On Thu, Jul 2, 2015 at 3:13 PM, Rostislav Pehlivanov <atomnu...@gmail.com> wrote: > This commit implements intensity stereo coding support to the native aac > encoder. This is a way to increase the efficiency of the encoder by zeroing > the right channel's spectral coefficients (in a channel pair) and rederiving > them in the decoder using information from the scalefactor indices of special > band types. This commit confomrs to the official ISO 13818-7 specifications, > although due to their ambiguity certain deviations have been taken to ensure > maximum sound quality. This commit has been extensively tested and has shown > to not result in audiable audio artifacts unless in extreme cases. This > commit also adds an option, aac_is, which has the value of 0 by default. > Intensity Stereo is part of the scalable aac profile and is thus non-default. > > The way IS coding works is that it rederives the right channel's spectral > coefficients from the left channel via the scalefactor index values left in > the right channel. Since an entire band's spectral coefficients do not need > to be coded, the encoder's efficiency jumps up and it unzeroes some high > frequency values which it previously did not have enough bits to encode. That > way less information is lost than the information lost by rederiving the > spectral coefficients with some error. This is why the filesize of files > encoded with IS do not decrease significantly. Users wishing that IS coding > should reduce filesize are expected to reduce their encoding bitrates > appropriately. > > This is V2 of the commit. The old version did not mark ms_mask as 0 since M/S > and IS coding are incompactible, which resulted in distortions with M/S > coding enabled. This version also improves phase detection by measuring it > for every spectral coefficient in the band and using a simple majority rule > to determine whether the coefficients are in or out of phase. Also, the > energy values per spectral coefficient were changed as to reflect the > official specifications.
This one also looks identical to a WIP I thoroughly tested, so I believe this means the whole set is good to be pushed, if a committer agrees. _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
