The abbreviation of AAC "advanced audio coding" is called "advanced audio coding" in Chinese and "2 1 century data compression mode" in mobile phone industry. The operation mode adopted by AAC is different from that of MP3. AAC can support up to 48 audio tracks, 15 low-frequency audio tracks, with more sampling rate and bit rate, and is compatible with many languages, with higher decoding efficiency.
Because "AAC" is a big family, it is divided into 9 specifications to meet the needs of different occasions:
A) MPEG-2 AAC LC low complexity specification.
B) main specifications of mpeg-2 AAC
C) MPEG-2 AAC SSR variable sampling rate (scalable sampling rate).
D) MPEG-4 AAC LC low complexity specification, which is included in the audio part of MP4 files commonly used in mobile phones.
E) main specifications of mpeg-4 AAC
F) MPEG-4 AAC SSR variable sampling rate specification (scalable sampling rate).
G) MPEG-4 AAC LTP long-term prediction specification.
H) MPEG-4 AAC LD low latency specification.
I) MPEG-4 AAC efficient specification.
Among the above specifications, the Main specification "main" contains all functions except gain control, and its sound quality is the best, while the low complexity specification is relatively simple, without gain control, but the coding efficiency is improved. The specification of SSR is basically the same as that of LC, but it has more gain control functions. In addition, MPEG-4/AAC/LTP/LD/HE are all in use. In particular, "He" supports Nero ACC encoder, which is a commonly used code rate recently. But generally speaking, there is little difference in sound quality between the main specification and the LC specification, so most of the AAC specifications currently used are "LC" specifications, because the memory capacity of mobile phones has not yet reached a reasonable level.
AAC is actually the abbreviation of Advanced Audio Coding. At present, only Apple's hard disk MP3 supports this format. AAC consists of Flawn Hof IIS-A, Dolby and AT&; An audio format developed by T * * * is a part of MPEG-2 specification. The algorithm adopted by AAC is different from MP3, and AAC improves the coding efficiency by combining other functions. AAC's audio algorithm far exceeds some previous compression algorithms (such as MP3, etc. ) in terms of compressibility. It also supports up to 48 tracks, 15 low-frequency tracks, more sampling rate and bit rate, multi-language compatibility and higher decoding efficiency. In a word, AAC can provide better sound quality on the premise that it is 30% smaller than MP3 files.
Because the AAC format family is huge, to understand it, we must first understand that it has different encoders. At present, it includes commercial and free AAC format audio encoders, several of which are as follows:
The authoritative encoder developed by FHG Fraunhofer IIS has good sound quality, but it is not open to the public.
B) Nero AAC is probably the most perfect AAC encoder at present, and supports the "LC AAC/HE AAC" specification. Nero AAC encoder provides the best quality "VBR LC AAC" format, and it also ensures the good performance of "HEAAAC" specification at low bit rate. At present, Sony Ericsson W800 and K750 mobile phones all support this kind of encoder.
C) Both media software of c)QuickTime/iTune Apple provide AAC coding function, and its coding technology comes from "DolbyLaboratories", which is the best medium bit rate encoder at present. The stereo sound quality of its coding is 20-25% better than other AAC encoders, and even the sound quality of Dolby AAC at 64 kbps is comparable to that of MP3 at 128 kbps.
D) FAAC is also a good command line encoder, which only supports "LC" specification, but the sound quality is quite good, close to the level of general commercial encoders, and this software is free. Sony Ericsson W800 and K750 also support this encoder.
In addition, there are AAC encoders such as Psytel, Coding Technologies, HHI/z Plane(Compaact) and Dolby AAC, others such as Panasonic, Nokia, AT & amp； Companies such as T and NEC also have specially developed encoders. After we understand these audio specifications and encoders in AAC format, it is not difficult to understand why the same Nokia mobile phone supports many file formats, which all seem to be audio in AAC format. For example, Nokia 6230i is a model that supports both AAC format and M4A format and MP4 format, and these audio file formats actually belong to AAC family members!