DAB+, a new digital radio standard using the HE-AAC encoding standard and announced by the WorldDAB organisation in 2006, is being considered for digital radio broadcasting in the UK.
HE-AAC encoding is more efficient than the MPEG-2 Layer II encoding standard currently used for DAB in the UK. DAB+ will either allow stations to broadcast at higher sound quality than currently possible with DAB, or allow more stations at the same quality currently broadcast on an ensemble.
The Digital Radio Development Bureau released a press statement in January 2007 in which they admitted that DAB+ will be used in the UK in future.
Ofcom has said that the UK will switch to DAB+, though not until the vast majority of radio sets can work with DAB+. If broadcasters and regulators dropped DAB in favour of DAB+ it is unlikely that current digital radio owners would be able to upgrade their model to receive the DAB+ signal.
Any change to DAB+ would have to be gradual, allowing those with non-upgradable receivers to obtain new equipment.
Ofcom has published a consultation with the intention to set up a new multiplex containing a mix of DAB and DAB+ services, with the intention of moving services to this format in the long term.
For more information see:
World DAB
Technical Information on DAB+ (PDF)
Economic Advantages of DAB+ (PDF)
DAB+ & DAB – The Differences
DAB audio signals are encoded in MP2, while DAB+ audio signals are encoded in High-Efficiency Advanced Audio Coding (HE-AAC) (formerly, aacPlus HE v2).
HE-AAC uses a number of clever techniques to make it more efficient, resulting in better audio quality at lower bitrates.
DAB+ and DAB can use the same transmitters, same multiplexing equipment, DAB and DAB+ signals can be placed on the same DAB multiplex. A DAB+ radio will also decode DAB signals as well (although a DAB radio won’t decode DAB+).
Many DAB radios can already cope with DAB+ automatically, or by downloading new firmware.
The main benefits of DAB+ are more stations, better sound quality, and lower bitrates. DAB+ signals are slightly more robust than those of DAB, DAB+ apparently removes the ‘bubbling mud’ noise that is associated with poor DAB signals.
Switching to DAB+ does not automatically mean better audio quality for listeners, this will depend on the individual broadcasters and on any standards set by Ofcom.
In countries where DAB+ is already in use, broadcasters often use 48 kbps HE-AAC, which is a roughly similar sound to 128 kbps MP2.
Broadcasters are charged for the bitrates they use, so there could also be economic incentives in the long term for many stations.
DAB+ is three times more “efficient” than DAB. This means that broadcasters can deliver the following via DAB+:
- DAB+ can deliver 3 times as many stations as DAB, or
- DAB+ can deliver the same number of stations at far higher audio quality than DAB is able to, or
- DAB+ can deliver more stations than DAB and all stations can be delivered at higher audio quality than on DAB
The final option, where DAB+ delivers a combination of more stations and at higher audio quality than DAB, is expected to be by far the most common way that DAB+
will be implemented.
DAB+ offers the following advantages over DAB:
High audio quality can be delivered affordably- 2 – 3 times as many radio stations can be carried on a multiplex, thus providing greater choice for listeners
- Transmission costs per station are 3 – 5 times lower
- Reception quality is far more robust
- Less spectrum is required
- Stations can deliver surround sound at a low additional cost
- Small local radio stations would be able to afford the cost of transmitting digitally when previously (small local DAB+ trials are already underway in the UK)
DAB+ delivers superior performance due to its adoption of modern technologies to replace DAB’s outdated ones:
- The AAC and AAC+ audio codecs – AAC is used to deliver high audio quality at higher bit rate levels (80 kbps and above), whereas AAC+ is the most efficient audio codec available today when used at low bit rate levels
- Reed-Solomon error correction coding – much stronger error correction than DAB’s, which leads to far more robust reception quality
- MPEG Surround format – added to allow broadcasters to deliver multichannel audio efficiently and cheaply
DAB+ in the UK
The introduction of DAB+ in the UK was broadly welcomed by the UK radio industry, it could lower costs, improve reception and sound quality.
Three stations are broadcasting in DAB+ on the second national commercial multiplex, these are Fun Kids, Jazz FM and Magic Chilled.
There are no plans to switch the majority of radio broadcasting over to DAB+ or switch off FM. So it is unlikely that the UK will be switching to DAB+ any time soon.
Some of the main proponents of DAB+ think it will result in better audio quality. It probably won’t
In countries that have adopted DAB+, 48kbps aacPlus is mainly used, DAB+ on the SDL multiplex is 32kbps.
A bitrate of between 40-64 kbps is required for an audio quality similar to FM. For better than FM quality a bitrate of between 56-96 kbps is required.
