AAC Audio File Format. Advanced Audio Coding.
Development of methods for data compression and psychoacoustics gradually led to the fact that the standard MP3 became an obstacle for the implementation of new ideas in the encoding process. As a result, by 1997 the Fraunhofer Institute (Fraunhofer IIS), which in the early 90's has created an MP3, as well as companies Dolby, AT & T, Sony and Nokia - have developed a new method of compressing audio - Advanced Audio Coding (AAC), which entered the MPEG-2 and MPEG-4.
The main differences from MP3 are:
- support a wider range of channels (up to 48 channels) and sampling rates (from 8 kHz to 96 kHz)
- more efficient and simple bank of filters: a hybrid filter bank has been replaced by the standard MDCT (modified discrete cosine transform)
- wider limits of variation in the frequency-time resolution of a bank of filters - eight times (in MP3 - three times) - have led to improved coding of transients and stationary parts of the audio signal
- better coding frequencies above 16 kHz
- a more flexible mode of stereo coding, which allows switches to M / S (joint stereo) independently in different frequency bands
- additional standard features that enhance the effectiveness of compression: Temporal Noise Shaping (TNS), the prediction coefficients of MDCT-time (long term prediction), parametric stereo, perceptual noise substitution, spectral band replication (SBR).
Due to these features, AAC standard is able to achieve a more flexible, efficient and higher quality audio coding. Due to widespread of MP3 format, AAC standard is still not comparable to the MP3 gained popularity. However, AAC is the basic format of iTunes Store, iPods iPod, iTunes, phone, iPhone, PlayStation 3, Nintendo Wii and a digital radio DAB / DRM. Raw AAC files not so popular. In most cases, AAC-encoded audio distributed as MP4 or M4A audio files.
Here you can find some tutorials how to convert AAC files: