving the DAB expansion in the northern Italian metropolitan areas. In Austria, however, the trial operation that had been running since 2000 was discontinued in 2008. In the Vienna area, a new pilot operation of DAB + took place from May 28, 2015 until further notice.
Idea and system development
Due to the physical conditions in analog terrestrial VHF transmitter networks, the same frequency can only be used again at a greater geographical distance - depending on the topography - for area-wide coverage with a radio program. At the time DAB was developed, it was only possible to operate six or seven nationwide transmitter chains (in addition to other individual transmitters for local or regional coverage) in the 87.5-108 MHz frequency range used by VHF.
The development of the dual broadcasting system in Germany from 1983 onwards had led to a number of additional radio programs produced by private broadcasters for which FM frequencies were mostly no longer available. The public service broadcasters in Germany felt compelled to develop a new, digital transmission system, also with a view to the susceptibility to interference with mobile VHF reception. The Institute for Broadcasting Technology (IRT) came up with the first proposals from which Digital Audio Broadcasting (DAB) later developed.
The two outstanding innovative development approaches of the digital terrestrial transmission system DAB are on the one hand the information compression of the audio signal (source coding) and on the other hand the technical mastery of the physically caused multipath propagation problems in the radio wave transmission. Both problems could only be solved technically and economically with the rapid progress of the development of microelectronics.
At the invitation of the Technical Director of the Bavarian Broadcasting Corporation, Frank Müller-Römer , a discussion event on the subject of “digital VHF broadcasting”, initiated by Wolfgang Klimek, member of the IDR working group (Initiative Digitaler Rundfunk), took place on December 16, 1981 Professors Hans Georg Musmann and Georg Plenge, Institute for Broadcasting Technology (IRT), Munich, reported on their thoughts on this topic. As a result, the opinion was expressed that in principle it should be possible in the VHF range to transmit a digitally coded radio signal in stereo instead of an analog signal. The IRT took up the suggestion and in the following years developed a concept for a digital transmission system in which the multipath reception problems with narrowband broadcasting could be avoided by a broadband broadcast bundle of programs. In 1985, the first transmission tests took place at the Gelbelsee transmitter of the Bavarian Broadcasting Corporation. Together with the IRT, co-channel and adjacent channel influences as well as the ranges of digitally transmitted signals were compared with VHF signals.
In 1986 it was decided at the European Ministerial Conference in Stockholm to develop a digital radio system in the EUREKA project 147. Germany took the lead (German Aerospace Research Institute, DLR, in Porz-Wahn ). The task and requirements for the DAB system were largely shaped by public broadcasting in Germany.
The frequency ranges are VHF band I (47–68 MHz, but no longer intended to be used for radio and television), VHF band III (174–230 MHz), and in some countries also “channel 13” (230–240 MHz) as well as parts of the L-band (around 1.47 GHz) for DAB. VHF band III is kept free for digital radio in Germany; TV stations still in this frequency band are to be relocated to the UHF range. The frequencies in the L-band are only suitable for local DAB coverage due to their limited technical range .
For digital audio broadcasting in Germany , the following frequency ranges are currently used for transmission:
- in VHF band III (174–230 MHz) the former television channels 5 to 12
- in the 1.5 GHz band (1.452–1.492 GHz) “L band”, the so-called local band. The term is derived from the radar frequency range "L" (1 to 2 GHz) and is not an official name. A direct line of sight to the transmitter is necessary, which keeps the range short. The L-band was hardly used for T-DAB and was therefore awarded in spring 2015 as E-UTRA band 32 to the LTE mobile operators Telekom Deutschland and Vodafone as part of the Digital Dividend II .
The T-DAB frequencies are divided into blocks. For example, VHF band III contains blocks 5A to 12D.
Band III is mainly used for nationally broadcast ensembles, while the L band was used by DAB to broadcast local ensembles due to higher costs. In the meantime, however, L-Band networks have been continuously "converted" into Band III networks. A very long-term use of the L-band for DAB was never guaranteed by law.
