[FFmpeg-trac] #9544(undetermined:new): adding empty channels while downsampling with dithering leads to noise on new channels

FFmpeg trac at avcodec.org
Thu Dec 2 12:42:04 EET 2021


#9544: adding empty channels while downsampling with dithering leads to noise on
new channels
-------------------------------------+-------------------------------------
             Reporter:  Harald       |                     Type:  defect
  Linden                             |
               Status:  new          |                 Priority:  normal
            Component:               |                  Version:
  undetermined                       |  unspecified
             Keywords:               |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
-------------------------------------+-------------------------------------
 Summary of the bug:

 1. Starting with a mono, 24bit, 48kHz audio file.

 2. Adding 3 new channels leads to 3 additional, silent channels.

 3. Downsampling the output to 16 bit with dithering leads to 1 channel
 with signal and 3 silent channels.

 4. Doing step 2 and 3 at once needs to accumulating noise on the silent
 channels.

 How to reproduce:

 {{{

 ./ffmpeg --version
 ffmpeg version N-104730-gbdc920574d Copyright (c) 2000-2021 the FFmpeg
 developers built with gcc 11 (Debian 11.2.0-12)


 ffmpeg -f lavfi -i "sine=frequency=1000:duration=1200" -c:a pcm_s24le
 sine_mono_24bit.wav
 ffmpeg -i sine_mono_24bit.wav -ac 4 sine_4c_24bit.wav
 ffmpeg -i sine_4c_24bit.wav -af
 aresample=osf=s16:dither_method=rectangular
 ./sine_4c_16bit_dither_noremap.wav
 ffmpeg -i sine_mono_24bit.wav -af
 aresample=osf=s16:dither_method=rectangular -ac 4
 ./sine_4c_16bit_dither_remap.wav
 }}}

 sine_4c_16bit_dither_noremap.wav will have one channel with signal and
 three silent ones.

 sine_4c_16bit_dither_remap.wav should be the same, but will have
 accumulating noise on the silent channels.



 I discovered this while reencoding a combined 24-bit/5.1/2.0 source to
 16-bit/5.1/5.1. The result had accumulating noise on the second audio
 stream.
-- 
Ticket URL: <https://trac.ffmpeg.org/ticket/9544>
FFmpeg <https://ffmpeg.org>
FFmpeg issue tracker


More information about the FFmpeg-trac mailing list