What is AAC?

MPEG-2 AAC (Advanced Audio Coding) is the latest MPEG standard on perceptual audio coding. It is an extension of existing standards towards optimum coding efficiency. AAC is not compatible with MPEG Audio Layers 1, 2, 3, that's why AAC was called NBC (Non Backwards Compatible).

There are three AAC profiles
• Main
• Low Complexity (LC)
• Scaleable Sampling Rate (SSR)

The Main profile includes all tools available and delivers the best audio quality of the three profiles. The LC (low complexity) profile comes along with a limited Temporal Noise Shaping tool and without prediction. This reduces audio quality for some special audio signals but saves a lot of computational power in both encoding and especially decoding. The SSR profile (Scaleable Sampling Rate) is a low complexity profile with a different filterbank and a special gain control tool.

AAC has sampling frequencies between 8 kHz and 96 kHz and any number of channels between 1 and 48, the AAC is well prepared for future developments in the audio and cinema sectors.

Like all perceptual coding schemes, MPEG-2 AAC basically makes use of the signal masking properties of the human ear in order to reduce the amount of data.

The AAC uses these techniques

• Huffman Coding
• Quantization and scaling
• M/S Matrixing
• Intensity Stereo
• Coupling Channel
• Backward adaptive prediction
• Temporal noise shaping (TNS)
• Modified discrete cosine transform (IMDCT)
• Gain control and hybrid filter bank (polyphase quadrature filter + IMDCT)

AAC Main - 128, gives significantly better performance than MP2 - 192 or MP3 128. In addition, AAC Main 96 gives better performance than MP3 128.

The AAC Main 128 was indistinguishable from the original for 80% of professional listeners (in tests performed by Mpeg organization).

The compression ratio is around a factor of 2 better compared to MPEG Layer 2 and still a factor of around 1.4 better than MPEG Layer 3 (MP3). This means that compared to Layer 3, you get the same quality at 70% of the bitrate.

You need a Pentium PC 90MHz or above to decode MPEG-2 AAC in realtime, and encoding time is like 4 times track length on a P2-300.

MPEG-2 AAC is the Data Compression for the 21st Century and AAC shall be incorporated (with Twin VQ) in MP4.Basically Mpeg2-AAC and MP4-AAC is the same and MP4 AAC players will play Mpeg2-AAC.MP4 shall have soem more features for better streaming in small bitrates and multi-bitrate for adapting to various streaming conditions.