A digital audio broadcasting system that is capable of providing a large number of high quality stereophonic channels to mobile receivers in an environment with severe multipath delay and fading. Optimum combination of frequency and time diversity to guarantee robust performance in the mobile channel, with its multipath delay and frequency-selective fading effects. The system is based upon a dynamic single channel per carrier assignment of each stereo channel to many carriers. Inter symbol interference degradations caused by multipath delay are mitigated via an adaptive equalizer in the receiver. This dynamic single channel per carrier system preserves the simplicity inherent in the single channel per carrier assignment while it incorporates the ability to address frequency-selective fading by providing substantial frequency diversity. The frequency diversity is achieved via a slow frequency hop implementation in which the assignment of a number of stereo channels to an equal number of carrier frequencies is changed periodically. The system offers the simplicity of single channel per carrier assignment of stereo channels while it achieves the powerful performance benefits of frequency diversity and adaptive equalization for the mobile channel.
What is Digital Audio Broadcasting (DAB)?
Digital Audio Broadcasting is a transmission technology first designed in the Ëœ80s as a method of bringing the benefits of digital to traditional analogue radio broadcasts.
Put simply, itâ„¢s the current successor to the FM and AM radio broadcasts youâ„¢ve been listening to for years.
Digital Audio Broadcasting is a digital radio technology for broadcasting radio stations, used in several countries, particularly in Europe. As of 2006, approximately 1,000 stations worldwide broadcast in the DAB format.
Digital audio broadcasting, DAB, is the most fundamental advancement in radio technology since that introduction of FM stereo radio. It gives listeners interference - free reception of CD quality sound, easy to use radios, and the potential for wider listening choice through many additional stations and services.
Considering that weâ„¢ve all been enjoying FM and AM broadcasts for decades, why is there a need to go digital? Well, as with all things in the technology world, gadgets evolve and DAB is seen as a natural progression as it brings numerous benefits to radio broadcasts â€œ including, in theory, less interference, better sound and a greater choice of radio channels. All sounds good (excuse the pun), so letâ„¢s see how it works.
How does DAB work?
As long as the sound is free of interference and of high quality, most users wouldnâ„¢t be interested in knowing exactly how those goals are achieved. For those who do want to know, however, hereâ„¢s a brief breakdown of the steps involved in processing a DAB broadcast.
Firstly, the analogue signals are converted to digital (i.e. a string of zeros and ones). That digital information is then compressed to an audio format known as MPEG-1 Layer 2 (sometimes referred to as MP2 or MUSICAM). Compressing audio to MP2 involves removing sounds that are imperceivable to the human ear, doing so reduces the size of transmitted data whilst largely retaining sound quality.
With the use of a multiplexer, the digital audio is then supplemented with information such as Programme Associated Data (PAD). Information included in PAD can consist of useful data such as song titles, artist names and general radio station details. Think of it as similar to FMâ„¢s RDS and youâ„¢re on the right track.
The multiplexer then takes the digitised signal and combines it with the digital signals of other radio stations, creating a single group transmission called a DAB ensemble.
That ensemble is then modulated using Coded Orthogonal Frequency-Division Multiplexing (COFDM), which involves splitting the signal across 1,536 unique carrier frequencies. Using multiple carrier frequencies, or sub-frequencies, lessens the impact of interference. Should one carrier be disturbed by interference, the receiver is still able to retrieve the original sound using the numerous other carriers.
It all sounds very complicated â€œ despite our gross simplification â€œ but all that magic then ultimately finds its way back to the listener via DAB radios (such as the Pure Evoke Flow picture above). A DAB radio is equipped with a decoder that puts the digital signals in chronological order, checks them for errors and then converts them back to analogue sound for output to the user. That, essentially, is the magic.
What are the advantages?
DAB broadcasts throughout the country â€œ where available â€œ use the same frequency and consequently do not require retuning. DAB radios automatically tune to all available stations, and theyâ„¢re sorted by name as opposed to a frequency number â€œ making it, in theory, easier to locate stations.
DAB stations also have the ability to display additional information courtesy of PAD. This can include real-time data such as the name of the track currently playing, or details of the radio station.
A greater choice of stations
Digital radio is more efficient than analogue, consequently allowing for more data to be sent in the same spectrum. This has enabled broadcasters to cram more stations into the same space, creating greater choice for the listener.
Due to the method of transmission, including COFDM modulation, DAB broadcasts are far less susceptible to interference and reception quality should be of a high standard. As a digital transmission, DAB broadcasts should also be free of the hissing and crackling associated with FM.
