From Hydrogenaudio Knowledgebase
These are some known AAC encoder implementations.
A commercial implementation of both LC AAC and HE AAC, Nero AAC is produced by Nero AG as part of their Nero Digital line of products. It is generally percieved to have the highest quality VBR LC AAC implementation (although QuickTime AAC beats it in CBR mode at 128kbps). The codec can also create HEv1/v2 AAC streams for extremely low bitrates and supports multi-channel surround sound encoding. As of August 2013, Nero AAC is available for free as a command line tool called "Nero AAC Codec" (formerly Nero Digital Audio) here, here and here.
 Recommended Nero AAC Presets
NOTE: Once a preset has been selected, the "Encoding Quality" option should be changed to the "Fast" mode. Despite the name implying worse quality then high, a test undertaken by guruboolez shows that the "Fast" mode offers significant quality advantages over the "High" (see the test here). In the forthcoming release of Nero AAC 3.0 (or a release soon afterwards), the "Fast" mode will become the default and the high quality mode will be removed.
 High Quality
- - VBR/Stereo - Streaming, 100-120 Kb/s (LC AAC) / Actual bitrate ~150kbps
- - VBR/Stereo - Internet, 90-100 Kb/s (LC AAC) / Actual bitrate ~128kbps
 Small Filesize
- - VBR/Stereo - Portable, 50-70 Kb/s (HE AAC) / Actual bitrate ~90kbps
The High Quality preset is for the archival of music, while the Small Filesize preset is for internet/streaming purposes.
 iTunes AAC
The codec is available for free through the iTunes Digital Jukebox.
More information can be found about Apple's AAC implementation on their AAC Audio information page.
The current recommended high quality encoding setting is 160kbps, or 128kbps for portable use.
The iTunes AAC encoder can be externally envoked via a command line tool created by Otto42 called iTunesEncode, which can be found at RareWares in the AAC section. This allows for the generation of iTunes AAC files from audio formats not inherently supported by iTunes using the format conversion functionality found in programs such as foobar2000.
FAAC is a free LC AAC encoder under the Lesser GPL license. Its quality has improved drastically over the last few years and FAAC is nowadays a viable alternative to the commercial encoders (although, at 128kbps or lower bitrates, not at the same quality level as some of them, according to Guruboolez's last listening test).
The default quality setting is -q 100 -c 16000 (~120kbps average bitrate), for better quality encodings use -q 150 -c 22000 (~175kbps average bitrate).
More information can be found at AudioCoding.
 HHI/zPlane (Compaact!)
Compaact is one of the newest AAC encoders. Like Nero AAC, compaact is not free, however it does offer an impressive feature set. Roberto Amorim's last AAC test showed that at 128kbps, Compaact! is tied with both the FAAC and Coding Technologies (Real) encoders. Compaact! features both the LC and Main Object Types, CBR, VBR, Multichannel, high resolution (24bit/96kHz) encoding, and command line support. Development on Compaact has stopped.
For portable encoding, try -q5 to -q6. For music archive purposes, try -q7 to -q8.
More information can be found at the Compaact website.
The creation of Ivan Dimkovic (who now works on Nero AAC), PsyTEL AAC was one of the first AAC encoders. Its multichannel support has bugs that make it unusable, but its stereo mode had the best quality available in its day. Since the implementation of Nero AAC, this codec has become obsolete. It's is now outclassed by both Nero AAC and iTunes - both offer higher quality and are much faster encoders.
The PsyTEL encoder can be found in the AAC section of
 Usability (Psytel aacenc/fastenc)
For music encoding. The quality ranges from -tape (lowest VBR quality) to -ultra (highest VBR quality). Ultra is considered overkill for most audio tracks, i.e: shouldn't be used except for extremely difficult music signals. Example: aacenc -extreme -if "audio file.wav"
 Encoder switches (Psytel aacenc/fastenc)
- Input filename. The name of the track to be encoded (must be a WAV file)
- Output filename. May be omitted, because encoder will automatically create the output file name from the input file name.
