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The Main profile of AAC is... terrible. It enables the use of delta coding across coefficients of two frames to try to increase compression, and it enabled one more pole for TNS filters. What the AAC authors failed to take into account were basic mathematics, as MDCT leakage (e.g. the spread of each frequency when represented in a discrete spectrum) is significant in most audio codecs. This leads to huge variations between each frame, basically rendering prediction completely pointless. In fact, enabling AAC-Main prediction does not, in general, even recoup the metadata losses from signalling the profile and prediction properties in the first place. So you lose efficiency by using AAC Main. The rumor is that it was put in the AAC spec for patent reasons, though patent-wise, it has about as much use as a patent for a bicycle designed for use by snakes. The only other thing AAC Main changes is it permits 3-pole TNS filters. When AAC's bands are absolutely tiny, except for very high frequency bands, where you're likely to use PNS instead. Just get rid of it.
FFmpeg README
FFmpeg is a collection of libraries and tools to process multimedia content such as audio, video, subtitles and related metadata.
Libraries
libavcodecprovides implementation of a wider range of codecs.libavformatimplements streaming protocols, container formats and basic I/O access.libavutilincludes hashers, decompressors and miscellaneous utility functions.libavfilterprovides means to alter decoded audio and video through a directed graph of connected filters.libavdeviceprovides an abstraction to access capture and playback devices.libswresampleimplements audio mixing and resampling routines.libswscaleimplements color conversion and scaling routines.
Tools
- ffmpeg is a command line toolbox to manipulate, convert and stream multimedia content.
- ffplay is a minimalistic multimedia player.
- ffprobe is a simple analysis tool to inspect multimedia content.
- Additional small tools such as
aviocat,ismindexandqt-faststart.
Documentation
The offline documentation is available in the doc/ directory.
The online documentation is available in the main website and in the wiki.
Examples
Coding examples are available in the doc/examples directory.
License
FFmpeg codebase is mainly LGPL-licensed with optional components licensed under GPL. Please refer to the LICENSE file for detailed information.
Contributing
Patches should be submitted to the ffmpeg-devel mailing list using
git format-patch or git send-email. Github pull requests should be
avoided because they are not part of our review process and will be ignored.