Since the frequencies in the L-band required a higher transmission power for equivalent broadcasting in the VHF band due to the high frequency, DAB in the L-band was broadcast with transmission powers of up to 4 kW.
From May 30, 2006, DMB was broadcast on a trial basis in band III and in the L band in some metropolitan areas, but the tests were discontinued in mid-2011 at the latest.
Market situation and competing systems
From 2004 onwards, there was a larger selection of DAB receivers available for interested consumers, which removed an obstacle from the early years. However, compared to FM receivers, the choices were still limited. For 2007, the University of Bonn named a number of 546,000 DAB receivers in German households.
In the meantime almost only receivers for DAB + are available in stores. Many manufacturers have equipped their models with new multi-standard chips. Switzerland assumes that with the migration of broadcasters from DAB to DAB +, the car equipment industry will follow suit and will also offer a sufficient range of car radio receivers for DAB + from 2012 onwards. Here, too, for economic reasons, it is hardly possible to completely replace FM radio with DAB alone.
After the decision of the commission to determine the financial needs of the broadcasting corporations (KEF) not to further support DAB broadcasting, radio over DVB-T was discussed in Germany as an alternative to DAB. Two radio stations came to Leipzig for the trial broadcast of 14 radio stations via DVB-T in the Berlin area. A divided DVB-T bouquet with up to 16 radio programs planned in Hamburg and Schleswig-Holstein did not materialize because the number of applicants was too low. In Berlin, the radio offer on DVB-T has meanwhile been reduced again. There are also no mobile receivers on the market, especially car radios. The main criticism of DVB-T radio is its incompatibility with European developments and poor mobility. According to the current specification, DVB-T becomes unusable from approx. 120 km / h.
The Digital Multimedia Broadcasting ( DMB ) system was not accepted by the market in Germany. It was never introduced in Austria or Switzerland. However, France uses this standard and the equipment industry has responded with multi-standard compatibility. There was also a radio transmission standard via satellite that was no longer used after years due to insufficient distribution.
DAB is successful in different countries. A good overview of the country-specific expansion can be found at Worlddab.org. An overview of competing standards, terrestrial and via satellite, can be found under digital radio, section Tabular overview . The current situation in Germany allows the conclusion that DAB + will be selected as the standard in the future and will establish itself as the sole radio platform. This would mean that the same standard would be selected for the European neighbors. DVB-T has not shown itself to be a replacement for DAB + due to various unsuccessful tenders. As a result, DAB + is likely to have emerged as the "winner" of the systems.
The official declared aim of the European Commission was to replace analog television and radio, including VHF radio, by 2012 (see analog switch-off ). This goal was not achieved.
In 2013 the share of DAB radios in Germany was 4.5%. That was around 2.7 million DAB devices. In 2014 there were around 5 million DAB devices across Germany. In the case of car radios in particular, there was an enormous increase in 2014 compared to the previous year by 108% to 1.3 million DAB devices.
In 2014, 7.5% of households in Germany received radio via DAB. In 2015, 10% of households in Germany received radio via DAB +, i.e. H. that around 4 million households in Germany received DAB + in 2015, around 1 million more than in 2014. In 2015 there were 6.4 million DAB + radios in Germany. Around 2 million of these are car radio devices, which corresponds to a growth rate of around 49% over the previous year. In 2015, 4.9% of all car radios in Germany were DAB + devices. Almost 6 million households in Germany had at least one DAB + radio set in 2017. This means that almost 11 million people in Germany have access to DAB + digital radio. The share of households with DAB + rose to 15.1% in 2017 from 12.6% in 2016.
Market overview between DAB and DAB +
The number of programs broadcast via DAB + has been growing in Germany since the end of 2011. In Switzerland, with a large range of DAB-Plus transmitters, the large department stores and electronics stores almost exclusively offer DAB + devices. Simple receivers for DAB + are sold there from around 55 euros, a larger selection is available from around 100 euros. The range of car radios in DIN sizes with DAB Plus compatibility (from 100 euros) is limited to one to three models from a handful of manufacturers. Similar prices apply to hi-fi components. Depending on the equipment, the price ranges are significantly higher than for VHF radio receivers. Due to the low demand, there is not yet well-functioning competition everywhere in Europe.
All devices available since November 2011 that can receive DAB + are downward compatible and can also receive DAB broadcasts using the conventional method (MPEG-1 Layer 2).
Manufacturers generally do not offer the option of upgrading DAB devices to DAB +.
If there is a USB connection on the radio, DAB + can be fed in via an additional device.
The transmitted signal is digital and essentially consists of a group of COFDM symbols that are combined into data frames (“frames”, see data frames ). The actual data is modulated using differential QPSK , which results in robust signal transmission with a relatively low data rate.
The duration of a frame depends on the selected transmission mode. The first symbol of a frame is the zero symbol, followed by the data-carrying COFDM symbols, which all have the same duration. The number of symbols and their duration is linked to the transmission mode used. The exact time units result from the base clock of 2.048 MHz on which the system is based.
The first two symbols are called the synchronization channel. While the zero symbol is being emitted, the emitted signal strength is greatly reduced. In the time domain (visualization e.g. with an oscilloscope ), this results in a clearly visible, regular signal interruption. With a very simple evaluation circuit, this gap can be recognized and the rough beginning of a frame can be determined. The zero symbol is often used in addition to signaling the transmitters in operation in a single frequency network (transmitter identification information signal). Individual carriers of the zero symbol are modulated here. The identification numbers of the transmitters can then be deduced from the distance between the modulated carriers. Together with optional information from the user data, the physical transmitter locations could be determined and the position of the receiver could be determined from the measured signal propagation times.
Each subsequent symbol consists of a usable length and a preceding guard interval , which contains a copy of almost a quarter of the end of the usable length. In the case of reflections or the use of several transmitters in the single-frequency network , deformations occur which, in stationary operation, can only be attributed to the different times of the arrival of the otherwise identical signals during the emission. The guard interval allows almost loss-free compensation of this signal deformation.
The second symbol contains the reference information, the content of which is specified in the standard. Deviations between the received and ideal signal course describe the deformation of the signal in the frequency range used. The deformation in the time domain can also be determined by comparing two consecutive reference symbols. The deformation in the frequency domain results from reflections and the possible use of several transmitters in the single-frequency network, which in the time domain is mainly due to the use of the receiver in motion.
The symbols following the synchronization channel initially contain the data of the Fast Information Channel and then the actual user data. The data of the Fast Information Channel may contain a. contain the names of the programs broadcast.
Signal structure and radiation
Linking the data to be transmitted to the individual carrier information is relatively complicated. In order to achieve a more even distribution of the bit values 0 and 1, the bit stream is linked with a pseudo-random sequence, which avoids high peak values in the output of the transmitter. The result of the link is then scrambled both in the frequency domain (carrier) and in the time domain (symbols). Finally, the selected carrier modulation is not used as absolute information, but as a difference to the previous carrier. These measures, together with the error correction used, lead to a strong immunity to typical signal disturbances such as lightning, which make individual symbols illegible, as well as individual frequency disturbances which can permanently overlay closely spaced carriers. The baseband signal generated in this way is now transposed to the target frequency. As a last measure, polarized radiation of the transmission signal is often used. In this way, external signals perpendicular to the radiation level can be significantly attenuated by suitable antennas.
In comparison to analog, frequency-modulated broadcasting, the entire process is significantly more robust against unwanted multi-path reception. This also makes it possible to cover large areas with just one frequency (single frequency network). Thus, the frequency economy, i.e. the consumption of spectrum per program, is usually significantly better with DAB than with conventional FM broadcasting.
