The MPEG-4 Low Delay Audio Coder (a.k.a. AAC Low Delay, or AAC-LD) is
audio compression standard designed to combine the advantages of perceptual audio coding with the low delay necessary for two-way communication. It is closely derived from the MPEG-2
Advanced Audio Coding
Advanced Audio Coding (AAC) is an audio coding standard for lossy digital audio compression. It was developed by Dolby, AT&T, Fraunhofer and Sony, originally as part of the MPEG-2 specification but later improved under MPEG-4.ISO (2006ISO/ ...
(AAC) standard. It was published in
MPEG-4 Audio Version 2 (
ISO
The International Organization for Standardization (ISO ; ; ) is an independent, non-governmental, international standard development organization composed of representatives from the national standards organizations of member countries.
Me ...
/
IEC 14496-3:1999/Amd 1:2000)
[ ][ ][ ] and in its later revisions.
AAC-LD uses a version of the
modified discrete cosine transform
The modified discrete cosine transform (MDCT) is a transform based on the type-IV discrete cosine transform (DCT-IV), with the additional property of being lapped: it is designed to be performed on consecutive blocks of a larger dataset, where s ...
(MDCT)
audio coding
An audio coding format (or sometimes audio compression format) is a content representation format for storage or transmission of digital audio (such as in digital television, digital radio and in audio and video files). Examples of audio coding f ...
technique called the LD-MDCT. AAC-LD is widely used by
Apple
An apple is a round, edible fruit produced by an apple tree (''Malus'' spp.). Fruit trees of the orchard or domestic apple (''Malus domestica''), the most widely grown in the genus, are agriculture, cultivated worldwide. The tree originated ...
as the
voice-over-IP (VoIP)
speech codec in
FaceTime
FaceTime is a proprietary videotelephony product developed by Apple. FaceTime is available on supported iOS mobile devices running iOS 4 and later and Mac computers that run and later. FaceTime supports any iOS device with a forward-facin ...
.
Real time CODEC requirements
The most stringent requirements are a maximum algorithmic delay of only 20 ms and a good audio quality for all kind of audio signals including speech and music.
* The AAC-LD coding scheme bridges the gap between speech coding schemes and high quality audio coding schemes.
Two-way communication with AAC-LD is possible on usual analog telephone lines and via ISDN connections. It can use a bit rate of 32 - 64 kbit/s or higher.
Compared to known speech coders, the codec is capable of coding both music and speech signals with good quality. Unlike speech coders, however, the achieved coding quality scales up with bitrate. Transparent quality can be achieved.
AAC-LD can also process stereo signals by using the advanced stereo coding tools of AAC. Thus it is possible to transmit a stereo signal with a bandwidth of 7 kHz via one ISDN line or with a bandwidth of 15 kHz via two ISDN lines.
See also
*
Advanced Audio Coding
Advanced Audio Coding (AAC) is an audio coding standard for lossy digital audio compression. It was developed by Dolby, AT&T, Fraunhofer and Sony, originally as part of the MPEG-2 specification but later improved under MPEG-4.ISO (2006ISO/ ...
(parent technology)
*
Opus an alternative codec for similar applications with lower delay and no licensing costs
*
MP3 (related older technology)
*
G.729 (
ITU-T
The International Telecommunication Union Telecommunication Standardization Sector (ITU-T) is one of the three Sectors (branches) of the International Telecommunication Union (ITU). It is responsible for coordinating Standardization, standards fo ...
's teleconferencing audio codec)
*
Ultra Low Delay Audio Coder ULD Audio codec (alternative technology also developed by Fraunhofer)
References
{{DEFAULTSORT:AAC-LD
Audio codecs
Computer file formats
MPEG-4
Lossy compression algorithms
Open standards covered by patents