Advanced Audio Coding: Meaning (information, definition, explanation, facts)

Advanced Audio Coding (AAC) is a lossy data compression scheme intended for audio streams. AAC was designed to replace MP3. AAC, ISO/IEC 13818-7, is an extension of the MPEG-2 international standard, ISO/IEC 13818-3. It was further improved in MPEG-4, MPEG-4 Version 2 and MPEG-4 Version 3, ISO/IEC 14496-3.

Some of its advances:

• Sample frequencies from 8 kHz to 96 kHz (official MP3: 16 to 48 kHz)
• Up to 48 channels
• Higher coding efficiency for stationary signals (blocksize: 576 -> 1024 samples)
• Higher coding efficiency for transient signals (blocksize: 192 -> 128 samples)
• Much better handling of frequencies above 16 kHz
• More flexible joint stereo (separate for every scale band)

What this all means to the listener is better and more stable quality than MP3 at equivalent or slightly lower bitrates.

Different MP3 encoders perform differently and they produce output of sometimes wildly varying quality. AAC, on the other hand, takes a modular approach to encoding. Depending on the complexity of the bitstream to be encoded, the desired performance and the acceptable output, implementers may create profiles to define which of a specific set of tools they want use for a particular application. The standard offers three default profiles:

• Main Profile (MAIN)
• Low Complexity Profile (LC) - the most widely used.
• Sample-rate Scalable (SRS) - also called Scalable Sample Rate (MPEG-4 AAC-SSR)

Depending on the AAC profile and the MP3 encoder, 96 kbit/s AAC can give nearly the same or better perceptional quality as 128 kbit/s MP3.

AAC in Apple's iPod

In April, 2003, Apple Computer brought mainstream attention to AAC by announcing that its iTunes and iPod products would support songs in AAC format (via a firmware update for older iPods), and that customers could download popular songs in this format via the iTunes Music Store. Optionally, a digital rights management scheme (named FairPlay) can be employed in tandem.