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• alac : Check that the channels fit at the given offset

  3 septembre 2013, par Martin Storsjö

  alac : Check that the channels fit at the given offset
  The code tries to decode a number of channels at the
  
  offset given by the ff_alac_channel_layout_offsets table.
  
  Even if the number of channels decoded so far doesn’t
  
  exceed the total number of channels, we need to check that
  
  we actually can decode that number of channels at this offset
  
  as well.
  Reported-by : Mateusz "j00ru" Jurczyk and Gynvael Coldwind
  
  CC : libav-stable@libav.org
  
  Signed-off-by : Martin Storsjö <martin@martin.st>
  

  • [DBH] libavcodec/alac.c

• Why low qmax value improve video quality ?

  14 novembre 2013, par theateist

  Maybe my questions doesn't make sense due to not understanding but please explain me what I miss because I did read posts and wiki and still it's not clear to me.

  As I understand setting low value for qmax will improve the quality by increasing the bitrate.
  Maybe I didn't understood something but isn't lowing the Q(quantization) will decrease the quantization levels and thus the bitrate which means degradation in quality ? Or in ffmpeg lowing Q means increasing the quantization levels ? If the last is true so it make sense that lower qmax improves the quality.

  If the above is true, so increasing qmax will decrease the quantization levels which means less bits for coding a quantization level. So, if number of bits for a level is lower, so total bits per frame will be lower, so how the encoder manage to get to the desired bitrate ?

• FFmpeg : AAC muxing into two different files in the same loop : The 2nd file is corrupted [on hold]

  1er septembre 2013, par user2677612

  We are using FFmpeg libraries git-ee94362 libavformat v55.2.100.
  Our purpose is to mux synthetic video and audio-file audio to 2 different files (in the same loop)
  with the the same format and parameters.

  With MP3 input, both M3U8 outputs are OK :

  Output file #0 : Total number of frames is : video = 229, audio = 347.

  Output file #1 : Total number of frames is : video = 229, audio = 347.

  But with AAC input, the 1st M3U8 output is OK, whereas the 2nd one - is not :

  Output file #0 : Total number of frames is : video = 403, audio = 690.

  Output file #1 : Total number of frames is : video = 3, audio = 690.

  It is exactly the same code in the main loop. We do not use the "aac_adtstoasc" filter in case of M3U output.
  By the way, the problem exist both with and without the filter,
  since in case of MP4 output, the 1st output file is OK, whereas the 2nd one - is not, - as well.

  Moreover :

  1) I have implemented the main loop as Windows threads, separated for every output file.

  2) I have separated input files, as well (i.e. use two identical input files).
  The problem remains !

  It seems that video PTS begins to be corrupted (wrong-calculated) for the 2nd (3rd, etc.) output file.

  If somebody is known about such a problem ? If yes, what can be a solution/workaround ?
  Thank you.