AAC(Advanced Audio Coding), Chinese name: advanced audio coding, appeared in 1997, based on MPEG-2 audio coding technology. By Fraunhofer IIS, Dolby Laboratories, AT&; T, Sony and other companies * * * developed together to replace MP3 format. In 2, after the appearance of MPEG-4 standard, AAC re-integrated its characteristics and added SBR technology and PS technology, which was also called MPEG-4 AAC to distinguish it from the traditional MPEG-2 AAC. Basic Introduction Chinese Name: Advanced Audio Coding mbth: Advanced Audio Coding appeared in: 1997 Development Company: FraunhoferIIS, AT&; T, Sony and other definitions, features, background, equipment support, Apple, Nokia, other products, features, coding, compression algorithm, algorithm, AAC+, definition AAC, full name Advanced Audio Coding, is a file compression format specially designed for sound data. Different from MP3, it adopts a brand-new algorithm for coding, which is more efficient and has a higher "cost performance". Using AAC format, people can feel that the sound quality is smaller without obvious reduction. Apple ipod and Nokia mobile phones support audio files in AAC format. Advantages of aac logo: Compared with mp3, AAC format has better sound quality and smaller files. Disadvantages: AAC is a lossy compression format, and there is an "essential" gap in sound quality compared with the popular lossless formats such as APE and FLAC. In addition, the faster transmission speed of USB3. and large-capacity MP3 over 16G is accelerating popularization, which also makes the "small" aura on AAC's head disappear. Features ① Improved compression ratio: higher sound quality can be obtained with smaller file size; (2) Supporting multi-channels: up to 48 full-range channels can be provided; ③ Higher resolution: up to 96KHz sampling frequency is supported; ④ Improved decoding efficiency: decoding and playing occupy less resources; Dolby Lab's conclusion ① AAC stereo music with ①128Kbps is considered by experts to be indistinguishable from the original uncompressed sound source; ②AAC format outperforms MP3 format at 96Kbps; ③ It is also 128Kbps, and 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 test items. Generally speaking, AAC can be said to be a very comprehensive coding method. On the one hand, the characteristics of 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, which is really an all-powerful coding method. Background As early as 1987, Fraunhofer IIS began the research and development of "EUREKA project EU147, Digital Audio Broadcasting(DAB)", which is the predecessor of MP3. Through cooperation with Professor Dieter Seitzer, they developed the famous ISO-MPEG Audio Layer-3 compression algorithm. In 1993, this algorithm was integrated into the MPEG-1 standard, and MP3 was put into use. At the end of 1996, Fraunhofer IIS obtained a patent for MP3 in the United States, and announced in 1998 that it would charge royalties for MP3. Since the beginning of 1999, MP3 format has been widely popular. In particular, there are many music websites that provide MP3 for free, and MP3 walkman has flooded into the market. Various factors have made MP3 become an extremely mainstream audio format. Although there were challengers such as VQF and WMA later, MP3' s solid foundation made it still firmly in the position of the boss. AAC Ruisheng Acoustics Technology Co., Ltd. With the passage of time, MP3 can't meet the needs more and more. For example, the compression rate 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 FraunhoferIIS and AT&; T, Sony, Dolby, Nokia and other companies have cooperated to develop the Advanced Audio Coding (AAC) audio format, which is known as the "data compression mode in the 21st century", to replace the position of MP3. Actually, the algorithm of AAC was completed in 1997, and it was called MPEG－2 AAC at that time, because it was still regarded as an extension of MPEG-2 (MP2) standard. However, with the formation of MPEG-4 (MP4) audio standard in 2, MPEG-2AAC has been taken as the core of its coding technology, and some new coding features have been added, so we also call it MPEG-4 AAC (M4A). The equipment supports Apple's heavyweight iPod and iPod mini, all of which can play AAC files at 16-32 kbps. In addition, the iTunes music player built by Apple has paved a shortcut for making and playing AAC files. Nokia Nokia promotes AAC format in the field of mobile phones, including Xpress Music series featuring music, such as 532XM, N-gage, 76 for 3G network, 77 for media mobile phone, 33 for fashion entertainment, 623 for new appearance, and HDR-1, a music player that can be used as files attached to mobile phones, all of which can play AAC files stored on MMC cards. Other Nokia products also have some manufacturers' products supporting AAC, such as Anychat developed by audio and video, DIVA GEM of Daisy Multimedia's flash memory Walkman, iPAQPA-1 supported by Compaq for SM card expansion, MEA21 of Toshiba's SD card walkman, MM-EX3 flash memory walkman of Aiwa, Walkman and PSP of Sony. Panasonic's e-Wear SD Audio Players and iriver's IDP－1 with DataPlay as storage medium, etc. Apple and Real also set up online music stores to compress the latest records into AAC format for download. The key difference between AAC and MP3 is that AAC is developed on the basis of MP3, so there are some similarities between the two coding systems. However, comparing the two coding flowcharts, we will find that the coding process of AAC is more complicated. ① Filter bank: it mainly completes the time-frequency conversion of signals. So as to obtain the spectral coefficients in the frequency domain. (2) Temporal Noise Shaping (TNS): This magical technology can trim the distribution of quantization noise in the time domain through prediction in the frequency domain. TNS technology has made great contributions to the improvement of sound quality in the quantization of some special voices and drastically changing signals! ③ Prediction: Predicting the audio signal can reduce the processing of redundant signals and improve the 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 efficiently. ⑤ Bit－stream format: In AAC, the transmission of information is subject to entropy coding 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 it 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 restores it when decoding, thus improving efficiency. As an audio compression algorithm with high compression ratio, AAC usually has a compression ratio of 18: 1, and some data say that it is 2: 1, which is far better than mp3. However, the sound quality is better than almost all traditional coding methods under the same specifications because of its multi-channel and low complexity description. However, until 26, not many people used this format to store audio, and even fewer mp3 players could play this format. Previously, only Apple iPod was known, while more mobile phones supported AAC. In addition, many audio playback software on computers supported AAC format, such as Apple iTunes. Arithmetic The algorithm adopted by AAC is different from that of MP3. AAC improves coding efficiency by combining other functions. AAC's audio algorithm far exceeds some previous compression algorithms (such as MP3, etc.) in compression ability. It also supports up to 48 audio tracks, 15 low-frequency audio tracks, more sampling rates and bit rates, multi-language compatibility and higher decoding efficiency. It is said that "it can accommodate up to 48 channels of audio tracks, with a sampling rate of 96 KHz, and can provide 5.1-channel music programs with the quality equivalent to ITU-R broadcasting at a data rate of 32Kbps". In a word, AAC can provide better sound quality with about 3% less storage space and bandwidth than MP3 files. However, AAC and mp3 are not the same in space and structure, and whether they like it or not is a matter of different opinions. AAC+ AAC+ is also called HE-AAC. HE means "high efficiency". HE-AAC is a mixture of AAC and SBR technology. SBR stands for Spectral Band Replication. The key of SBR is to provide full bandwidth coding at low bit rate without generating redundant signals. Traditionally, it is considered that audio coding at low bit rate means reducing bandwidth and sampling rate (see MP3 FAQ #7) or generating unpleasant noise signals. The solution of SBR is to let the core code de-encode the low-frequency signal, while the SBR decoder generates the high-frequency signal and some guidance signals (usually the bit stream is extremely low, ~2 kbps) by analyzing the low-frequency signal. This is the reason for adopting SBR-free decoder, so that your frequency response (frequency echo) will be seriously wasted. This is why it is called Spectral Band Replication, which only increases the bandwidth of audio, not reconstruction.