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. Designed to be the successor of the MP3 format, AAC generally achieves higher sound quality than MP3 encoders at the same bit rate.
AAC has been stan ...
(AAC) standard. It was published in
MPEG-4 Audio
MPEG-4 Part 3 or MPEG-4 Audio (formally ISO/IEC 14496-3) is the third part of the ISO/IEC MPEG-4 international standard developed by Moving Picture Experts Group. It specifies audio coding methods. The first version of ISO/IEC 14496-3 was publish ...
Version 2 (
ISO
ISO is the most common abbreviation for the International Organization for Standardization.
ISO or Iso may also refer to: Business and finance
* Iso (supermarket), a chain of Danish supermarkets incorporated into the SuperBest chain in 2007
* Iso ...
/
IEC
The International Electrotechnical Commission (IEC; in French: ''Commission électrotechnique internationale'') is an international standards organization that prepares and publishes international standards for all electrical, electronic and r ...
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 ...
(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 an edible fruit produced by an apple tree (''Malus domestica''). Apple trees are cultivated worldwide and are the most widely grown species in the genus '' Malus''. The tree originated in Central Asia, where its wild ancest ...
as the
voice-over-IP
Voice over Internet Protocol (VoIP), also called IP telephony, is a method and group of technologies for the delivery of voice communications and multimedia sessions over Internet Protocol (IP) networks, such as the Internet. The terms Internet ...
(VoIP)
speech codec
Speech coding is an application of data compression of digital audio signals containing speech. Speech coding uses speech-specific parameter estimation using audio signal processing techniques to model the speech signal, combined with generic d ...
in
FaceTime.
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 - 64kbit/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. Designed to be the successor of the MP3 format, AAC generally achieves higher sound quality than MP3 encoders at the same bit rate.
AAC has been stan ...
(parent technology)
*
Opus
''Opus'' (pl. ''opera'') is a Latin word meaning "work". Italian equivalents are ''opera'' (singular) and ''opere'' (pl.).
Opus or OPUS may refer to:
Arts and entertainment Music
* Opus number, (abbr. Op.) specifying order of (usually) publicatio ...
an alternative codec for similar applications with lower delay and no licensing costs
*
MP3
MP3 (formally MPEG-1 Audio Layer III or MPEG-2 Audio Layer III) is a coding format for digital audio developed largely by the Fraunhofer Society in Germany, with support from other digital scientists in the United States and elsewhere. Origin ...
(related older technology)
*
G.729
G.729 is a royalty-free narrow-band vocoder-based audio data compression algorithm using a frame length of 10 milliseconds. It is officially described as ''Coding of speech at 8 kbit/s using code-excited linear prediction'' speech coding (CS-ACEL ...
(
ITU-T
The ITU Telecommunication Standardization Sector (ITU-T) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunications and Information Co ...
'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