Switching to DAB+ would free up capacity on the three national ensembles, allowing many mono stations to broadcast in stereo.
Changing to DAB+ will not automatically mean a reduction in transmission costs, a patent fee is paid to Fraunhofer.
DAB broadcasters are charged for the bitrate they use, so quality will almost certainly suffer in order to reduce costs, as is currently the case with DAB.
DAB+ Technical Specifications
DAB+ broadcasts in VHF Band III (174-235MHz), a higher frequency than both AM and FM resulting in shorter wavelengths. This spectrum efficient technology allows broadcasters to offer their listeners new digital-only stations using less spectrum than analogue and DAB radio.
DAB+ uses a robust modulation which is designed for radio reception in mobile environments such as vehicles. DAB+ also provides new features such as scrolling text, EPG, slideshow and in some receivers, animation.
Receivers must be capable of decoding at least one audio sub-channel.
A receiver must be able to decode a DAB audio service contained in a sub-channel of a size up to and including 280 Capacity Units (e.g. 256 kbps@UEP1).
DAB audio services are defined in ETSI EN 300 401.
A receiver must be able to decode a DAB+ audio service contained in a sub-channel of a size up to and including 144 Capacity Units (e.g. 96 kbps@EEP1A).
DAB+ audio services are defined in ETSI TS 102 563.
Coding
DAB+ audio signals are encoded in High-Efficiency Advanced Audio Coding (HE-AAC).
HE-AAC v2 combines three technologies:
- The core audio codec AAC (Advanced Audio Coding)
- A bandwidth extension tool SBR (Spectral Band Replication), which enhances efficiency by using most of the available bit rate for the lower frequencies (low band) of the audio signal. The decoder generates the higher frequencies (high band) by analysing the low band and side information provided by the encoder. This side information needs considerably less bit rate than would be required to encode the high band with the core audio codec
- Parametric stereo (PS): a mono down-mix and side information are encoded as opposed to a conventional stereo signal. The decoder reconstructs the stereo signal from the mono signal using the side information HE-AAC v2 is a superset of the AAC core codec. This superset structure permits to use plain AAC for high bitrates, AAC and SBR (HE-AAC) for medium bit rates or AAC, SBR and PS (HE-AAC v2) for low bit rates. Therefore HE-AAC v2 provides the highest level of flexibility for the broadcaster.
- MPEG Audio Layer II and HE-AAC v2 radio services can coexist in one ensemble. However, legacy receivers might list HE-AAC v2 radio services even though they will not be able to decode them.
HE-AAC v2 provides the same perceived audio quality at about one-third of the subchannel bit rate needed by MPEG Audio Layer II. The same audio coding is also used in DRM and DMB e.g. for television audio. Devices, which also include DMB can benefit from the fact that the audio coding for this range of technologies is essentially the same.
Latency
Latency is a part of digital systems generally and applies equally to digital TV. Approximately 3-4 seconds of latency for DAB+ digital radio is unavoidable resulting from system implementation delays.
There is a trade-off between reducing these delays and the likelihood of errors being transmitted.
Buffering delays may also be experienced in digital receivers; these are not related to the actual DAB+ broadcasts.
DAB+ Functionalities
Functionalities available in DAB+:
- Service following (e.g. to FM or other DAB ensembles)
- Traffic announcements
- PAD multimedia (dynamic labels such as title artist information or news headlines
- Still images such as weather charts, images and other multimedia content.)
- Service language and programme type information (e.g. Classical Music, Rock Music, Sport) etc.
Coverage
The geographical coverage area of radio services using HE-AAC v2 is slightly larger than that for radio services using MPEG Audio Layer II.
The multimedia information carried in PAD of a HE-AAC v2 radio service is much better protected against transmission errors than PAD data of a radio service using MPEG Audio Layer II (DAB).
Short Zapping Delay
An important design criterion for DAB+ was a short “zapping” delay. Both the time it takes to switch from one radio service to another station on the same DAB ensemble, as well as the time it takes to tune to a radio service on another DAB ensemble, was minimized.
Surround Sound
Currently, all DAB radio services are mono or stereo. However, DAB+ also provides the means to broadcast surround sound in a backwards compatible way. Using MPEG Surround it is possible to broadcast a stereo signal together with surround side information (e.g. 5 kbps side information). Standard stereo radios will ignore this side information and decode the stereo signal. MPEG Surround receivers will evaluate the side information and reproduce surround sound. So at a comparatively low additional bit rate, the broadcaster can increase the audio experience on surround sound receivers, and still provide high-quality sound to all other radios.
