AAC (Advanced Audio Coding) and MP3 (MPEG-1 Audio Layer 3) are lossy formats for audio files. MP3, an audio-specific format, is now the de facto standard of digital audio compression for the transfer and playback of music on digital audio players. Designed to be the successor of the MP3 format, AAC generally achieves better sound quality than MP3 at similar bit rates. This difference in quality is more obvious at lower bitrates.
edit AAC vs MP3 audio quality
The AAC format was designed to be an improvement over MP3 in the following aspects:
- More sample frequencies (from 8 kHz to 96 kHz) than MP3 (16 kHz to 48 kHz)
- Up to 48 channels (MP3 supports up to two channels in MPEG-1 mode and up to 5.1 channels in MPEG-2 mode)
- Arbitrary bit-rates and variable frame length. Standardized constant bit rate with bit reservoir.
- Higher efficiency and simpler filterbank (rather than MP3's hybrid coding, AAC uses a pure MDCT)
- Higher coding efficiency for stationary signals (AAC uses a blocksize of 1024 or 960 samples, allowing more efficient coding than MP3's 576 sample blocks)
- Higher coding accuracy for transient signals (AAC uses a blocksize of 128 or 120 samples, allowing more accurate coding than MP3's 192 sample blocks)
- Can use Kaiser-Bessel derived window function to eliminate spectral leakage at the expense of widening the main lobe
- Much better handling of audio frequencies above 16 kHz
- More flexible joint stereo (different methods can be used in different frequency ranges)
- Adds additional modules (tools) to increase compression efficiency: TNS, Backwards Prediction, PNS etc. These modules can be combined to constitute different encoding profiles.
Overall, the AAC format allows developers more flexibility to design codecs than MP3 does, and corrects many of the design choices made in the original MPEG-1 audio specification. This increased flexibility often leads to more concurrent encoding strategies and, as a result, to more efficient compression.
The MP3 specification, although antiquated, has proven surprisingly robust in spite of considerable flaws. AAC and HE-AAC are better than MP3 at low bit rates (typically less than 128 kilobits per second). This is especially true at very low bit rates where the superior stereo coding, pure MDCT, and more optimal transform window sizes leave MP3 unable to compete. However, as bit rate increases, the efficiency of an audio format becomes less important relative to the efficiency of the encoder's implementation, and the intrinsic advantage AAC holds over MP3 no longer dominates audio quality.
edit Licensing and Patents for AAC and MP3
No licenses or payments are required to be able to stream or distribute content in AAC format. This makes AAC a much more attractive format to distribute content than MP3, particularly for streaming content like Internet radio. However, a patent license is required for all manufacturers or developers of AAC codecs. It is for this reason FOSS implementations such as FAAC and FAAD are distributed in source form only, in order to avoid patent infringement.
On the other hand, Thomson, Fraunhofer IIS, Sisvel (and its U.S. subsidiary Audio MPEG), Texas MP3 Technologies, and Alcatel-Lucent all claim legal control of relevant MP3 patents related to decoders. So the legal status of MP3 remains unclear in countries where those patents are valid. However, while these patent and licensing issues affect companies, consumers are largely unconcerned and the popularity of the MP3 format has not abated.