There are four transmission modes for the transmission, which differ in various properties. Mode I is mostly used in Germany. The following table shows the system parameters of the four transmission modes of DAB.
|Maximum transmitter distance||96 km||24 km||12 km||48 km|
|OFDM symbols per frame
(without zero symbol)
|Number of carriers used||1536||384||192||768|
|Clock period (T)||1 ⁄ 2.048MHz (≈ 488.3ns)|
|Duration of a frame||196,608 T (96,000 µs)||49,152 T(24,000 µs)||49,152 T(24,000 µs)||98,304 T(48,000 µs)|
|Duration zero symbol||2656 T(≈ 1297 µs)||664 T(≈ 324 µs)||345 T(≈ 168 µs)||1328 T(≈ 648 µs)|
|Duration OFDM symbols||2552 T(≈ 1246 µs)||638 T(≈ 312 µs)||319 T(≈ 156 µs)||1276 T(≈ 623 µs)|
|Useful life of the OFDM symbol (Tu)||2048 T(1000 µs)||512 T(250 µs)||256 T(125 µs)||1024 T(500 µs)|
|Duration of the guard interval (Tg)||504 T(≈ 246 µs)||126 T(≈ 62 µs)||63 T(≈ 31 µs)||252 T(≈ 123 µs)|
|Bandwidth||1.536 MHz (This allows a maximum of four channels to be accommodated in a 7 MHz TV channel block)|
|Net data rate||2304 kbit / s|
DAB has four country-specific transmission modes (I, II, III and IV). For a receiver to be used worldwide, it must support all modes. The selection of the transmission mode depends on the operating conditions.
- Mode I for bands I , II and III , terrestrial (use in single-frequency networks and local broadcasts)
- Mode II for I, II, III, IV, V and L-band, terrestrial and satellite (local broadcasts)
- Mode III for frequencies below 3 GHz, terrestrial and satellite (transmissions in cable networks)
- Mode IV for I, II, III, IV, V and L-band, terrestrial and satellite (local broadcasts)
Audio coding method
The reception of stations that use coding according to HE-AAC v2 is only possible with DAB receivers that are also equipped with a corresponding decoder. With the aim of differentiating the receivers, which can also decode HE-AAC v2 in addition to MUSICAM, WorldDMB introduced the designation "DAB +". However, this is a pure marketing name that is not part of the standard.
The audio data of the programs are at first by means of DAB MUSICAM ( MPEG-1 Audio Layer 2 alias MP2) with data rates of 32 to 256 kbit / s coded. The bit rate of 160 kbit / s (frequently used standard), which was often used before most stations switched to DAB +, is 7.5 times lower than that of an audio CD, but should achieve a quality that comes close to that of an audio CD (cf. . Lossy audio data compression ).
For DAB transmission, several audio data streams are combined with pure data services that are also possible to form a so-called ensemble with a high data rate. The resulting multiplex is modulated and broadcast as described above.
A disadvantage compared to the analogue VHF reception is the higher energy consumption of the DAB receivers, which can be seen above all from the low battery life of portable DAB devices. According to initial experience, this also applies to all DAB + receivers.
