AAC Family Name

Name and Algorithm Name

AAC is often hard for everyone to understand, and many tools call the AAC version strange, even some encoder/player or even
Mislead everyone. For example, some regard he AAC as a AAC-LC, in fact there is no error, but not accurate. The following is a document about the AAC family:
A clear definition of the name:

AAC = MPEG2 AAC ~ = MP3 + TNS + TP (it is not an upgrade of MP3 since it is not backward compatible but uses all MP3's features in a better way ).

MPEG4 AAC = MPEG2 AAC + LTP + PNS
There are several profliles depending on the decoding/encoding complexity, required power, delay, bandwith characteristics, error resilience characteristics, Etc... the most used profile in the PC arena is the aac lc (low complexity) = MPEG4 AAC without LTP.

HE-AAC = SBR + aac lc.
Coding Technologies, developers of SBR, named this coding aacplus, also known as AAC +, HE-AAC, aacp, AAC-LC + SBR, Etc... SBR technology was Prevously introduced in the mp3pro codec.

HE-AAC v2 = Ps + HE-AAC
Coding Technologies, developers of the mpeg parametric stereo, named this coding aacplus v2 as a new revision of the previous release. it is also known as AAC ++, EAAC +, enhanced HE-AAC, eaacp, HE-AAC + PS, Etc... recently it was standarized by ISO as HE-AAC V2.

S-AAC... (just guessing, not yet released but in Reference Model 0 stage)
Since MPEG is focusing in multichannel, the next standard will be something based in the spatial audio coding tool standarized as MPEG surround, that allows to do someting similar to PS but aimed to 5.1ch or 7.1ch content. this cocould be named as S-AAC, AAC surround or AACs, surround HE-AAC, [put your favorite name here]. there isn't an official name for it yet.

Terms and acronyms:

AAC Advanced Audio Coding, developed by Dolby Laboratories.

TNS Temporal Noise Shaping is a tool designed to control the location, in time, of the quantization noise by transmission of filtering coefficients.

TP temporal prediction is a tool designed to enhance compressibility of stationnary signals.

LTP long term prediction is once again a prediction tool. This one requires less computation power but it is far more complex than the one used in MPEG-2 AAC, while providing comparable coding performance.

PNS perceptual noise substitution, allows to replace coding of noise-like parts of the signal by some noise generated on the decoder side, so the decoding result is not deterministic among multiple decoding processes of the same encoded data.

SBR spectral band replication is a tool that creates associated higher frequency content based on the lower frequencies and coding it as statistical information: Level, distribution and ranges. each of these parameters is encoded separately, taking account of their distinctive characteristics. it involves reconstruction of a noise-like frequency spectrum by employee a noise generator with some statistical information (Level, distribution, ranges ), so the decoding result is not deterministic among multiple decoding processes of the same encoded data. both ideas are based on the principle that the human brain tends to consider high frequencies to be either harmonic phenomena associated with lower frequencies or noise, and is thus less sensitive to the exact content of high frequencies in audio signals.

PS parametric stereo, the stereo image information is separated from the mono signal being represented as a small amount of high quality parametric stereo information. the scheme relies on dissecting the incoming audio signal into three 'objects' that are a common constituent of all audio signals: transients, sinusoids and noise the stereo information is efficiently parameterized. each of these objects is encoded separately, taking account of their distinctive characteristics. like PNs and SBR the decoding result is not deterministic among multiple decoding processes of the same encoded data.

SAC spatial audio coding exploits inter-channel differences in level, phase and coherence to capture the spatial image of a multi-channel audio signal relative to a transmitted downmix signal. it encodes each of these cues separately taking account of their distinctive characteristics such that the cues, And the transmitted signal, can be decoded to synthesize a high quality multi-channel representation allowing higher compression than separate channel coding.

Algorithm complexity analysis of AAC fixed-point decoder

[Test Environment]: CPU @ 128 MHz, 32 K dcache, with no multiplier.
[Test vector]: AAC Lc + SBR, faac Encoding
[Test Results]:
AAC decoder Total: Running 3c21b cycles
Requant: 000004225 cycles 6.8%
Spectral_data: 00000618b cycles 10.1%
Scalefactors: 1000000a21 cycles 1.0%
Output: 000000000 cycles 0.0%
FB: 12722dd0 cycles 58.0%
Mdct [0256]: 000005fd5 cycles 9.9%
Cfft [0064]: 0000047e6 cycles 7.4%
Mdct [2048]: 000008663 cycles 13.9%
Cfft [0512]: 0000091fd cycles 15.2%
Mdct [1024]: 000000000 cycles 0.0%
Cfft [0256]: 000000000 cycles 0.0%

To sum up, filterbank is the most complex algorithm in SBR.