Outline of the DAB+ System

DAB+ is Three Times as Efficient as DAB

DAB+ uses MPEG-4 High Efficiency AAC v2 profile (HE-AAC v2). This audio codec is the most efficient audio compression scheme available worldwide and allows for up to three times as many services per multiplex as the original DAB: A 40 kbps subchannel with HE-AAC v2 provides a similar audio quality (even slightly better in most cases) as MPEG Audio Layer II at 128 kbps.

HE-AAC v2 combines three technologies:

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 or DRM can benefit from the fact that the audio coding for this range of technologies is essentially the same. Other systems using AAC include:

DAB+ Functionalities

All the functionalities available for MPEG Audio Layer II services are also available for DAB+:

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.

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 an HE-AAC v2 radio service is much better protected against transmission errors than PAD data of a radio service using MPEG Audio Layer II.

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.

The Performance of DAB+ in Field Tests

During the standardisation process, field tests were conducted in the UK and Australia. They gave a number of interesting results:

To determine the audio quality at low bitrates, listening tests were performed by the EBU (European Broadcasting Union) in 2003. For stereophonic audio, the listening tests show that

At the time of these tests, HE-AAC v2 was not yet available, but it can safely be assumed that its performance will be similar to or even better than that of HE-AAC.

The PS (parametric stereo) tool, which was added after the EBU tests were completed, significantly increases the perceived audio quality at lower bit rates. It should be noted that the bit rates cited from these listening tests are pure audio bit rates and not DAB subchannel bit rates. In order to carry audio in a DAB multiplex using the new specification a 10% overhead should be taken into account.

Audio comparison tests performed in Australia in 2005 confirmed that HE-AAC v2 provides similar perceived audio quality at about one third of the subchannel bit rate needed by MPEG Audio Layer II.

Broadcasters will want to make further tests before taking operational decisions about the appropriate bit rate for a particular service.