[FFmpeg-trac] #2144(avfilter:new): libavfilter ebur128 loudness inaccuracy, irregular time interval, LFE interference

[FFmpeg]

#2144: libavfilter ebur128 loudness inaccuracy, irregular time interval, LFE
interference
-------------------------------------+-------------------------------------
             Reporter:  sedacca      |                     Type:  defect
               Status:  new          |                 Priority:  normal
            Component:  avfilter     |                  Version:  git-
             Keywords:  ebur128 EBU  |  master
  R128 f_ebur128.c                   |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
-------------------------------------+-------------------------------------
 Summary of the bug:
 (1) The time interval between logged data rows is often greater than the
 expected 100 milliseconds.
 (2) For some channel layouts the wrong weighting factors are applied to
 channels, leading to incorrect reported loudness.
 (3) Loudness computation is disturbed by the presence of the LFE channel,
 which is not supposed to affect reported loudness.

 How to reproduce the problem:
 {{{
 % ffmpeg -nostats -i seq-3341-6-6channels-WAVEEX-16bit.wav -filter_complex
 ebur128 -f null dummyout.txt 2>f_ebur128_c_2013-01-15_bug.txt
 }}}

 The input WAV file is from http://tech.ebu.ch/docs/testmaterial/ebu-
 loudness-test-setv03.zip.

 The results in `ebur_results_before_bugfix_6ch.txt` show irregular time
 increments, for instance from t: 9.98898 to t: 10.0955 (a difference of
 106.5 msec instead of the expected 100 msec). They also show an integrated
 loudness level of -22.6 LUFS instead of the expected -23.0 LUFS.

 Version:
 {{{
 % ffmpeg -v 9 -loglevel 99 -i seq-3341-6-6channels-WAVEEX-16bit.wav
 ffmpeg version N-48928-g918b411 Copyright (c) 2000-2013 the FFmpeg
 developers
   built on Jan 15 2013 15:34:46 with gcc 4.6 (Ubuntu/Linaro
 4.6.3-1ubuntu5)
   configuration: --enable-gpl
   libavutil      52. 15.100 / 52. 15.100
   libavcodec     54. 89.100 / 54. 89.100
   libavformat    54. 60.100 / 54. 60.100
   libavdevice    54.  3.102 / 54.  3.102
   libavfilter     3. 32.100 /  3. 32.100
   libswscale      2.  1.103 /  2.  1.103
   libswresample   0. 17.102 /  0. 17.102
   libpostproc    52.  2.100 / 52.  2.100
 Splitting the commandline.
 Reading option '-v' ... matched as option 'v' (set libav* logging level)
 with argument '9'.
 Reading option '-loglevel' ... matched as option 'loglevel' (set libav*
 logging level) with argument '99'.
 Reading option '-i' ... matched as input file with argument 'seq-3341-6
 -6channels-WAVEEX-16bit.wav'.
 Finished splitting the commandline.
 Parsing a group of options: global .
 Applying option v (set libav* logging level) with argument 9.
 Successfully parsed a group of options.
 Parsing a group of options: input file seq-3341-6-6channels-WAVEEX-
 16bit.wav.
 Successfully parsed a group of options.
 Opening an input file: seq-3341-6-6channels-WAVEEX-16bit.wav.
 [wav @ 0x2eef4c0] Format wav probed with size=2048 and score=99
 [wav @ 0x2eef4c0] File position before avformat_find_stream_info() is 80
 [wav @ 0x2eef4c0] parser not found for codec pcm_s16le, packets or times
 may be invalid.
 [wav @ 0x2eef4c0] probing stream 0 pp:4
 [wav @ 0x2eef4c0] probing stream 0 pp:3
 [wav @ 0x2eef4c0] probing stream 0 pp:2
 [wav @ 0x2eef4c0] probing stream 0 pp:1
 [wav @ 0x2eef4c0] probed stream 0
 [wav @ 0x2eef4c0] parser not found for codec pcm_s16le, packets or times
 may be invalid.
 [wav @ 0x2eef4c0] max_analyze_duration 5000000 reached at 5001333
 microseconds
 [wav @ 0x2eef4c0] File position after avformat_find_stream_info() is
 2893124
 Input #0, wav, from 'seq-3341-6-6channels-WAVEEX-16bit.wav':
   Duration: 00:00:20.00, bitrate: 4608 kb/s
     Stream #0:0, 706, 1/48000: Audio: pcm_s16le ([1][0][0][0] / 0x0001),
 48000 Hz, 5.1, s16, 4608 kb/s
 Successfully openened the file.
 At least one output file must be specified
 Statistics: 2916352 bytes read, 0 seeks
 }}}

 Causes in file `f_ebur128.c`:
 (1) For unexpected time intervals: In the function `filter_samples()`, the
 `for` loop encompassing lines 449 to 638 has control variable `i` that is
 incorrectly reused and altered by inner `for` loops at lines 537, 558,
 566, and 577.
 (2) For unexpected channel weightings: the examination of the bitmap for
 channel layout does not properly scan for bits that are set and skip over
 bits that are cleared.
 (3) For wrong loudness indication when LFE is present:  the increment
 operation to `*samples` is wrongly skipped when the channel index `ch` has
 the value for the LFE channel. This causes the LFE samples to
 inadvertently contribute to the loudness computation when the channel
 index next has the value for a non-LFE channel, and distorts the time and
 weighting of samples.

 Suggested fixes to file `f_ebur128.c` (a suggested patch will be posted to
 the developer list):
 (1) Add a new control variable `idx_insample` for the outermost `for` loop
 in the function `filter_samples()`;
 (2) Add new logic to scan the `channel_layout` bitmap for the next nonzero
 entry, and update the macro `BACK_MASK` to include all non-front and all
 non-LFE channels.
 (3) Move the existing increment operation of `*samples` to precede the
 `continue` operation that skips unneeded expensive computations for the
 LFE channel.

-- 
Ticket URL: <https://ffmpeg.org/trac/ffmpeg/ticket/2144>
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