Pause and rewind
Modern DAB radios include the ability to pause and rewind live radio broadcasts. Think of it like Sky+ TV, only, on the radio!
Are there any disadvantages?
It all sounds good thus far, but DAB isnâ„¢t without its faults.
DAB isnâ„¢t yet available to everyone in the UK. Although coverage has increased rapidly in recent years, many areas still suffer from poor coverage or no coverage at all.
One of the benefits of DAB broadcasts should be an increase in quality, but that doesnâ„¢t always prove to be the case. It is widely documented that many UK DAB stations broadcast in the MP2 format at a bit rate of just 128kbps, resulting on occasion in better sound from age-old FM. The MP2 format does support bit rates of up to 384kbps, so why such drastic compression? Itâ„¢s done simply to create greater room for more stations; a case of quantity over quality, sadly.
DAB broadcasts canâ„¢t be received by traditional FM/AM radios. Although DAB radios are now readily available, they are yet to become common place and their supremacy may not be realized until 2015 â€œ thatâ„¢s the year in which the UK government plans to cease analogue broadcasts. Today, many car manufacturers continue to use FM radios as standard.
Criticisms of DAB
Music radio stations broadcasting in mono
A number of music radio stations and stations that carry drama on DAB in the UK are being broadcast in mono. These stations are often available in stereo on other digital platforms, where capacity is not as constrained, and on FM where applicable.
The reception quality on DAB can be poor even for people that live well within the coverage area. The reason for this is that the old version of DAB uses weak error correction coding so that when there are a lot of errors with the received data not enough of the errors can be corrected and a "bubbling mud" sound occurs. In some cases a complete loss of signal can happen. This situation will be improved upon in the new DAB standard (DAB+, discussed below) that uses stronger error correction coding and as additional transmitters are built.
The nature of a SFN is such that the transmitters in a network must broadcast the same signal at the same time. To achieve synchronization, the broadcaster must counter any differences in propagation time incurred by the different methods and distances involved in carrying the signal from the multiplexer to the different 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. Also delays in the receiver due to digital processing (e.g. deinterleaving) add to the overall delay to the listener. This delays the signal to the listener by about 2 seconds (depending on the decoding circuitry used). This has two disadvantages: (i) DAB radios are out of step with live events so time signals are not accurate and the experience of listening to live commentaries on events being watched is impaired, and (ii) listeners using a combination of FM and DAB radios (e.g. in different rooms of a house) will not hear an intelligible signal when both receivers are within earshot.
As DAB is at a relatively early stage of deployment, DAB coverage is poor in nearly all countries in comparison to the high population coverage provided by FM.
Transmission on DAB is far more expensive than on FM, and measures taken by broadcasters to limit their costs have resulted in some DAB ensembles having to carry too many channels, forcing bit rates to be reduced to levels that deliver sound quality inferior to traditional FM However, a DAB transmitter can carry several channels allowing for lower maintenance costs over time.
In 2006 tests finally began using the much improved HE-AAC codec for DAB+. Virtually none of the current receivers in the field support the new codec, however, thus making them partially obsolete once DAB+ broadcasts begin and completely obsolete once the old MPEG-1 Layer 2 stations are switched off.
As DAB requires digital signal processing techniques to convert from the received digitally encoded signal to the analogue audio content, the complexity of the electronic circuitry required to do this is high. This translates into needing more power to effect this conversion than compared to an analogue FM to audio conversion, meaning that portable receiving equipment will tend to have a shorter battery life, or require higher power (and hence more bulk).
As an indicator of this increased power consumption, dual FM/DAB radios quote the length of time they can play on a single charge. For DAB, this is often between one-sixth and one-twelfth of the time they can play when in FM mode.
Closing thoughts and the future of DAB
In closing, the numerous advantages of DAB outweigh the handful of disadvantages, but would-be buyers should be aware of another revision to digital radio known as DAB+.
DAB+, announced in 2006, is an upgraded version of DAB that makes use of improved error correction and the HE-AAC audio codec (commonly known as AAC+). HE-AAC is more efficient than MP2 and should result in higher-quality broadcasts.
Readers, however, should be aware that DAB+ is not backward compatible with the existing DAB standards. Consequently, current DAB radios may not support future DAB+ broadcasts, and those that do may require a firmware upgrade. We recommend readers check for DAB+ compatibility prior to making a purchase.
Exactly when DAB+ broadcasts will air in the UK remains unknown, but they're expected to be made available alongside existing DAB transmissions. Although firm time frames are currently unavailable, it is widely believed that standard DAB broadcasts will cease some time after the widespread adoption of DAB+.