Advanced Audio Coding

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

With the 26 years that had passed since the creation of the MP3 format, many improvements had 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 and seeking 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 (iTunes and QuickTime AAC), Real Networks and Nero AG (Creators of Nero Burning Rom), or the open source FAAC which is analogous to the LAME encoder. AAC is supported on some hardware players, most notably the Apple iPod and some cell phones, and is available in Apple's online store.

In terms of quality, the AAC format is on par with (Ogg) Vorbis, LAME MP3, WMA Pro and other modern codecs, and with added SBR coding (HE AAC) it can provide quite high quality at low bitrates.

Recent developments have led to aacPlus, later standardized as MPEG-4 HE-AAC, which is able to give subjectively good results at low bitrates. The website Tuner2 has several Internet radio stations which are sending out streams at low rates – such as 40 kbps – and some of these are surprisingly good considering the bit rates used.

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 free and high quality ones (iTunes or Nero Digital)
• Reaches transparency in most samples and for most users at around 150 kbps
• Part of MPEG-4 specs
• Anyone can create his or her own implementation (specifications and demo sources available)
• Almost everything supports it, including Android devices, Apple devices, most of the modern portable players, etc.

Cons

• Problem cases that trip out all transform codecs
• 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 are valid but your player won't play them or play at a reduced quality.

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. Some of the various Object Types are (see Wikipedia for a full list):

• MPEG-2 AAC LC / Low Complexity
• MPEG-2 AAC Main
• MPEG-2 AAC SSR / Scalable Sampling Rate
• MPEG-4 AAC LC / Low Complexity
• MPEG-4 AAC Main
• MPEG-4 AAC SSR / Scalable Sampling Rate
• MPEG-4 AAC LTP / Long Term Prediction
• MPEG-4 AAC HE / High Efficiency
• MPEG-4 AAC LD / Low Delay
• USAC / Unified Speech and Audio Coding ["xHE-AAC" being a profile]

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 and supported by most decoders. However, the High Efficiency (HE) Object Type has become more popular recently with its addition to the Nero and Quicktime AAC encoder.

Currently all players support the LC Object Type, although some will work on only MPEG2 or MPEG4 streams. 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 Prediction (LTP) – MPEG4 AAC only
• Perceptual Noise Substitution (PNS) – MPEG4 AAC only
• Spectral Band Replication (SBR) – HE AAC
  • Enhanced SBR (eSBR) - USAC/xHE-AAC
• Parametric stereo (PS) – HE AAC
  • "MPEG" parametric surround - USAC/xHE-AAC
• Algebraic code-excited linear prediction (ACELP)-like - USAC/xHE-AAC (speech-optimized method)

Encoders / Decoders (Supported Platforms)

Current AAC encoders (most to least recommended)
1	Apple AAC M/W
2	FhG AAC (Winamp) W
3	Fraunhofer FDK AAC S/L/M/W
4	Nero AAC L/W
5	FFmpeg 3.0+ AAC encoder S/L/M/W
6	FAAC S/L/M/W
7	Libav (pre-3.0 FFmpeg) AAC encoder S/L/M/W
S Source code available; L Linux; M macOS; W Windows
List of AAC encoders

Current

• Fraunhofer "FhG" AAC (Windows) Distributed as binary library only, included in Winamp. Can be extracted and used with a CLI wrapper.
• Fraunhofer FDK AAC (Android, Linux, others)
• Nero AAC (Linux,Windows)
  • No longer developed nor maintained by Nero, but the latest release is stable and very good.
• Apple AAC (MacOS X,Windows)
• FFmpeg 3.0+ native AAC encoder (Multiplatform)
• exhale (xHE-AAC only, Multiplatform, experimental)

Obsolete

• Libav/FFmpeg pre-3.0 native AAC encoder (Multiplatform)
  • Use new FFmpeg encoder.
• FAAC/FAAD (Multiplatform)
  • Development appears stagnant, but the latest release is stable.

Past

• aacplusenc (Multiplatform) (Dead?)
• PsyTEL, developed into Nero AAC, now obsolete. (Windows)
• HHI/zPlane Compaact!, short-lived closed-source project, disappeared. (Windows)

Patent situation

The owners of the AAC patent pool expects to charge a per-unit fee from codec makers. In theory at least, compiling your own version of an encoder or decoder means you are "making" an unlicensed product. To this day very few people even care about this: FFmpeg happily links you to binary builds that contain its native decoder, capable of decoding even AAC-LD, which has a patent that lasts till 2030.

External References

• AAC FAQ
• Read the AAC guide to learn how to obtain AAC/MP4 files out of WAV files and CDs.
• Detailed AAC comparisons can be found at Roberto's listening tests page.