Advanced Audio Coding
Advanced Audio Coding (AAC) is a lossy data compression scheme intended for audio streams.
Contents
Overview
AAC is a wideband audio coding algorithm that exploits two primary coding strategies to dramatically reduce the amount of data needed to convey high-quality digital audio. First, signal components that are perceptually irrelevant and can be discarded without a perceived loss of audio quality are removed. Next, redundancies in the coded audio signal are eliminated.
Why AAC was designed
AAC was designed as an improved-performance codec relative to MP3 (which was specified in MPEG-1) and MPEG-2 Part 3 (which is also known as "MPEG-2 Audio" or ISO/IEC 13818-3).
AAC ISO Standard
AAC, which was first specified in the standard known formally as ISO/IEC 13818-7, was published in 1997 as a new "part" (distinct from ISO/IEC 13818-3) in the MPEG-2 family of international standards.
Codec improvements
The codec design was further improved in MPEG-4 Part 3, known formally as ISO/IEC 14496-3, with the addition of Perceptual Noise Substitution (PNS) and a Long Term Predictor (LTP).
Bifurcations in the AAC standard
Although the AAC codec specified in MPEG-2 Part 7 and the AAC specified in MPEG-4 Part 3 are somewhat different, they are both informally known as AAC (for clarity it is best to refer specifically either to MPEG-2 AAC or to MPEG-4 AAC).
AAC's improvements over MP3
Some of its advances:
- Sample frequencies from 8 Hz to 96 kHz (official MP3: 16 Hz to 48 kHz)
- Up to 48 channels
- Higher efficiency and simpler filterbank (hybrid → pure MDCT)
- 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.
Modular Coding
AAC 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 four default profiles:
- Low Complexity Profile (LC) - the simplest and most widely used and supported.
- Main Profile (MAIN), which expands upon LC with backwards prediction.
- Sample-rate Scalable (SRS), also called Scalable Sample Rate (MPEG-4 AAC-SSR).
- Long Term Prediction (LTP), added in MPEG-4, an improvement of the MAIN profile using a forward predictor with lower computational complexity
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.
Products that support AAC
iTunes and iPod
In April, 2003, Apple Computer brought mainstream attention to AAC by announcing that its iTunes and iPod products would support songs in MPEG-4 AAC format (via a firmware update for older iPods), and that customers could download popular songs in a protected version of the format via the iTunes Music Store. AAC has now become so associated with Apple hardware and software that people are commonly of the mistaken belief that AAC expands to "Apple Audio Codec." Optionally, a digital rights management scheme (named FairPlay) can be employed in tandem.
Apple has added support for VBR encoding of AAC tracks in iTunes v5.0 (released 9/7/2005).
PSP
Sony PSP has added AAC support with version 2.0 firmware update, which was released in August 2005.
Extensions
- High Efficiency AAC (MPEG-4 HE-AAC) - SBR technology has been applied to AAC, and was incorporated into the standard to form High Efficiency AAC v1, also known as aacPlus v1. Another recent introduction to AAC is Parametric Stereo (aacPlus v2 or Enhanced aacPlus). These technologies significantly improve the performance of AAC at lower bitrates, and are used, for example, in Digital Radio Mondiale.
See also
- Audio data compression
- *.m4a file extension
Some External links
- Apple's page on MPEG-4 AAC
- EE Times article on AAC
- Fraunhofer MPEG-2 AAC Information
- International Standards Organization
- AAC Licensing
- Open Source AAC codec FAAC (encoder) and FAAD2 (decoder)
- Roberto's public listening tests - blind, controlled listening tests of lossy compression formats including AAC
