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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.
Knowledge article
1. From MP3 to AAC
As early as 1987, Fraunhofer IIS started the research and development of "EUREKA project EU 147, Digital Audio Broadcasting (DAB)", which is the predecessor of MP3. Through the cooperation with Professor Dieter Seitzer, they developed the famous third-layer compression algorithm of ISO-MPEG audio. 1993, this algorithm was integrated into the MPEG- 1 standard, and MP3 was put into use. At the end of 1996, Flawn Hof IIS obtained a patent for MP3 in the United States, and announced on 1998 that it would collect royalties for MP3. Since 1999, MP3 format has become popular. In particular, there are many music websites that provide MP3 for free, and MP3 walkman has flooded into the market. Various factors make MP3 an extremely mainstream audio format. Although some challengers such as VQF and WMA appeared later, MP3' s solid foundation kept it firmly in the top position.
But audio formats, like computer software and hardware, will eventually be updated. Isn't the tape eliminated by CD? CD will be replaced by DVD-Audio. As time goes by, MP3 can't meet our needs more and more. For example, the compression ratio lags behind Ogg, WMA, VQF and other formats, and the sound quality is not ideal (especially at low bit rate), with only two channels ... So Flawn Hof IIS cooperated with AT&T, Sony, Dolby, Nokia and other companies to jointly develop "2 1 century". In fact, the algorithm of AAC was completed in 1997, which was called MPEG-2 AAC at that time, because it was also regarded as an extension of MPEG-2 standard. However, with the formation of MPEG-4 audio standard in 2000, MPEG-2 AAC has been regarded as the core of its coding technology, and some new coding features have been added, so we call it MPEG-4 AAC.
Support status of 2.2. gas concrete
At present, there are few products supporting AAC, mainly because the patent royalties greatly limit the development of AAC! Fortunately, with the support of Nokia, Apple and Panasonic, the scene was not deserted.
(1) Nokia
Nokia promotes AAC formats in the mobile phone field, including N-gage, 3G network 7600, media mobile phone 7700, fashion entertainment 3300, new look 6230 and music player HDR- 1 as mobile phone accessories, which can play AAC files stored on MMC cards.
(2) Apple
Heavyweight iPod and iPod mini can play 16-320 kbps AAC files, and Apple's built-in iTune music player paved a shortcut for making and playing AAC files.
3) Panasonic
Panasonic's e-Wear SD audio player series walkman is a highlight of supporting AAC. At present, there are many products such as SV-SD05, SV-SD50, SV-SD75 and SV-SD85.
4) Other products
In addition, some manufacturers' products support AAC, such as DIVA GEM of Daisy Multimedia, IPAQPA- 1 of Compaq supporting SM card expansion, MEA2 10 of Toshiba's SD card walkman, MM-EX300 of Aihua, IDP- 100 of iriver using DataPlay as storage medium and so on.
Apple and Real also set up an online music store to compress the latest records into AAC format for download. Of course there is a charge, but the sales volume is very good. Apple's iTune online music store, in particular, can sell 3.3 million songs in a week. Not only promoted the development of the recording industry, but also safeguarded the copyright, which greatly promoted the popularization of AAC!
Advanced functions of 3.3. gas concrete
AAC is developed on the basis of MP3, so their coding system has some points. But comparing the two coding flowcharts, you will find that the coding process of AAC is more complicated.
MP3 encoding process
AAC coding process
(1) The main differences between AAC and MP3 are:
① Filter bank:
(2) Time Noise Shaping (TNS): This magical technology can trim the distribution of quantization noise in time domain through frequency domain prediction. TNS technology has made great contributions to the improvement of sound quality in the quantization of some special parts and the dramatically changing signals!
③ Prediction: Predicting audio signals can reduce the processing of redundant signals and improve efficiency.
④ Quantization: The quantization process of AAC is to use two nested loops for repeated operations. By controlling the quantization analysis well, the bit rate can be used more effectively.
⑤ Bit stream format: In AAC, information transmission is entropy coded to ensure as little redundancy as possible. In addition, AAC has an elastic bit stream structure, which further improves the coding efficiency.
⑥ Long-term prediction (LTP): This tool is only available in MPEG-4 AAC. It is used to reduce the signal redundancy between two consecutive coded frames and is very effective for processing low bit rate speech.
⑦ Perceptual Noise Substitution (PNS): This tool is only available in MPEG-4 AAC. When the encoder finds a signal similar to noise, it does not quantize it, but marks it and ignores the past, and then recovers it when decoding, thus improving efficiency.
(2) 2) Features of AAC:
① Improvement of compression ratio: You can get higher sound quality with smaller file size;
(2) Multi-channel support: up to 48 full-range channels can be provided;
③ Higher resolution: up to 96KHz sampling frequency;
④ decoding efficiency is improved: decoding and playing occupy less resources;
(3) Dolby Lab's conclusion:
① AAC stereo music of128 kbps is considered by experts to be indistinguishable from the original uncompressed sound source;
②AAC format is better than MP3 format of 128Kbps at the bit rate of 96Kbps;
③ The same as 128Kbps, the sound quality of AAC format is obviously better than that of MP3；;
④AAC is the only network broadcast format that can get "excellent" in all EBU audition tests.
Generally speaking, AAC can be said to be a very comprehensive coding method. On the one hand, multi-channel and high sampling rate make it very suitable for future DVD-Audio；; . On the other hand, the high sound quality at low bit rate makes it suitable for mobile communication, VoIP, online broadcasting and other fields. It's really versatile