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AAC is short for Advanced Audio Coding, which appeared in 1997 and was originally an audio coding technology based on MPEG-2. Co-developed by Fraunhofer IIS, Dolby Laboratories, AT&T, Sony and other companies to replace the MP3 format. In 2000, the MPEG-4 standard was introduced, and AAC reintegrated other technologies (PS, SBR). To distinguish it from the traditional MPEG-2 AAC, AAC with SBR or PS features is also called MPEG-4 AAC.<\/p>\n
![\"AAC](\"https:\/\/miro.medium.com\/proxy\/1*DJcdKo4CnADZzLdYOrmqlQ.jpeg\")
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AAC is a new generation of lossy audio compression technology. It derives three main encodings, LC-AAC, HE-AAC, and HE-AACv2 through some additional encoding technologies (such as PS, SBR, etc.). LC-AAC is the Compared to traditional AAC, in relative terms, it is mainly used for medium and high code rate (>= 80 Kbps), HE-AAC (equivalent to AAC+SBR) is mainly used for medium and low code (<= 80 Kbps), and the recently released HE-AACv2 (equivalent to AAC+SBR+PS) is mainly used for low bit rates (<=48 Kbps); in fact, most encoders are set to <=48 Kbps to automatically enable PS technology, and >48 Kbps does not add PS, which is equivalent to ordinary HE-AAC.<\/p>\n
2. Brief description of AAC specification<\/p>\n
AAC has a total of 9 specifications to meet the needs of different occasions:<\/p>\n
MPEG-2 AAC LC Low Complexity Specification (Low Complexity): Relatively simple, no gain control, but improved<\/p>\n
Encoding efficiency, you can find a balance point in the encoding efficiency of medium bit rate and sound quality<\/p>\n
Main Main Specification MPEG-2 AAC<\/p>\n
Scalable sample rate MPEG-2 AAC SSR<\/p>\n
MPEG-4 AAC LC Low Complexity Specification (Low Complexity) —— the current mobile phone is more common in the MP4 file<\/p>\n
The audio part of the file includes the audio file of this specification<\/p>\n
Main Main Specification MPEG-4 AAC —— It contains all the functions except gain control, and its sound quality is the best<\/p>\n
Scalable sample rate MPEG-4 AAC SSR<\/p>\n
MPEG-4 AAC LTP Long Term Prediction<\/p>\n
Low Delay Specification MPEG-4 AAC LD (Low Delay)<\/p>\n
MPEG-4 AAC HE (High Efficiency) High Efficiency Specification—-This specification is suitable for low bit rate encoding, with<\/p>\n
Nero ACC encoder support<\/p>\n
Currently the most used are LC and HE (suitable for low bit rates). The popular Nero AAC encoding program only supports the three specifications of LC, HE and HEv 2. For AAC audio encoded, the specifications are displayed in LC. HE is actually AAC(LC)+SBR technology, HEv2 is AAC(LC)+SBR+PS technology;<\/p>\n
Hev1 and HEv2 are simply represented by this diagram:<\/p>\n
(AAC in the figure refers to the original AAC-LC)<\/p>\n
HE: “High Efficiency.” HE-AAC v1 (also known as AACPlusV1, SBR) implements AAC(LC)+SBR technology by means of a wrapper. SBR actually stands for Spectral Band Replication. Briefly describe, the main spectrum of music is concentrated in the low-frequency band, and the high-frequency band has a small but important amplitude, which determines the sound quality. If the whole frequency band is encoded, if it is to protect the high frequency, the low frequency band will be encoded too precisely and the file will be huge; if the main low-frequency component is preserved and the high-frequency component is lost, the sound quality will be lost. SBR cuts the frequency spectrum, the low frequency is encoded separately to save the main components, and the high frequency is amplified and encoded separately to save the sound quality. It is “built-in”, and the sound quality is also preserved in the case of reducing the file size, which perfectly resolves this contradiction.<\/p>\n","protected":false},"excerpt":{"rendered":"