On Tue, Apr 14, 2015 at 12:42:21PM -0300, Claudio Freire wrote: > On Tue, Apr 14, 2015 at 6:10 AM, Michael Niedermayer <michae...@gmx.at> wrote: > > On Tue, Apr 14, 2015 at 12:33:50AM +0100, Rostislav Pehlivanov wrote: > >> This commit implements the perceptual noise substitution AAC extension. > >> This is a proof of concept implementation, and as such, is not enabled by > >> default. This is the second revision of this patch, made after some > >> discussion via non-public email due to a mistake. Any changes made since > >> the first revision have been indicated. > >> > >> In order to extend the encoder to use an additional codebook, the array > >> holding each codebook has been modified with two additional entries - 13 > >> for the NOISE_BT codebook and 12 which has a placeholder function. The > >> cost system was modified to skip the 12th entry using an array to map the > >> input and outputs it has. It also does not accept using the 13th codebook > >> for any band which is not marked as containing noise, thereby restricting > >> its ability to arbitrarily choose it for bands. The use of arrays allows > >> the system to be easily extended to allow for intensity stereo encoding, > >> which uses additional codebooks. > >> > >> The 12th entry in the codebook function array points to a function which > >> stops the execution of the program by calling an assert with an always > >> 'false' argument. After a discussion, it was pointed out in an email > >> discussion with Claudio Freire that having a 'NULL' entry can result in > >> unexpected behaviour and could be used as a security hole. There is no > >> danger of this function being called during encoding due to the codebook > >> maps introduced. > >> > >> Another change from version 1 of the patch is the addition of an argument > >> to the encoder, '-aac_pns' to enable and disable the PNS. This currently > >> defaults to disable the PNS, as it is experimental. The switch will be > >> removed in the future, when the algorithm to select noise bands has been > >> improved. The current algorithm simply compares the energy to the > >> threshold (multiplied by a constant) to determine noise, however the > >> FFPsyBand structure contains other useful figures to determine which bands > >> carry noise more accurately. > >> > >> Finally, the way energy values are converted to scalefactor indices has > >> changed since the first commit, as per the suggestion of Claudio Freire. > >> This may still have some drawbacks, but unlike the first commit it works > >> without having redundant offsets and outputs what the decoder expects to > >> have, in terms of the ranges of the scalefactor indices. > >> > > > >> Some spectral comparisons: https://0x0.st/T7.png (original), > >> https://0x0.st/Th.png (encoded without PNS), https://0x0.st/A1.png > >> (encoded with PNS, const = 1.2), https://0x0.st/Aj.png (spectral > >> difference). The constant is the value which multiplies the threshold when > >> it gets compared to the energy, larger values means more noise will be > >> substituded by PNS values. Example when const = 2.2: https://0x0.st/Ae.png > > > > its probably better to upload and link to places that are more permanent > > than 0x0.st as someone in the future might see this discussion or > > commit and could want to see the pictures too > > > ffmpeg's wiki?
yes, possible [...] -- Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB Rewriting code that is poorly written but fully understood is good. Rewriting code that one doesnt understand is a sign that one is less smart then the original author, trying to rewrite it will not make it better.
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