aresample=matrix_encoding=dplii doesn't downmix backleft and center correctly

[Peter Robertson]

Summary of the bug: DPLII files have too-loud right channel.
Seems to be some problem with the interaction of back left and center channel
How to reproduce:

ffmpeg -i surround_blbrc.wav -af "aresample=matrix_encoding=dplii" -ac 2 dplii_blbrc.wav
ffmpeg version git-2014-03-09-8dfd0e6
built on Mar  9 2014 22:53:26 with gcc 4.8.2 (Gentoo 4.8.2 p1.3r1, pie-0.5.8r1)

You can see the problem easiest by crafting a 5.1 surround wave file with empty channels except the center channel with noise and the two back channels, first half of the back left with noise and the second half of the back right with noise. However, all surround files that use the bl and center channels at the same time will be too loud in the resulting file's right channel.

Can provide example files if you like.

[Carl Eugen Hoyos]

Please provide your failing command line together with the complete, uncut console output and please provide a sample.

[Peter Robertson]

Input sample for #3455

[Peter Robertson]

Output sample for #3455

[Peter Robertson]

peter@Chip /run/media/peter/FRAPs/LetsPlay $ ~/mplayer/ffmpeg/ffmpeg -i surround_blbrc.wav -af "aresample=matrix_encoding=dplii" -ac 2 dplii_blbrc.wav
ffmpeg version git-2014-03-09-8dfd0e6 Copyright (c) 2000-2014 the FFmpeg developers
  built on Mar  9 2014 22:53:26 with gcc 4.8.2 (Gentoo 4.8.2 p1.3r1, pie-0.5.8r1)
  configuration: --enable-libx264 --enable-gpl
  libavutil      52. 66.101 / 52. 66.101
  libavcodec     55. 52.102 / 55. 52.102
  libavformat    55. 34.100 / 55. 34.100
  libavdevice    55. 11.100 / 55. 11.100
  libavfilter     4.  3.100 /  4.  3.100
  libswscale      2.  5.101 /  2.  5.101
  libswresample   0. 18.100 /  0. 18.100
  libpostproc    52.  3.100 / 52.  3.100
Guessed Channel Layout for  Input Stream #0.0 : 5.1
Input #0, wav, from 'surround_blbrc.wav':
  Duration: 00:00:01.01, bitrate: 4608 kb/s
    Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 48000 Hz, 5.1, s16, 4608 kb/s
File 'dplii_blbrc.wav' already exists. Overwrite ? [y/N] y
Output #0, wav, to 'dplii_blbrc.wav':
  Metadata:
    ISFT            : Lavf55.34.100
    Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 48000 Hz, stereo, s16, 1536 kb/s
Stream mapping:
  Stream #0:0 -> #0:0 (pcm_s16le -> pcm_s16le)
Press [q] to stop, [?] for help
size=     189kB time=00:00:01.01 bitrate=1536.6kbits/s    
video:0kB audio:189kB subtitle:0 data:0 global headers:0kB muxing overhead 0.041252%

[Peter Robertson]

I can fix this problem by inverting the back-left channel of my source files before letting ffmpeg do its thing - it seems like ffmpeg is subtracting the back left from the center channel rather than adding it which means it is inverting the back-left when it shouldn't or not inverting it when it should.

I'll do some tinkering over the weekend and see if I can't figure out some demonstration to attach.

[Carl Eugen Hoyos]

The sample you uploaded is too short to do any test imo, I tested with a file that you should be able to find with Google:

$ ffmpeg -i Test\ AC3\ v2.0.avi -af "aresample=matrix_encoding=dplii" -ac 2 out.wav
$ ffmpeg -i Test\ AC3\ v2.0.avi -af "aresample=matrix_encoding=dplii" -ac 2 -ab 640k out.ac3
$ ffmpeg -i Test\ AC3\ v2.0.avi -af "aresample=matrix_encoding=dplii" -ac 2 -ab 640k out.eac3

I now set my receiver to "PLII" and played the three files over the digital connection (HDMI): The surround sound sounded as expected (surround sound although I used stereo files). I finally tested the wav file over the analog connection and the stereo signal also played like a surround signal.

Or do I misunderstand the problem?

[Carl Eugen Hoyos]

The suggested patch can be found here:
​http://thread.gmane.org/gmane.comp.video.ffmpeg.devel/175700
​http://ffmpeg.org/pipermail/ffmpeg-devel/2014-March/155559.html

[Carl Eugen Hoyos]

See also #4175.