Advanced Audio Coding

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Advanced Audio Coding (AAC) is a standardized, lossy compression and encoding scheme for digital audio.

AAC usually achieves better sound quality than the more popular MP3 format when compared at the same bitrate, especially for bitrates below about 100 kbit/s.

It is the default, and most commonly used format for compressing audio CDs for Apple's iPod and iTunes (Extension .m4a). Apple uses the AAC format for the files they sell on the iTunes Store; however, these files are restricted with FairPlay, a DRM system, and stored in a proprietary .m4p container that is only compatible with Apple's own iPod device, or iTunes.

AAC is also used as the standard audio file for Sony's Playstation 3 and as the default audio codec for the .m4v format that Apple employs in its iTunes Store video files.

1 History
- 1.1 Standardization
- 1.2 AAC's improvements over MP3
2 How AAC works
3 Promoting aspects
4 Extensions and improvements
5 See also
6 External links
7 Notes

[edit] History

AAC was developed with the cooperation and contributions of companies including Dolby, Fraunhofer (FhG), AT&T, Sony and Nokia, and was officially declared an international standard by the Moving Pictures Experts Group in April 1997.

[edit] Standardization

It is specified both as Part 7 of the MPEG-2 standard, and Part 3 of the MPEG-4 standard. As such, it can be referred to as MPEG-2 Part 7 and MPEG-4 Part 3 depending on its implementation, however it is most often referred to as MPEG-4 AAC, or AAC for short.

AAC was first specified in the standard MPEG-2 Part 7 (known formally as ISO/IEC 13818-7:1997) in 1997 as a new "part" (distinct from ISO/IEC 13818-3) in the MPEG-2 family of international standards.

It was updated in MPEG-4 Part 3 (known formally as ISO/IEC 14496-3:1999) in 1999. The reference software specified in MPEG-4 Part 4 and the conformance bitstreams are specified in MPEG-4 Part 5. A notable addition in this version of the standard is Perceptual Noise Substitution (PNS).

HE-AAC (AAC with SBR) was first standardized in ISO/IEC 14496-3:2001/Amd.1. HE-AAC v2 (AAC with Parametric Stereo) was first specified in ISO/IEC 14496-3:2001/Amd.4. ^[1]

The current version of the AAC standard is ISO/IEC 14496-3:2005 (with 14496-3:2005/Amd.2. for HE-AAC v2^[2])

AacPlus v2 is also standardized by ETSI (European Telecommunications Standards Institute) as TS 102005^[1].

The MPEG4 standard also contains other ways of compressing sound. These are low bit rate and generally used for speech.

[edit] AAC's improvements over MP3

AAC was designed to have better performance than MP3 (which was specified in MPEG-1 and MPEG-2) by the ISO/IEC in 11172-3 and 13818-3.

Improvements include:

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 bitrates and variable frame length. Standardized constant bit rate with bit reservoir.
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)
Can use Kaiser-Bessel derived window function to eliminate spectral leakage at the expense of widening the main lobe
Much better handling of frequencies above 16 kHz
More flexible joint stereo (separate for every scale band)
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. This increased flexibility often leads to more concurrent encoding strategies and, as a result, to more efficient compression. However in terms of whether AAC is better than MP3, the advantages of AAC are not entirely conclusive, and the MP3 specification, while outdated, has proven surprisingly robust. AAC and HE-AAC are better than MP3 at low bit-rates (typically < 128 kbps). At medium to higher bit-rates (typically > 128 kbps in stereo), the two formats are more comparable in most fields.

[edit] How AAC works

AAC is a wideband audio coding algorithm that exploits two primary coding strategies to dramatically reduce the amount of data needed to represent high-quality digital audio.

Signal components that are perceptually irrelevant are discarded;
Redundancies in the coded audio signal are eliminated.

Furthermore:

The signal is processed by a modified discrete cosine transform (MDCT) according to its complexity;
Internal error correction codes are added;
The signal is stored or transmitted.
In order to prevent corrupt samples, a modern implementation of the Luhn mod N algorithm is applied to each frame

The MPEG-4 audio standard does not define a single or small set of highly efficient compression schemes but rather a complex toolbox to perform a wide range of operations from low bitrate speech coding to high-quality audio coding and music synthesis.

The MPEG-4 audio coding algorithm family spans the range from low bitrate speech encoding (down to 2 kbit/s) to high-quality audio coding (at 64 kbit/s per channel and higher).
AAC offers sampling frequencies between 8 kHz and 96 kHz and any number of channels between 1 and 48.
In contrast to MP3's hybrid filter bank, AAC uses the modified discrete cosine transform (MDCT) together with the increased window lengths of 1024 points. AAC is much more capable of encoding audio with streams of complex pulses and square waves than MP3 or MP2.

