Advanced Audio Coding (AAC) is an audio coding standard for lossy digital audio compression. Designed to be the successor of the MP3 format, AAC generally achieves better sound quality than MP3 at similar bit rates.
AAC Standard
AAC主要分为MPEG-2 AAC和MPEG-4 AAC, 分别由MPEG-2 Part7和MPEG-4 Part3来定义.
*MPEG-2 Part 7 *
1997年AAC首次在MPEG-2 Part 7出现, 在其中介绍了三种Profiles:
- AAC-LC (应用最广 兼容性最好)
- AAC Main
- AAC-SSR
MPEG-4 Part 3
In 1999, MPEG-2 Part 7 was updated and included in the MPEG-4 family of standards and became known as MPEG-4 Part 3, MPEG-4 Audio or ISO/IEC 14496-3:1999.
1999年的时候, MPEG-2 Part 7被纳入MPEG-4 Part 3(ISO/IEC 14496-3:1999).
该版本引入了一个非常重要的概念Audio Object Types(AOT)，通过AOT对MPEG-2 Part 7介绍的三种Profiles(AAC-LC, AAC Main and AAC-SSR)进行兼容, 后续历次版本增加的每种Profile相当于增加一个Audio Object Type.
比如ISO/IEC 14496-3:2001/Amd 1:2003引入了HE-AAC Profile,相当于增加了一种Audio Object Type: SBR, 而HE-AAC = AAC-LC + SBR(可直观这么理解).
The following table shows an overview about where the Elementary Stream payloads for the Audio Object Types can be found and where the detailed syntax is defined.
**Audio Object Types(部分) in ISO/IEC 14496-3:2009(E) **
Profile vs Audio Object Type
Profile = MPEG-4 Audio Object Type - 1
比如AAC-LC的Audio Object Type的值为2， 其Profile为1.
Perferences:
https://en.wikipedia.org/wiki/Advanced_Audio_Coding
ISO/IEC 13818-7:2006(E)
ISO/IEC 14496-3:2009(E)
