2. Overview of DAB+ features

The features discussed in this section are supported by the suite of DAB+ standards defined in ETSI Technical Specifications. A full list of the Technical Specifications can be found in §15.

2.1. Audio

Audio can be delivered in two ways using DAB and DAB+. The original DAB audio encoding used MPEG-2 while DAB+ audio encoding uses HE-AAC v2. DAB+ audio encoding also includes additional concatenated Forward Error Correction through a Reed-Solomon code along with a virtual interleaver which provides additional error protection. DAB+ is the preferred audio encoding solution as for most audio types the required bit rate to achieve suitable audio quality is 40% to 50% less than the original DAB MPEG-2 encoding.

2.2. Metadata

Metadata is becoming increasingly important to both listeners and broadcasters. DAB provides a range of metadata, including text (DL), images (SLS), programme information (PI) and service information (SI), such as logos.

Metadata is essential to ensure that modern graphical user interfaces provide a suitable range of features which are easily viewed and used. This is particularly the case for new cars, many of which have large colour screens.

Metadata is delivered both through Programme Associated Data (PAD) incorporated into the audio service as well as through data services and can also be delivered via IP connections. Metadata is particularly important in vehicles; see the WorldDAB metadata explainer video on the WorldDAB website. 2

2 See the WorldDAB website at www.worlddab.org

3 Image courtesy of Telestar

2.3. Features

DAB digital radio includes a wide range of features to provide services and enhance the listening experience:

• Announcements

The DAB Announcement system provides the ability to signal receivers to change to a different service. This is typically used for regular information delivery, such as traffic and news flashes. The use of such announcements is under the control of the listener, who can enable or disable this feature.

The announcement system is also used for Emergency Warnings. In this case, the Alarm announcement is used. It is not user configurable and all receivers must have the built-in ability to change services to the service indicated in the Alarm automatically. See [27]

• Service Following

Service Following provides the ability to switch to a different audio source when the current signal becomes too weak to deliver error-free audio reliably. The Service Following feature can be used to switch to other DAB ensembles, DRM or FM-RDS. See [27]

• Programme Information

The EPG is a Programme Information (PI) feature which provides the ability for broadcasters to inform their listeners of the programmes that they offer, for example the daily or weekly schedule of programmes.

• Service Information

A range of Service Information can be delivered by broadcast and/or IP. Information includes service logos (multiple sizes), geolocation, programme type, genre, service names and groups, keywords, phenomes and IP links.

This metadata can add considerable functionality for hybrid DAB Broadcast / IP connected radio receivers, particularly in vehicles.

See [14] for details of SI for both broadcast and hybrid delivery.

2.4. Transmission and multiplexing

DAB is transmitted in a Time Division Multiplexed format using Orthogonal Frequency Division Multiplexing with concatenated Forward Error Correction coding. As such, it is a robust signal which was specifically designed to provide radio services in difficult environments which are predominantly shadowed from the transmission site.

DAB+ has several different FEC code rates that can be used to trade delivered data capacity with robustness, with the inner convolutional code rates ranging from R=1/4 (strongest) to R=4/5 (weakest). For DAB+ the most commonly used FEC code rate is R=1/2, also called Equal Error Protection level 3A (EEP-3A), which results in a total user data capacity of 1.152 Mbps per ensemble.

The DAB broadcast system has considerable flexibility and can deliver up to 64 services, each of which can be defined independently in terms of sub-channel bit rate, audio mode and the amount of PAD to carry metadata such as DL and SLS. There is no such thing as a typical DAB+ ensemble; there are many variations across established DAB markets. For example, the Australian market initially had a general configuration of 18 services at the same 64 kbps coding rate. That configuration has been adjusted considerably with the evolution of services and now has a mixture of sub-channel bit rates for different types of audio content, as is also the case in Europe. We observe classical, jazz and folk music usually being delivered at higher sub-channel bit rates due to the more frequent presence of pure music tones, and pop music and speech usually delivered at lower sub-channel bit rates, due to the ease of coding.

Multiplexing systems can allow ensemble capacity to be grouped by Service Provider (SP) who can control their capacity independently from other SPs. For example, an ensemble may have three SPs which have 256, 256 and 512 kbps of capacity assigned respectively. Each SP is free to define the number of services which will be delivered using their allocated capacity and can change the configuration of services, including the number of services and their bit rates, at any time they choose.