AAC encoders can switch dynamically between a single MDCT block of length 1024 points or 8 blocks of 128 points.

If a signal change or a transient occurs, 8 shorter windows of 128 points each are chosen for their better temporal resolution.
By default, the longer 1024-point window is otherwise used because the increased frequency resolution allows for a more sophisticated psychoacoustic model, resulting in improved coding efficiency.

[edit] Modular encoding

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 (LC) - the simplest and most widely used and supported;
Main Profile (MAIN) - like the LC profile, with the addition of backwards prediction;
Sample-Rate Scalable (SRS), a.k.a. Scalable Sample Rate (MPEG-4 AAC-SSR);
Long Term Prediction (LTP); added in the MPEG-4 standard - 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.[2]

[edit] AAC Low Delay

The MPEG-4 Low Delay Audio Coder (AAC-LD) is designed to combine the advantages of perceptual audio coding with the low delay necessary for two-way communication. It is closely derived from the MPEG-2 Advanced Audio Coding (AAC) format.

The most stringent requirements are a maximum algorithmic delay of only 20 ms and a good audio quality for all kind of audio signals including speech and music. The AAC-LD coding scheme bridges the gap between speech coding schemes and high quality audio coding schemes.

AAC Low Delay compared to normal AAC codecs and ITU speech audio compression systems.

[edit] AAC error protection toolkit

Applying error protection enables error correction up to a certain extent. Error correcting codes are usually applied equally to the whole payload. However since different parts of an AAC payload show different sensitivity to transmission errors, this would not be a very efficient approach.

The AAC payload can be subdivided into parts with different error sensitivities.

Independent error correcting codes can be applied to any of these parts using the Error Protection (EP) tool defined in MPEG-4 Audio.
This toolkit provides the error correcting capability to the most sensitive parts of the payload in order to keep the additional overhead low.

[edit] Error Resilient (ER) AAC

Error Resilience (ER) techniques can be used to make the coding scheme itself more robust against errors.

For AAC, three custom-tailored methods were developed and defined in MPEG-4 Audio

Huffman Codeword Reordering (HCR) to avoid error propagation within spectral data;
Virtual Codebooks (VCB11) to detect serious errors within spectral data;
Reversible Variable Length Code (RVLC) to reduce error propagation within scale factor data.

[edit] Promoting aspects

[edit] Licensing and patents

In contrast with the MP3 format, which requires royalty payments on distributed content, no licenses or payments are required to be able to stream or distribute content in AAC format. ^[3] This reason alone makes AAC a much more attractive format for distributing content, particularly streaming content (such as Internet radio).

However, a patent license is required for all manufacturers or developers of AAC codecs. ^[4] It is for this reason FOSS implementations such as FAAC and FAAD are distributed in source form only, in order to avoid patent infringement.

AAC requires a patent license, and thus uses proprietary technology. But contrary to popular belief, it is not the property of a single company, having been developed in a standards-making organization.

[edit] Marketing aspects

Ironically, AAC was promoted as the successor to MPEG 1.0 audio, layer III for audio coding at medium to high bitrates, although lower bitrates are its forte. While much less popular than mp3, its sustainability is currently maintained by it being the default Apple iTunes codec, the media player which powers the iPod, the most popular digital audio player on the market.^[5] Furthermore, the iTunes Store, whose sales account for 85% of the market for legal online downloads, ^[6] sells AAC-encoded songs (encapsulated within FairPlay Digital Rights Management).

[edit] Products that support AAC

iTunes and iPod

iTunes AAC file, the most popular usage of the format

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 DRM-restricted form (see FairPlay) via the iTunes Store.

Apple added support for VBR encoding of AAC tracks in iTunes v5.0. It also added certain enhancements in higher-end iPods such as chapters (bookmarks that can incorporate web links and pictures set to appear at certain times during playback of audio books and podcasts) which are not features of AAC itself, but of the proprietary Apple file format that wraps the AAC bitstream.^{[citation needed]}

Other Portable Players

Microsoft Zune: Microsoft's Zune portable media player supports AAC among other audio and video formats.
SanDisk Sansa e200R: The new Rhapsody-branded SanDisk Sansa e200R series contains updated firmware allowing for support and playback of MP4, M4A, and RealAudio, using the AAC codec.
Sony PlayStation Portable (PSP): The PSP has had support for MP4 AAC files since the version 2.0 firmware update (released August 2005), but initially for files with a .mp4 extension only, meaning .m4a files needed to be renamed. This was fixed in the 2.7 firmware update.
Sony Walkman (Walkman): The Walkman S series of MP3 players can use AAC encoded files. Older series may be able to add support with a firmware update.
Sony Ericsson phones such as the P990, K800, and the Walkman-branded W series music phones such as the W950 and the W810 support MP4 files with audio encoded using AAC-LC, HE-AAC v1 and HE-AAC v2.
Palm OS PDAs: Many Palm OS based PDAs and smartphones can play AAC and HE-AAC with the 3rd party software Pocket Tunes. Version 4.0, released in December 2006, added support for native AAC and HE-AAC files. The AAC codec for TCPMP, a popular video player, was withdrawn after version 0.66 due to patent issues, but can still be downloaded from sites other than corecodec.org. BetaPlayer, the commercial follow-on to TCPMP, will presumably include AAC support.
Nokia Nseries multimedia phones: also support AAC format.

