Introduction

AAC or 'Advanced Audio Coding' forms part of the latest specifications from the MPEG comittee, and is their official successor to the popular MP3 format. As with MP3, the AAC format is an international standard, and is backed up by several big-name companies, including Dolby, Sony and Nokia.

With the 8 (this is just a guess) years that passed since the creation of the MP3 format, many improvements have been realised leading to a seemingly complex specification with several flavours of AAC available. To potentially add to the confusion, AAC is usually wrapped inside an MP4 container to provide tagging, seeking and possibly other benefits?? For this reason, AAC can also be referred to as MP4 audio..

There are several AAC encoders to choose from, coming from large names such as Apple and Nero, or the open source FAAC which is analogous to the LAME encoder. AAC is supported on many hardware players, and is available in online stores..

In terms of quality, it outperforms MP3 by a nice margin, being on par with OGG and other great codecs, and with AAC-HE provide the best low bitrate quality.

Pros

An international standard approved by the ISO
Flexible: supports several sampling rates (8000-96000 Hz), bit depths, and multichannel (up to 48 channels)
Several implementations, including a free and high quality one (iTunes)
Reaches transparency in most samples and for most users at around 150kbps
Part of MPEG4 specs
Fast decoding (using FAAD)
Anyone can create it's own implementation (Specifications and demo sources available)
Some portable players support it (Philips Expanium, iPod, cell phones from Nokia)

Cons

Problem cases that trip out all transform codecs
Relatively slow encoding
Heavily patented
Increased complexity
AAC comes in different "flavors" (object types: AAC LC, AAC HE, AAC PS etc.). Many (especially portable) players only support LC (at the moment) so you can have files that a valid but your player won't play them.

Technical Information

AAC stands for 'Advanced Audio Coding' and is part of the MPEG-4 Systems Standard. Originally known as MPEG-2 Non-Backwards Compatible (As apposed to MPEG-2 Backwards Compatible) it is the succesor to MPEG-1/2 Layer III (MP3). It uses the MP4 container (which is based on Apple's MOV container) to store metadata (i.e. tag information).

As part of the MPEG-4 Systems Standard, an AAC encoded file can include up to 48 full-bandwith audio channels (up to 96 kHz) and 15 Low Frequency Enhancement channels (limited to 120 Hz) plus 15 data streams.

AAC encoding methods are organised into Profiles (MPEG-2) or Object Types (MPEG-4). These different Object Types are not necessarily compatible with each other and may not be playable with various decoders. The various Object Types are:

MPEG-2 AAC LC / Low Complexity
MPEG-2 AAC Main
MPEG-2 AAC SSR / Scalable Sampling Rate
MPEG-4 AAC LC
MPEG-4 AAC Main
MPEG-4 AAC SSR
MPEG-4 AAC LTP / Long Term Prediction
MPEG-4 AAC HE / High Efficiency
MPEG-4 AAC LD / Low Delay

Different Object Types vary in complexity. Some take longer to encode/decode as a result of the different complexities. Furthermore, the benefits of the more complex profiles are often not worth the CPU power required to encode/decode them. As a result the Low Complexity/LC Object Type has become the profile used by most encoders. However, the High Efficiency Object Type has become more popular recently with its addition to the Nero AAC encoder which now supports HE AAC encoding.

Currently all players support the LC Object Type. Players based on the FAAD2 decoder (eg. foobar2000, Winamp Plugins) support almost all Object Types including HE AAC. 3ivX also supports all Object Types except SSR.

Technologies used for compression

Huffman coding
Quantization and scaling
M/S matrixing
Intensity stereo
Channel coupling
Backward adaptive prediction
Temporal Noise Shaping (TNS)
Modified Discrete Cosine Transform (IMDCT)
Gain control and hybrid filter bank (polyphase quadrature filter (IPQF)+IMDCT)
Long Term Predictor (LTP) - MPEG4 AAC only
Perceptual Noise Substitution (PNS) - MPEG4 AAC only
Spectral Band Replication (SBR) - HE AAC
Parametric Stereo (PS) - HE AAC

Encoders

There are several encoders listed at AAC implementations.

Decoders

FAAD

FAQ

Great, so you've given me all the technical stuff, but what is AAC really?

AAC is the culmination of the current state of the art audio encoding techniques. It is designed to improve upon and replace MP3 as the defacto Audio Encoding standard. It usually offers (depending on the codec) equivalent quality to MP3 at a lower bitrate.

What is the difference between .MP4 and .M4A?

Besides the extension, absolutely nothing. Apple came up with extension to distiguish between files with Video and Audio (the MP4 extension) and files with Audio only (the M4A extension). As far as the internal structure of the file, nothing is different.

What extensions does the Apple iPod Accept?

The iPod accepts files with both the MP4 extension and the M4A extension. It will not accept unwrapped AAC files (files with the .AAC extension).

What is the difference between LC (Low Complexity) and HE (High Efficiency)?

These are two of the various Object Types in the MPEG4 Systems Standard. LC is the most popular Object Type with all encoders/decoders supporting it. Currently, Nero, Coding Technolgies, and Panasonic have incorporated the HE AAC standard into their encoders, which allows for higher quality sound at lower bitrates then the LC Object Type does (at the same bitrate). The HE Object Type is only used for music with a bitrate of less than ~80kbps.

What's the best AAC encoder?

There is no best AAC encoder as such. It can be said with reasonable confidence (based on Roberto's last test, see above) that QuickTime/iTunes is the best AAC encoder at 128kbps. However, the quality of any encoder is not linear and therefore these results can not be extrapolated to other bitrates. It can also be said with reasonable confidence that both the iTunes encoder and the Nero AAC encoder are 'mature' and should not fail badly on any particular sample at an average bitrate of 128kbps (i.e. Internet Profile for Nero AAC) or above. Beyond that, only you can decide through ABX testing. See the Audio format guide for more information. However, that being said, QuickTime/iTunes and Nero AAC are considered to be the "safe" encoders if you wish to archive your music collection on your computer.

Do AAC encoded files play back gaplessly?

Gapless playback is not part of the AAC standard and as such is not mandatory. However, certain companies can choose to add gapless encoding/decoding if they desire, providing it doesn't break compatibility with previous decoders. This is what Ahead have done with their Nero AAC codec. The files get encoded with information that allows the gap heard between files to be removed. This however is only possible with supported players (currently these include foobar2000 and Nero ShowTime). Currently Nero AAC is the only AAC codec to have gapless encoding/decoding support.

What players can play back AAC music?

There are now a number of players that can play back this new format. foobar2000 is considered by many to be the most powerful audio player in existence, and it is certainly capable of playing back AAC encoded files. Other players include the iTunes Digital Jukebox, [http://www.winamp.com Winamp] and Real Player.