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The LTP profile of AAC is... terrible. It was an early 90's attempt at bridging the gap between speech codecs and general purpose codecs. It did so by trying to exploit the fact that most speech patterns are regular. Unfortunately, it went about it the same way as AAC Main, by taking the previous frame's samples, modifying them through an LPC filter, transforming them back using a forward MDCT, putting the output coefficients back into the current frame, and using delta coding. But once again, they ignored basic mathematics and MDCT leakage. Thankfully, because AAC LTP is meant to operate at very low bitrates, the extreme quantization results in most leakage being irrelevant. Unfortunately, the result is that the output sounds pretty much terrible regardless of whether LTP is enabled or not. This was the first attempt at trying to couple speech coding into AAC. No, the second attempt did not succeed either. Nnnneither did the third. Or fourth. For the fifth one, they literally just jammed a speech codec into AAC with USAC once they saw Opus do it. Just drop support for encoding AAC LTP. It was always experimental to begin with.
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.