|
| DAB+ |
DAB+ is based on the original DAB standard but uses a more efficient audio codec. Whereas DAB uses MPEG Audio Layer II (better known as MP2), DAB+ uses HE-AAC v2 (better known as MP4 or AAC+). This allows equivalent or better subjective audio quality to be broadcast at lower bit rates. The increased efficiency offers benefits for Governments and Regulators (even better spectrum efficiency), broadcasters (lower costs per station) and consumers (a wider choice of stations).
DAB+ is designed to provide the same functionality as the original DAB radio services including services following (e.g. to the same service on another DAB ensemble or its FM simulcast), traffic announcements and PAD multimedia data (e.g. dynamic labels such as title artist information or news headlines; complementary graphics and images etc.)
|
| |
| Benefits of DAB+ |
- Latest MPEG-4 audio codec delivers exceptional performance efficiency
- More stations can be broadcast on a multiplex
- Greater station choice for consumer
- More efficient use of radio spectrum
- Lower transmission costs for digital stations
- New receivers backwards compatible with existing MPEG Audio Layer II broadcasts
- Current MPEG Audio Layer II services and consumers unaffected
- Compatible with existing scrolling text and multimedia services
- Robust audio delivery
- Optimised for live broadcast radio
- Broadcasters/regulators can select either standard MPEG Audio Layer II, or the additional audio coding, or both, to suit their country
- Fast re-tuning response time (low zapping delay)
- MPEG Surround is possible
|
| |
| DAB+ Explained |
| DAB+ is Three Times as Efficient as DAB, it 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: |
- 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 is 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 bit rates, 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. A detailed description of HE-AAC v2 is available on the EBU website1. An introduction to MPEG-4 is available on the MPEG Industry Forum website2.
|
| 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. |
| |
| DAB+ Functionalities |
| All the functionalities available for MPEG Audio Layer II services are also available for 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.
|
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. |
| |
| Glossary |
| HE-AAC v2 (High Efficiency Advanced Audio Coding version 2) |
Synonyms: AAC+, MP4, MPEG-4, aacPlus
High Efficiency AAC (HE-AAC) is a lossy data compression scheme for digital audio. It is an extension of Low Complexity AAC (AAC LC) optimized for low-bitrate applications such as streaming audio. HE-AAC version 1 (HE-AAC v1) uses Spectral Band Replication (SBR) to enhance the compression efficiency in the frequency domain. HE-AAC version 2 (HE-AAC v2) couples SBR with Parametric Stereo (PS) to enhance the compression efficiency of stereo signals. |
| |
| MPEG-4 |
Synonyms: H.264, MPEG-4 AVC
Standard for video compression used in DMB. The two standard documents for MPEG-4 AVC (ISO/IEC 14496-10) and H.264 (ITU-T H.264 standard) are jointly maintained so that they have identical technical content. |
| |
| MPEG Audio Layer II |
Synonyms: MP2, MPEG-1 Layer II, MUSICAM
MPEG Audio Layer II is an audio codec defined by ISO/IEC 11172-3. It is a dominant encoding standard for audio broadcasting as part of the DAB digital radio and DVB digital television standards, and it is the core of the better known MP3 standard. |
| |
| Parametric Stereo (PS) |
| Feature used in HE-AAC v2 to further enhance efficiency in low bandwidth stereo media. It performs sparse coding in the spatial domain, somewhat similar to what SBR does in the frequency domain. An HE-AAC v2 bitstream is obtained by downmixing the stereo audio to mono at the encoder along with 2-3 kbit/s of side info (the Parametric Stereo information) in order to describe the spatial intensity stereo generation and ambience regeneration at the decoder. By having the Parametric Stereo side info along with the mono audio stream, the decoder (player) can regenerate a faithful spatial approximation of the original stereo panorama at very low bitrates. |
| |
| Spectral Band Replication (SBR) |
| Technology to enhance audio or speech codecs, especially at low bit rates and is based on harmonic redundancy in the frequency domain. SBR replicates higher frequency content by transposing up harmonics from the lower and mid-frequencies at the decoder. |
|