Sony PlayStation 3

With the launch of the PlayStation 3 (November 11, 2006), Sony announced support for the AAC format for music (MPEG-4 AAC) and video with AAC audio. This allows for easy interoperability with the PSP system that supports AAC already. 128 kbps AAC is the default format for audio CDs imported to the PS3 hard disk.^[7]

Other Software Media Players

Almost all current computer media players include built-in decoders for AAC, or can utilize a library to decode it. On Microsoft Windows, DirectShow can be utilized this way with the corresponding filters to enable AAC playback in any DirectShow based player. Software player applications of particular note include:

ffdshow is a free open source DirectShow filter for Microsoft Windows operating systems that uses FAAD2 to support AAC decoding
foobar2000 is a freeware audio player for Windows that supports LC and HE AAC
Winamp for Windows, which includes an AAC encoder that supports LC and HE AAC;
MPlayer or xine are often used as AAC decoders on Linux
RealPlayer includes RealNetworks's RealAudio 10 AAC encoder
Another Real product, Rhapsody supports the RealAudio AAC codec, in addition to offering subscription tracks encoded with AAC.
VLC media player supports playback of MP4 and AAC files
Media Player Classic
XBMC (XBox Media Center) supports both AAC (LC and HE) on modified Xbox game-consoles.
KSP Sound Player also supports AAC
Sony SonicStage also support AAC in the version 4,
The KMPlayer also supports AAC
XMMS supports mp4 playback using a plugin provided by the faad2 library.

Nero Digital Audio

In May 2006, Nero AG released a free AAC encoding tool, Nero Digital Audio, which is capable of encoding LC, HE and HEv2 AAC streams. The tool is a Command Line Interface tool only, and a separate utility is included to decode to PCM WAV.

Various tools including the Foobar2000 audio player and MeGUI can provide a GUI for the encoder.

FAAD and FAAC

FAAD and FAAC standing for Freeware Advanced Audio Decoder and Coder respectively, collectively make up an opensource implementation of AAC. They are available from AudioCoding.com.

Phones

For a number of years, many mobile phones from manufacturers such as Nokia, Motorola, Samsung, Sony Ericsson, BenQ-Siemens and Philips have supported AAC playback. The first such phone was the Nokia 5510 released in 2002 which also plays mp3's. However this phone was a commercial failure and such phones with integrated music players did not gain mainstream popularity until 2005 when the trend of having AAC as well as MP3 support continued. Most smartphones and music-themed phones nowadays support these codecs and sometimes additional codecs such as WMA and FLAC

Other Devices

Epson supports AAC playback in the P-2000 and P-4000 Multimedia/Photo Storage Viewers. This support is not available with their older models, however.

Vosonic supports AAC recording and playback in the VP8350, VP8360 and VP8390 MultiMedia Viewers.

The Sony Reader portable eBook plays M4A files containing AAC, and displays metadata created by iTunes. Other Sony products, including the A and E series Network Walkmans, support AAC with firmware updates (released May 2006). While the S series supports it out of the box.

The high end of Pioneer's latest range of car stereo headunits also contain AAC decoding support along with MP3 and WMA. Products like the DEH-P75BT will playback .m4a files recorded onto CD in a data format.

The Samsung SGH-X630 for example plays AAC only as raw files and refuses to play when AAC is wrapped in the standard MP4 container. It plays LC files as well as HE-AAC and HE-AAC v2 files.

[edit] Extensions and improvements

Some extensions have been added to the original AAC standard:

Perceptual Noise Substitution (PNS) - added in MPEG-4. It allows the coding of noise as pseudorandom data.
MPEG-4 Scalable To Lossless (SLS);
High Efficiency AAC (HE-AAC), a.k.a. aacPlus v1 or AAC+ - the combination of SBR (Spectral Band Replication) and AAC; used for low bitrates;
HE-AAC v.2, a.k.a. aacPlus v2 or eAAC+ - the combination of Parametric Stereo (PS) and HE-AAC;
Long Term Predictor (LTP) - added in MPEG-4.