Audio equalizer filter problem - clipping with negative gain

[Alex-Frst]

How to reproduce:

% ffmpeg -f lavfi -i aevalsrc="1.98*random(0)-0.99" -to 60 noise.wav
...

% ffmpeg -i noise.wav -af equalizer=2000:q:1:-5  noise2.wav
ffmpeg version N-54499-g63d7684 Copyright (c) 2000-2013 the FFmpeg developers
  built on Jul  6 2013 10:35:18 with gcc 4.7.3 (GCC)
  configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-avisynth --enable-bzlib --enable-fontconfig --enab
le-frei0r --enable-gnutls --enable-iconv --enable-libass --enable-libbluray --enable-libcaca --enable-libfreetype --enable-libgs
m --enable-libilbc --enable-libmodplug --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libope
njpeg --enable-libopus --enable-librtmp --enable-libschroedinger --enable-libsoxr --enable-libspeex --enable-libtheora --enable-
libtwolame --enable-libvo-aacenc --enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libxavs --
enable-libxvid --enable-zlib
  libavutil      52. 38.100 / 52. 38.100
  libavcodec     55. 18.102 / 55. 18.102
  libavformat    55. 11.101 / 55. 11.101
  libavdevice    55.  2.100 / 55.  2.100
  libavfilter     3. 78.102 /  3. 78.102
  libswscale      2.  3.100 /  2.  3.100
  libswresample   0. 17.102 /  0. 17.102
  libpostproc    52.  3.100 / 52.  3.100
Guessed Channel Layout for  Input Stream #0.0 : mono
Input #0, wav, from 'noise.wav':
  Metadata:
    encoder         : Lavf55.11.101
  Duration: 00:01:00.00, bitrate: 705 kb/s
    Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 44100 Hz, mono, s16, 705 kb/s
Output #0, wav, to 'noise2.wav':
  Metadata:
    ISFT            : Lavf55.11.101
    Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 44100 Hz, mono, s16, 705 kb/s
Stream mapping:
  Stream #0:0 -> #0:0 (pcm_s16le -> pcm_s16le)
Press [q] to stop, [?] for help
clipping
    Last message repeated 654 times
clipping  120kB time=00:00:01.39 bitrate= 706.1kbits/s
    Last message repeated 651 times
clipping  236kB time=00:00:02.73 bitrate= 705.8kbits/s
    Last message repeated 653 times
clipping  352kB time=00:00:04.08 bitrate= 705.8kbits/s
    Last message repeated 662 times
clipping  476kB time=00:00:05.52 bitrate= 705.7kbits/s
    Last message repeated 653 times
clipping  604kB time=00:00:07.01 bitrate= 705.7kbits/s
    Last message repeated 649 times
clipping  720kB time=00:00:08.35 bitrate= 705.7kbits/s
    Last message repeated 654 times
clipping  848kB time=00:00:09.84 bitrate= 705.7kbits/s
    Last message repeated 672 times
.....

[Elon Musk]

It is obvious that your command cause clipping, as its wrong.

[Alex-Frst]

How can equalizing with negative gain (-5dB in the example) at some frequency cause clipping (or message about clipping)?

% ffmpeg -i noise.wav -filter:a "astats" -vn -f null NUL 
....
[Parsed_astats_0 @ 000000000263ac60] Overall
[Parsed_astats_0 @ 000000000263ac60] DC offset: 0.000367
[Parsed_astats_0 @ 000000000263ac60] Min level: -0.989990
[Parsed_astats_0 @ 000000000263ac60] Max level: 0.989990

After equalizing:

% ffmpeg -i noise2.wav -filter:a "astats" -vn -f null NUL 
....
[Parsed_astats_0 @ 000000000036ac60] Overall
[Parsed_astats_0 @ 000000000036ac60] DC offset: 0.000367
[Parsed_astats_0 @ 000000000036ac60] Min level: -0.989990
[Parsed_astats_0 @ 000000000036ac60] Max level: 0.989990

Where is clipping?

[Elon Musk]

Obvious clipping is in source, as its not normalized (between -1 and 1). Thank you very much for wasting my time on this instead of working on among others, new filter(s).

[Alex-Frst]

Yes, it not normalaized (-0.989990...0.989990). But problem is not in source. With +0.1dB gain there is no any messages:

% ffmpeg -i noise.wav -af equalizer=2000:q:1:0.1  noise3.wav
...
Guessed Channel Layout for  Input Stream #0.0 : mono
Input #0, wav, from 'noise.wav':
  Metadata:
    encoder         : Lavf55.11.100
  Duration: 00:01:00.00, bitrate: 705 kb/s
    Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 44100 Hz, mono, s16, 705 kb/s
Output #0, wav, to 'noise2.wav':
  Metadata:
    ISFT            : Lavf55.11.100
    Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 44100 Hz, mono, s16, 705 kb/s
Stream mapping:
  Stream #0:0 -> #0:0 (pcm_s16le -> pcm_s16le)
Press [q] to stop, [?] for help
size=    5168kB time=00:01:00.00 bitrate= 705.6kbits/s
video:0kB audio:5168kB subtitle:0 global headers:0kB muxing overhead 0.001512%

Stats after +0.1dB:

% ffmpeg -i noise3.wav -filter:a "astats" -vn -f null NUL 
...
[Parsed_astats_0 @ 00000000026486c0] Overall
[Parsed_astats_0 @ 00000000026486c0] DC offset: 0.000367
[Parsed_astats_0 @ 00000000026486c0] Min level: -0.997406
[Parsed_astats_0 @ 00000000026486c0] Max level: 0.997650

All OK. It's -0.997406..0.997650 now, little gain and no clipping messages.

Let's test normilized sourse:

% sox noise.wav noise4.wav norm
...
% ffmpeg -i noise4.wav -filter:a "astats" -vn -f null NUL 
...
[Parsed_astats_0 @ 00000000024e86c0] Overall
[Parsed_astats_0 @ 00000000024e86c0] DC offset: 0.000371
[Parsed_astats_0 @ 00000000024e86c0] Min level: -1.000000
[Parsed_astats_0 @ 00000000024e86c0] Max level: 0.999969

% ffmpeg -i noise4.wav -af equalizer=2000:q:1:-5 -y noise5.wav
....
Guessed Channel Layout for  Input Stream #0.0 : mono
Input #0, wav, from 'noise4.wav':
  Duration: 00:01:00.00, bitrate: 705 kb/s
    Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 44100 Hz, mono, s16, 705 kb/s
Output #0, wav, to 'noise5.wav':
  Metadata:
    ISFT            : Lavf55.11.100
    Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 44100 Hz, mono, s16, 705 kb/s
Stream mapping:
  Stream #0:0 -> #0:0 (pcm_s16le -> pcm_s16le)
Press [q] to stop, [?] for help
clipping
    Last message repeated 679 times
clipping  100kB time=00:00:01.16 bitrate= 706.2kbits/s
    Last message repeated 643 times
clipping  196kB time=00:00:02.27 bitrate= 705.9kbits/s
    Last message repeated 651 times
....

clipping again.

[Alex-Frst]

Looks like it's because of equalizing algorithm, dither or something else... Sox clipping with this parameters too. I was confised by "Beware of clipping when using a positive gain" in documentation.
Closed. No more wasting your time.