In order to be able to meet the quality requirements even with low bit rates, WorldDMB submitted the HE-AAC v2 process as a supplementary coding process for DAB for standardization. An additional error protection ( Reed-Solomon code ) is added. DAB + uses the same audio codec and a similar error protection as DMB , but is otherwise technically different. A comparison of the necessary data rates between MUSICAM (DAB) and HE-AAC v2 (DAB +) is less a question of the technical specification, but depends above all on the audio quality and the audio content to be transmitted. Before the introduction of DAB +, a net data rate of 160 kbit / s was established when using MUSICAM in Germany, with 128 kbit / s often also being accepted. In order to achieve similar quality with HE AAC v2, around 80 kbit / s or 72 kbit / s is assumed, whereby the estimates often vary greatly in practice. HE-AAC v2 is certainly suitable for enabling an acceptable, but no longer necessarily artifact-free , audio transmission even at relatively low bit rates . DAB + was introduced at 80 kbit / s and can therefore transmit around twice as many audio programs in an ensemble as the conventional DAB transmission method. In practical terms, that means around 12 to 18 audio programs per DAB ensemble for DAB +. Extensive practical experience has been made in test ensembles. DAB + achieved a higher level of acceptance. It was positive to note in the tests that the audio signal generated from it was not disturbed even at a very low level of the received signal. From around 10 to 15 percent signal level, however, nothing could be heard anymore, because with DAB + there is no noise (as with FM) or "bubbles" (DAB), but reception abruptly breaks off.
DAB Surround enables surround sound with 5.1 or 7.1 channels. This is achieved by combining a mono or stereo signal in MPEG-1 Audio Layer 2 (DAB) or HE-AACv2 (DAB +) with MPEG Surround. Devices that do not support MPEG Surround only reproduce the mono / stereo signal in this case.
In addition to pure audio transmission, the following data services and types are already specified in DAB:
- (Multimedia Object Transfer Protocol, ETSI standard EN 301 234): MOT is a protocol for transmitting any files to all recipients in a broadcast process. Unlike FTP and other IP -related protocols MOT takes into account the difficulties in a unidirectional connection. Files are transmitted as segments that can be repeated so that the recipient can assemble the entire file over time (similar to teletext ). Special additional information (in the MOT header ) describes the transferred object and other attributes ( compression , application type, etc.). MOT is the basis for the broadcast website procedure (BWS), with which an entire HTML tree with start pages and interactive elements can be transmitted to a recipient . Radio stations can also use the MOT SlideShow (SLS) to transmit additional information to their listeners in graphical form. The distribution of Journaline - data service , the hierarchically organized text messaging provides also available by MOT.
- MOT can either be transmitted accompanying the program in the data stream of an audio channel (PAD, Program Associated Data) or as an independent pure data service in a packet data channel, sometimes called N-PAD (non-program accompanying data). In both cases it is part of the multiplex signal of a DAB ensemble
- ( Dynamic Label Segment ): Transmission of radio text- like information (interpreter etc.) in an audio program as program-accompanying data (PAD). A maximum of 128 characters can be transmitted per message.
- IP over DAB
- (ETSI standard EN 101 735): Transmission of IP packets via DAB; this allows IP-based services ( e.g. video streams ) to be transmitted to the recipient. Without a return channel, however, only broadcast / multicast data make sense.
- ( Traffic Message Channel ): transmission of coded and highly compressed traffic information taken from RDS , which can be converted back into readable text or assistance for navigation systems via a code book .
- ( Transport Protocol Experts Group ): Multimodal traffic and travel information.
Other services can be transmitted in DAB without any problems, since they can be signaled via special management information in the multiplex .
DAB / DMB thus opens up the possibility of a fast data channel on which, in addition to TMC data (Traffic Message Channel), considerably larger amounts of data can be transmitted at a speed that is 100 times higher. This not only enables the transmission of much more detailed messages, but also inner-city messages, which can no longer be transmitted via TMC due to the high data volume and a top-limited location list. TPEG is currently in the TISA ( Traveler Information Services Association ) specification. The TISA is an amalgamation of the TMC forum under ERTICO in Brussels and the TPEG group at the EBU in Geneva . In addition, there is the “mobile.info” working group in Germany with the participation of BMW , Daimler , VW - Audi , Bosch - Blaupunkt , FhG , GEWI , Navteq , Tele Atlas , T-Systems and VDO-Siemens . In coordination with TISA, this group specifies a particularly lean TPEG Automotive, tailored to automotive needs, which is characterized by very low distribution costs and high efficiency.
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