Advanced audio coding (AAC) is a technique used for compressing and encoding scheme digital audio files.
AAC technology can be used for coding audio files at medium to high bit rates. AAC is built to be the logical successor to MP3 (ISO/MPEG Audio Layer -3) and claims to offer better sound quality than its predecessor at the same bit rate.
What is an AAC file?
AAC (Advanced Audio Coding) refers to digital audio coding standards that represent audio files based on lossy audio compression. It was launched as successor of MP3 file format keeping in view that the lateral faced issues for the implementation of new ideas in the encoding process based on the development of methods for data compression. AAC achieves better sound quality as compared to MP3 at the same bit rate. It was defined in MPEG-2 Part 7 (ISO/IEC 13818-7), and in an updated form in MPEG-4 Part 3 (ISO/IEC 14496-3). The format was adopted as default media format by YouTube, iPhone, iPod, iPad, Apple iTunes and several other platforms. Several applications and APIs are available for conversion of AAC file format to other formats such as MP3, WMA, M4A, WAV and others.
AAC file format is an enhanced version of MP3 with some improvements. Contributed by several companies including the Fraunhofer Institute (Fraunhofer IIS – developers of MP3), Nokia, Dolby, AT&T and Sony, the format was declared as an international standard by Moving Picture Experts Group (MPEG) in Apr, 1997. Later in 1999, the MPEG-2 Part 7 was updated and included in the MPEG-4 family of standards. It was known as MPEG-4 Part 3, identified as ISO/IEC 14496-3: 1999 or MPEG-4 Audio.
Techopedia Explains Advanced Audio Coding (AAC)
AAC technique involves exploiting two primary coding strategies in order to minimize the amount of data required to impart high-quality digital audio. Signal components that are irrelevant are discarded. Redundancies in the coded audio signal are wiped out.
AAC File Format Specifications
The AAC file format specifications allow more flexibility to design codec than MP3 does, resulting in more concurrent encoding strategies and efficient compression. The format has been choice of selection by a number of hardware platforms for its improvements over MP3 in terms of providing support for more options even at less bitrates. The AAC file format specifications are available as MPEG-2 Part 7 and MPEG-4 Part 3 (not free to download). The format uses following media types:
The main differences between AAC and MP3 in terms of improvements are as follow:
- AAC supports a wider range of channels (up to 48) and sampling rates (from 8 kHz to 96 kHz)
- AAC has better coding frequencies above 16 kHz
- AAC has wider limits of variation in the frequency-time resolution of a bank of filters that have lead to improved coding of transients and stationary parts of the audio signal
- More efficient and simple bank of filters: a hybrid filter bank has been replaced by the standard MDCT (modified discrete cosine transform)
- Supports enhanced effectiveness of compression usingTemporal Noise Shaping (TNS), the prediction coefficients of MDCT-time (long term prediction), parametric stereo, perceptual noise substitution, spectral band replication (SBR).
- more flexible joint stereo (different methods can be used in different frequency ranges);
The encoding of digital audio files involves the following steps:
- Modified discrete cosine transform (MDCT) is used to convert the signal from time-domain to frequency-domain. 2. Filter banks are used to convert an exact number of time samples into frequency samples.
- The frequency domain is quantized using a psychoacoustic model and is then encoded.
- Appropriate internal error correction codes are applied.
The signal is stacked or transmitted.
- Luhn mod N algorithm is used for each frame to avoid sample corruption
AAC can sample frequencies ranging from 8Hz to 96kHz and up to 48 channels. It is also able to compress audio that contains streams of complex pulses and square waves better than MP3.
AAC is an international standard used by some major companies like Dolby Laboratories Inc., Sony Corp, and Nokia Corp. AAC is also used as a default audio codec for .m4v format by Apple in iTunes Store video files.