MPEG-4 High Efficiency Advanced Audio Coding
| High Efficiency - Advanced Audio Coding | |
|---|---|
| File extension : |
.aac, .3gp, .mp4, .m4a, .m4p
|
| Developed by: | MPEG |
| Type: | Lossy audio compression |
| Standard (s) : | ISO / IEC 14496-3: 2003 / Amd.1 (HE-AAC v1) ISO / IEC 14496-3: 2005 / Amd.2 (HE-AAC v2) |
MPEG-4 High Efficiency Advanced Audio Coding ( HE-AAC for short ), also called AAC + v1 , is an audio codec of the Moving Picture Experts Group (MPEG) that requires a license . The standard delivers comparatively good results at low bit rates and is therefore particularly suitable for live streams .
Basics
HE-AAC is based on MPEG-4 AAC , but has been expanded to include a technique called Spectral Band Replication (SBR). This technology was already used in the mp3PRO codec. Thanks to SBR, HE-AAC delivers better sound quality than the "normal" AAC or other (older) audio codecs, especially at low bit rates (96 kbit / s and lower). Low bit rates, such as those used in live streaming or cellular communications, are therefore the main area of application for HE-AAC. At higher bit rates (over ~ 96 kbps), AAC is still used in the Main or Low Complexity (LC) profiles because of the better quality .
HE-AAC supports up to 48 audio channels and digital rights management (DRM). HE-AAC can be used with constant or variable bit rate. HE-AAC is backward compatible with AAC to a limited extent. This means that all AAC decoders can also decode HE-AAC streams; without special SBR support, however, only with half the sampling rate. Since SBR only generates high frequencies synthetically, HE-AAC, in contrast to AAC (main or LC profile), cannot achieve transparency .
AAC + v2
In addition to AAC + v1 (HE-AAC), there is now a new version of the codec called AAC + v2 or eAAC + . In contrast to AAC + v1 , the AAC + v2 codec also uses a technique called Parametric Stereo (PS) to compress stereo signals . This allows stereo signals to be compressed even more effectively, which further improves the sound quality at low bit rates. Accordingly, PS is not used for pure mono signals. Also, AAC + v2 is backwards compatible. However, decoders without special support for PS can only decode AAC + v2 streams as mono.
According to the manufacturer, AAC + v2 only delivers better sound results at data rates below 48 kbps. For data rates above 48 kbps, however, the manufacturer continues to recommend the use of AAC + v1 for reasons of quality.
DVD-Audio
HE-AAC alias aacPlus v1 was standardized in July 2004 by Working Group 4 of the DVD Forum as the official audio codec for the so-called “Compressed Zone” of DVD-Audio . A highly compressed version of the music content of the DVD-Audio is stored in this area, which can be transferred to portable playback devices with DRM protection.
Abbreviations
There are different abbreviations for the "AAC +" variants with identical meanings:
- AAC + SBR = HE-AAC = aacPlus v1 (= AAC +)
- AAC + SBR + PS = HE-AAC v2 = aacPlus v2 (= enhancedAAC + = eAAC +)
Web links
- http://www.dvdforum.org/
- Manufacturer's website ( Memento from January 18, 2005 in the Internet Archive )
- Internet radios streaming with AAC +
- Codec plugins page
Individual evidence
- ↑ MPEG-4 HE-AAC v2 - audio coding for today's digital media world ( Memento of the original from March 16, 2007 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF file; 395 kB), Article in the EBU Technical Review (01/2006, page 9)
- ↑ Diagram ( Memento from March 4, 2007 in the Internet Archive ) on the manufacturer's website ( Memento from January 18, 2005 in the Internet Archive )
- ↑ mp3-tech 48kbps AAC Public Test
- ↑ Scheme ( Memento from December 25, 2005 in the Internet Archive ) on the manufacturer's website ( Memento from January 18, 2005 in the Internet Archive )
