[[PageOutline(1, Contents)]] [[http://en.wikipedia.org/wiki/Advanced_Audio_Coding|Advanced Audio Coding (AAC)]] is the successor format to MP3, and is defined in MPEG-4 part 3 (ISO/IEC 14496-3). It is often used within an MP4 container format; for music the `.m4a` extension is customarily used. The second-most common use is within MKV (Matroska) files because it has better support for embedded text-based soft subtitles than MP4. The examples in this guide will use the extensions MP4 and M4A. FFmpeg supports two AAC-LC encoders (`aac` and `libfdk_aac`) and one HE-AAC (v1/2) encoder (`libfdk_aac`). The license of `libfdk_aac` is not compatible with GPL, so the GPL does not permit distribution of binaries containing incompatible code when GPL-licensed code is also included. Therefore this encoder have been designated as "non-free", and you cannot download a pre-built ffmpeg that supports it. This can be resolved by [[CompilationGuide|compiling ffmpeg]] yourself. See also [[Encode/HighQualityAudio]] for general guidelines on FFmpeg audio encoding (which also includes a comparison of which AAC encoder is best quality). ---- = Fraunhofer FDK AAC (`libfdk_aac`) =#fdk_aac The Fraunhofer FDK AAC codec library. This is currently the highest-quality AAC encoder available with ffmpeg. Requires ffmpeg to be configured with `--enable-libfdk-aac` (and additionally `--enable-nonfree` if you're also using `--enable-gpl`). Detailed information about the FDK AAC library (not FFmpeg specific) can be found at [http://wiki.hydrogenaud.io/index.php?title=Fraunhofer_FDK_AAC HydrogenAudio Knowledgebase: Fraunhofer FDK AAC]. {{{ #!div style="border: 1px solid #e5c7c7; margin: 1em; background-color: #ffd;" '''Note:''' `libfdk_aac` defaults to a low-pass filter of around 14kHz ([[http://wiki.hydrogenaud.io/index.php?title=Fraunhofer_FDK_AAC#Bandwidth|details]]). If you want to preserve higher frequencies, use `-cutoff 18000`. Adjust the number to the upper frequency limit only if you need to; keeping in mind that a higher limit may audibly reduce the overall quality. }}} == Constant Bit Rate (CBR) mode ==#fdk_cbr These settings target a specific bit rate, with less variation between samples. It gives you greater control over file size, and it is compatible with the HE-AAC profile. As a rule of thumb, for audible transparency, use 64 kBit/s for each channel (so 128 kBit/s for stereo, 384 kBit/s for 5.1 surround sound). Set the bit rate with the `-b:a` option. === Examples === Convert and audio file to AAC in an M4A (MP4) container: {{{ ffmpeg -i input.wav -c:a libfdk_aac -b:a 128k output.m4a }}} Convert 5.1 surround sound audio of a video, leaving the video alone: {{{ ffmpeg -i input.mp4 -c:v copy -c:a libfdk_aac -b:a 384k output.mp4 }}} Convert the video with [[Encode/H.264#twopass|libx264]], with a target of fitting a 90-minute movie on a 700 MB (=5734400 KB) CD-ROM, mixing the audio down to two channels (Windows users should use `NUL` rather than `/dev/null` and `^` instead of `\`): {{{ ffmpeg -y -i input.mp4 -c:v libx264 -b:v 933k -preset:v veryfast -pass 1 -an /dev/null && \ ffmpeg -i input.mp4 -c:v libx264 -b:v 933k -preset:v veryfast -pass 2 \ -ac 2 -c:a libfdk_aac -b:a 128k output.mp4 }}} == Variable Bit Rate (VBR) mode ==#fdk_vbr Target a ''quality'', rather than a specific bit rate. 1 is lowest quality and 5 is highest quality. Set the VBR level with the `-vbr` flag. VBR modes gives roughly the following bit rates per channel ([[http://wiki.hydrogenaud.io/index.php?title=Fraunhofer_FDK_AAC#Bitrate_Modes|details]]): ||VBR||kbps/channel||AOTs|| ||1||20-32||LC,[[#fdk_he|HE,HEv2]]|| ||2||32-40||LC,HE,HEv2|| ||3||48-56||LC,HE,HEv2|| ||4||64-72||LC|| ||5||96-112||LC|| HE bit rates will be much lower. {{{ #!div style="border: 1px solid #e5c7c7; margin: 1em; background-color: #ffd;" '''Note:''' A bug exists in libfdk-aac 0.1.3 and earlier that will cause a crash when using high sample rates, such as 96kHz, with VBR mode 5. ([[http://wiki.hydrogenaud.io/index.php?title=Fraunhofer_FDK_AAC#Issues|details]]). }}} === Examples === Convert an audio file to AAC in an M4A (MP4) container: {{{ ffmpeg -i input.wav -c:a libfdk_aac -vbr 3 output.m4a }}} From a video file, convert only the audio stream: {{{ ffmpeg -i input.mp4 -c:v copy -c:a libfdk_aac -vbr 3 output.mp4 }}} Convert the video with [[Encode/H.264#crf|libx264]], and mix down audio to two channels: {{{ ffmpeg -i input.mp4 -c:v libx264 -crf 22 -preset:v veryfast \ -ac 2 -c:a libfdk_aac -vbr 3 output.mp4 }}} == High-Efficiency AAC ==#fdk_he This is a pair of AAC profiles tailored for low bit rates (version 1 and version 2). HE-AAC version 1 is suited for bit rates below 64kb/s (for stereo audio) down to about 48 kb/s, while HE-AAC version 2 is suited for bit rates as low as 32 kb/s (again, for stereo). {{{ #!div style="border: 1px solid #e5c7c7; margin: 1em; background-color: #ffd;" '''Note:''' HE-AAC version 2 only handles stereo. If you have mono, or want to down-mix to mono, use HE-AAC version 1. }}} Unfortunately, many devices that can play AAC-LC (the default profile for `libfdk_aac`) simply cannot play either version of HE-AAC, so this is not recommended for surround sound audio, which normally needs to be compatible with such hardware players. If you are only going to play it on your computer, or you are sure that your hardware player supports HE-AAC, you can aim for a bit rate of 160kb/s for version 1, or 128kb/s for version 2. As always, experiment to see what works for your ears. === Examples === HE-AAC version 1: {{{ ffmpeg -i input.wav -c:a libfdk_aac -profile:a aac_he -b:a 64k output.m4a }}} HE-AAC version 2: {{{ ffmpeg -i input.wav -c:a libfdk_aac -profile:a aac_he_v2 -b:a 32k output.m4a }}} ---- = Native FFmpeg AAC Encoder = The native FFmpeg AAC encoder. This is currently the second highest-quality AAC encoder available in FFmpeg and does not require an external library like the other AAC encoders described here. This is the default AAC encoder. === Examples === Constant bit rate using `-b:a`: {{{ ffmpeg -i input.wav -c:a aac -b:a 160k output.m4a }}} Variable bit rate (using `-q:a`: {{{ ffmpeg -i input.wav -c:a aac -q:a 2 output.m4a }}} Effective range for `-q:a` is around 0.1-2. This VBR is experimental and likely to get even worse results than the CBR. = Metadata = You can add metadata to any of the examples on this guide: {{{ ffmpeg -i input ... \ -metadata author="FFmpeg Bayou Jug Band" \ -metadata title="Decode my Heart (Let's Mux)" \ output.mp4 }}} For more info, see [https://www.ffmpeg.org/doxygen/3.2/group__metadata__api.html the Metadata API description] and the [https://wiki.multimedia.cx/index.php/FFmpeg_Metadata MultimediaWiki entry on FFmpeg metadata]. ---- = Progressive Download = By default the MP4 muxer writes the 'moov' atom after the audio stream ('mdat' atom) at the end of the file. This results in the user requiring to download the file completely before playback can occur. Relocating this moov atom to the beginning of the file can facilitate playback before the file is completely downloaded by the client. You can do this with the `-movflags +faststart` option: {{{ ffmpeg -i input.wav -c:a libfdk_aac -movflags +faststart output.m4a }}} You can also use this option on existing MP4/M4A files. Since the audio is simply being [https://ffmpeg.org/ffmpeg.html#Stream-copy stream copied] there is no re-encoding occurring, just re-muxing, so therefore there is no quality loss: {{{ ffmpeg -i input.m4a -c:a copy -movflags +faststart output.m4a }}} ---- = FAQ = == Which encoder provides the best quality? == For AAC-LC: `libfdk_aac` > Native FFmpeg AAC encoder (`aac`). For HE-AAC you have one choice: `libfdk_aac`. == Should I use AAC-LC or HE-AAC? == If you require a low audio bitrate, such as ≤ 32kbs/channel, then HE-AAC would be worth considering if your player or device can support HE-AAC decoding. Anything higher may benefit more from AAC-LC due to less processing. If in doubt use AAC-LC. All players supporting HE-AAC also support AAC-LC. ---- = Also See = * [[Encode/HighQualityAudio|Guidelines for high quality lossy audio encoding]]