Inside Dolby Pulse

AAC and HE AAC

At the heart of Dolby® Pulse are the high-efficient MPEG audio technologies AAC LC, HE AAC v1, and HE AAC v2. Advanced Audio Coding, Low Complexity (AAC LC) is one of the most advanced audio formats available today. HE AAC extends its applicability to lower bit rates while maintaining the highest audio fidelity by enhancing AAC with Spectral Band Replication (SBR) and Parametric Stereo (PS). Dolby Pulse enhances these audio formats further with metadata.

Standardization

HE AAC is widely adopted by many international standardization bodies. HE AAC is standardized by 3GPP2, ISMA (Internet Streaming Media Alliance), DVB (Digital Video Broadcasting), the DVD Forum, Digital Radio Mondiale, and many others.

As an integral part of MPEG-4 Audio, HE AAC is ideal for deployment with the new H.264/AVC video codec standardized in MPEG-4 Part 10.

Dolby Pulse brings additional functionality, flexibility, and reliability to the format while remaining fully backward- and forward-compatible with MPEG-4 AAC, MPEG-4 HE AAC v1, and MPEG-4 HE AAC v2 as defined in ISO/IEC 14496-3:2005.

Example Applications

Dolby Pulse can improve the audio quality of a number of applications, including:

• Digital satellite and cable broadcasting
  
• IPTV
  
• Digital radio

Spectral Band Replication

Spectral Band Replication is an audio coding enhancement tool that is standardized in ISO/IEC 14496-3:2001/Amd.1:2003. It improves the performance of low-bit-rate audio and speech codecs by either increasing the audio bandwidth at a given bit rate or by improving coding efficiency at a given quality level.

From a technical point of view, Spectral Band Replication is a method for highly efficient coding of high frequencies in audio compression algorithms. The higher frequencies are generated by the Spectral Band Replication decoder, which is mainly a postprocess decoder. Instead of transmitting the spectrum, Spectral Band Replication reconstructs the higher frequencies in the decoder based on an analysis of the lower frequencies transmitted in the underlying coder. To ensure an accurate reconstruction, some guidance information is transmitted in the encoded bitstream at a very low data rate.

The reconstruction is efficient for harmonic as well as for noise-like components and allows for proper shaping in the time domain and the frequency domain. As a result, Spectral Band Replication allows full bandwidth audio coding at very low data rates, thus offering a significant increase in compression efficiency compared to the core coder.

Parametric Stereo

The Parametric Stereo technology enhances the efficiency of audio compression for low-bit-rate stereo signals. Parametric Stereo is fully standardized in MPEG-4. The Parametric Stereo encoder extracts a parametric representation of the stereo image of an audio signal, whereas only a monaural representation of the original signal is encoded in a conventional fashion.

The stereo image information is represented as a small amount of high-quality Parametric Stereo information and is transmitted along with the monaural signal in the bitstream. Based on the Parametric Stereo information, the decoder is capable of regenerating the stereo image.

As a result, the perceived audio quality of a low-bit-rate audio bitstream incorporating Parametric Stereo is significantly higher compared to the quality of a similar bitstream without Parametric Stereo.

Metadata

In broadcast, one of the main differences between Dolby Pulse and standard MPEG-4 HE AAC is the support for Dolby audio metadata. The specification for MPEG-4 HE AAC describes a number of optional metadata parameters to control the reproduction of the audio signal in a consumer device such as a set-top box or television set. Dolby Pulse integrates all required metadata parameters included in Dolby Digital and Dolby Digital Plus, allowing it to function exactly the same as these current broadcast audio technologies.