- Bitrate switch (CBR mode). Sets the number of bits utilized per second for the encoding process. Example: aacenc -br 192 -if "audio file.wav"
- High quality VBR mode. Can be used with -br switch to select base BitRate. If -br is not specified, it takes as default 64kbps/channel. Example: aacenc -br 192 -vbrhi -if "audio file.wav"
- Lower quality VBR mode. Recommended for internet streaming. Example: aacenc -vr -if "audio file.wav"
- LowPassFilter cut-off (in Hertz). Not recommended. Example: aacenc -br 128 -c 15995 -if "audio file.wav"
- Encoder quality level (1 to 9). 9 is usually taken as default, but you can use smaller numbers if you need high speed and high quality isn't essential. Example: aacenc -br 192 -qual 9 -if "audio file.wav"
- Use adif instead of adts (default) headers. For compatibility with some decoder software and hardware players. Example: aacenc -br 192 -adif -if "audio file.wav"
- No headers (raw iso aac stream). For decoder compatibility. Example: aacenc -br 192 -nh -if "audio file.wav"
- -profile "x"
- Choose iso aac encoding profile:
- 0 - low complexity (default, recommended)
- 1 - main (not recommended, buggy)
- 2 - main ltp (mpeg-4 only)
- Only lc profile is playable on hardware players so far. Example: aacenc -br 192 -profile 2 -if "audio file.wav"
- Improved human speech coding. Best for human voice encoding. Not recommended for low Bitrates or [CBR] coding. Example: aacenc -vbrhi -br 192 -ihsc -if "audio file.wav"
- Tells encoder to use highest sensitivity threshold of audibility. Not recommended on Bitrates lower than 192kbps. Example: aacenc -br 192 -low_ath -if "audio file.wav"
- (perceptual noise substitution) - Improves the quality at very low Bitrates. Should be used only at 64kbps or less. Example: aacenc -br 56 -pns -if "audio file.wav"
Imagine Technology provided an MPEG-4 LC AAC plugin for Adobe Audition. This plugin provided file input and output for the MPEG-4 AAC specification, defined in ISO/IEC 14496-3. After Imagine was bought by Ingenient Technologies, they stopped marketing the Audition plugin.
 Coding Technologies
Coding Technologies (CT) is a Swedish/German company that works close to FhG IIS in development and research of new audio compression techniques.
Thet have distinguished themselves in development of parametric coding methods, such as SBR and Parametric Stereo. SBR is the technology behind the quality boost in MP3pro and HE AAC/AACplus.
They have licensed their encoding and decoding tools to several companies - E.G, Real Networks and Magix.
Does exist an encoder, Aacplusenc, which is based on the Coding Technologies reference code
This encoder is developed by Fraunhofer IIS. It has two names because they have different origins:
- The FhG encoder is included in WinAMP.
- The libfdk-aac encoder is an open-source encoder originally included in Android (like libvo-aacenc) then someone repackaged it to form libfdk-aac. This encoder can be used from FFmpeg/Libav.
There is no official references about whether these two encoders are the same, but their VBR bitrate is essentially the same.
According to the latest AAC listening test @ 96 kbps by IgorC, the FhG encoder is better than Nero and tied with the QuickTime encoder.
Emuzed develops and sells various products and technologies for the PC multimedia and embedded multimedia markets. They have ported and optimized codecs for MPEG-4 ASP and AAC LC for a chip vendor preparing to offer bundled multimedia hardware and software. More info can be found at their encoders & decoders page.
NEC Corporation has developed an LC AAC decoding algorithm for mobile devices. They have also developed a codec named MPEG-4 AAC Ext.1, which they claim decreases bitrate while maintaining the same audio quality. The new MPEG-4 AAC Ext.1 coding technology also features high compatibility with current MPEG-4 AAC. For more information, see NEC's press release.
Panasonic has developed an HE AAC codec together with NEC and Coding Technologies as described in this MPEG Industry Forum paper.
 Real/Helix Producer
RealNetworks has incorporated Coding Technologies/FhG's MPEG-4 AAC / aacPlus™ technology and software within RealNetworks’ software products. As a result, in the newest version of RealProducer 10, AAC has replaced ATRAC3 as the high bitrate audio codec, and that software can encode AAC files wrapped in the MP4 container. In addition, the Producer SDK on Windows also includes HE-AAC encoding. More info can be found at RealNetworks' press release, as well as Coding Technologies' aacPlus page.