AAC format analysis

AAC, the full name Advanced Audio Coding, is a file compression format specially designed for sound data. Unlike MP3, it uses a brand-new algorithm for encoding, which is more efficient and has a higher "price ratio". Using the AAC format can make people feel that the sound quality is not significantly reduced, and it is more compact. Apple iPod and Nokia mobile phones support audio files in AAC format.

1.Features

①Improved compression rate: higher sound quality can be obtained with a smaller file size;
②Support multi-channel: can provide up to 48 full-range channels;
③Higher resolution: up to 96KHz sampling frequency;
④Improved decoding efficiency: less resources are occupied by decoding and playback;

Conclusion from Dolby Laboratories
①128Kbps AAC stereo music is considered by experts to be difficult to detect the difference from the original uncompressed sound source;
②The performance of the AAC format at the 96Kbps bit rate exceeds that of the 128Kbps MP3 format;
③It is also 128Kbps, and the sound quality of AAC format is obviously better than that of MP3;
④AAC is the only one that can obtain "excellent" webcast format in all EBU audition test items.
In general, AAC can be said to be a very comprehensive encoding 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 It is also suitable for fields such as mobile communications, Internet telephony, online broadcasting, etc. It is really a versatile coding method.
 

2.characteristic

  The key difference from MP3
        AAC is developed on the basis of MP3, so the two encoding systems have some similarities. But comparing the two coding flowcharts, you will find that the AAC coding process is more complicated.
①Filter bank: mainly completes the time-frequency conversion of the signal. Thereby, the spectral coefficients in the frequency domain are obtained.
②Temporal Noise Shaping (TNS): This magical technique can modify the distribution of quantization noise in the time domain through prediction in the frequency domain. In the quantization of some special speech and drastically changing signals, TNS technology has contributed a lot to the improvement of sound quality!
③Prediction: Prediction of audio signals can reduce the processing of repeated redundant signals and improve efficiency.
④Quantization: The quantization process of AAC uses two nested loops for repeated operations. Through good control of quantitative analysis, the bit rate can be used more efficiently.
⑤Bit-stream format: In AAC, the transmission of information must be entropy-coded to ensure as little redundancy as possible. In addition, AAC has a flexible bitstream structure, which further improves coding efficiency.
⑥Long Term Prediction (LTP): This is a tool only available in MPEG-4 AAC. It is used to reduce the signal redundancy between two consecutive coded sound frames. It is very useful for processing low bit rate speech. effective.
⑦Perceptual Noise Substitution (PNS): This is also a tool only available in MPEG-4 AAC. When the encoder finds a signal similar to noise, it does not quantify it, but makes a mark and ignores the past. It is restored when decoding, which improves efficiency.

3.coding

(1) Compression algorithm
        As a high compression ratio audio compression algorithm, AAC usually has a compression ratio of 18:1, and some data say that it is 20:1, which is far better than mp3. The sound quality is due to the use of multi-channel and low-complexity description methods. It is better than almost all traditional encoding methods in the same specification. However, until 2006, there were not many using this format to store audio, and there were very few mp3 players that could play this format. Only Apple iPods were known before, and relatively more mobile phones support AAC. In addition, Many audio playback software on the computer support AAC format, such as Apple iTunes.
(2) Algorithm
        The algorithm used by AAC is different from that of MP3. AAC combines other functions to improve coding efficiency. AAC's audio algorithm far exceeds some previous compression algorithms (such as MP3, etc.) in compression capabilities. It also supports up to 48 audio tracks, 15 low-frequency audio tracks, more sample rates and bit rates, multi-language compatibility, and higher decoding efficiency. It claims that it can hold up to 48 channels of audio tracks, the sampling rate is 96KHz, and it can provide 5.1-channel music programs with a quality equivalent to ITU-R broadcasting at a data rate of 320Kbps.
        In short, AAC can provide better sound quality while saving about 30% of storage space and bandwidth than MP3 files. But in terms of space and structure, the encoding style of AAC and mp3 is not the same. Whether you like it or not is a matter of whether you like it or not.

 

 

 

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About Us FMUSER.ORG is a system integration company focusing on RF wireless transmission / studio video audio equipment / streaming and data processing .We are providing everything from advice and consultancy through rack integration to installation, commissioning and training.   We offer FM Transmitter, Analog TV Transmitter, Digital TV transmitter, VHF UHF Transmitter, Antennas, Coaxial Cable Connectors, STL, On Air Processing, Broadcast Products for the Studio, RF Signal Monitoring, RDS Encoders, Audio Processors and Remote Site Control Units, IPTV Products, Video / Audio Encoder / Decoder, designed to meet the needs of both large international broadcast networks and small private stations alike.   Our solution has FM Radio Station / Analog TV Station / Digital TV Station / Audio Video Studio Equipment / Studio Transmitter Link / Transmitter Telemetry System / Hotel TV System / IPTV Live Broadcasting / Streaming Live Broadcast / Video Conference / CATV Broadcasting system.   We are using advanced technology products for all the systems, because we know the high reliability and high performance are so important for the system and solution . 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