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• aaccoder : Implement Perceptual Noise Substitution for AAC

  15 avril 2015, par Rostislav Pehlivanov

  aaccoder : Implement Perceptual Noise Substitution for AAC
  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 fourth revision of this patch,
  
  made after some problems were noted out. Any changes made since the previous revisions 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. 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.
  Some of the sample files provided triggered an assertion when the parameter to tune the threshold was set to
  
  a value of ’2.2’. Claudio Freire reported the problem’s source could be in the range of the scalefactor
  
  indices for noise and advised to measure the minimal index and clip anything above the maximum allowed
  
  value. This has been implemented and all the files which used to trigger the asserion now encode without error.
  The third revision of the problem also removes unneded variabes and comparisons. All of them were
  
  redundant and were of little use for when the PNS implementation would be extended.
  The fourth revision moved the clipping of the noise scalefactors outside the second loop of the two-loop
  
  algorithm in order to prevent their redundant calculations. Also, freq_mult has been changed to a float
  
  variable due to the fact that rounding errors can prove to be a problem at low frequencies.
  
  Considerations were taken whether the entire expression could be evaluated inside the expression
  
  , but in the end it was decided that it would be for the best if just the type of the variable were
  
  to change. Claudio Freire reported the two problems. There is no change of functionality
  
  (except for low sampling frequencies) so the spectral demonstrations at the end of this commit’s message were not updated.
  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://trac.ffmpeg.org/attachment/wiki/Encode/AAC/Original.png (original),
  
  https://trac.ffmpeg.org/attachment/wiki/Encode/AAC/PNS_NO.png (encoded without PNS),
  
  https://trac.ffmpeg.org/attachment/wiki/Encode/AAC/PNS1.2.png (encoded with PNS, const = 1.2),
  
  https://trac.ffmpeg.org/attachment/wiki/Encode/AAC/Difference1.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://trac.ffmpeg.org/attachment/wiki/Encode/AAC/PNS_2.2.png
  Reviewed-by : Claudio Freire <klaussfreire@gmail.com>
  
  Signed-off-by : Michael Niedermayer <michaelni@gmx.at>
  

  • [DH] libavcodec/aaccoder.c
  • [DH] libavcodec/aacenc.c
  • [DH] libavcodec/aacenc.h

• aacpsy : correct calculation of minath in psy_3gpp_init

  21 avril 2015, par Andreas Cadhalpun

  aacpsy : correct calculation of minath in psy_3gpp_init
  The minimum of the ath(x, ATH_ADD) function depends on ATH_ADD.
  
  This patch uses the first order approximation to determine it.
  For ATH_ADD = 4 this results in the value at 3407.06812 (-5.24241638)
  
  not the one at 3410 (-5.24237967).
  CC : libav-stabl@libav.org
  
  Signed-off-by : Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
  
  Signed-off-by : Anton Khirnov <anton@khirnov.net>
  

  • [DH] libavcodec/aacpsy.c

• aacpsy : correct calculation of minath in psy_3gpp_init

  21 avril 2015, par Andreas Cadhalpun

  aacpsy : correct calculation of minath in psy_3gpp_init
  The minimum of the ath(x, ATH_ADD) function depends on ATH_ADD.
  
  This patch uses the first order approximation to determine it.
  For ATH_ADD = 4 this results in the value at 3407.06812 (-5.24241638)
  
  not the one at 3410 (-5.24237967).
  CC : libav-stabl@libav.org
  
  Signed-off-by : Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
  
  Signed-off-by : Anton Khirnov <anton@khirnov.net>
  

  • [DBH] libavcodec/aacpsy.c