[FFmpeg-devel] On libavfilter: A Summary of Issues

[Jan Ehrhardt]

Clément Bœsch in gmane.comp.video.ffmpeg.devel (Mon, 18 Feb 2013
03:37:04 +0100):
>The main problem is that you need to adjust ebur128 so it is resizing
>frames to windows to something like 100ms (IIRC), which might require some
>thinking in the filter. The reason is that a loudness result is associated
>for this time frame, so you need to transmit metadata to volume at this
>rate.

I just tested the ebur128 filter on one of my videos. The filter reports
the loudnesses every 100ms:

[Parsed_ebur128_1 @ 0000000003fb1e40] t: 1809.21    M: -13.8 S: -14.8 I:
-11.9 LUFS     LRA:   7.5 LU
[Parsed_ebur128_1 @ 0000000003fb1e40] t: 1809.31    M: -12.7 S: -14.7 I:
-11.9 LUFS     LRA:   7.5 LU
frame=54280 fps=535 q=30.0 size=   85021kB time=00:30:09.23 bitrate=
385.0kbits/
t: 1809.41    M: -12.9 S: -14.7     I: -11.9 LUFS     LRA:   7.5 LU
[Parsed_ebur128_1 @ 0000000003fb1e40] t: 1809.51    M: -14.1 S: -14.8 I:
-11.9 LUFS     LRA:   7.5 LU
[Parsed_ebur128_1 @ 0000000003fb1e40] t: 1809.61    M: -15.2 S: -14.9 I:
-11.9 LUFS     LRA:   7.5 LU
[Parsed_ebur128_1 @ 0000000003fb1e40] t: 1809.71    M: -16.4 S: -14.9 I:
-11.9 LUFS     LRA:   7.5 LU
[Parsed_ebur128_1 @ 0000000003fb1e40] t: 1809.81    M: -14.0 S: -14.6 I:
-11.9 LUFS     LRA:   7.5 LU
[Parsed_ebur128_1 @ 0000000003fb1e40] t: 1809.91    M: -14.1 S: -14.5 I:
-11.9 LUFS     LRA:   7.5 LU
[Parsed_ebur128_1 @ 0000000003fb1e40] t: 1810.01    M: -14.0 S: -14.5 I:
-11.9 LUFS     LRA:   7.5 LU
[Parsed_ebur128_1 @ 0000000003fb1e40] t: 1810.11    M: -13.8 S: -14.6 I:
-11.9 LUFS     LRA:   7.5 LU

[snip]

[Parsed_ebur128_1 @ 00000000040e2d40] Summary:
  Integrated loudness:
    I:         -11.9 LUFS
    Threshold: -22.3 LUFS
  Loudness range:
    LRA:         7.9 LU
    Threshold: -32.3 LUFS
    LRA low:   -17.1 LUFS
    LRA high:   -9.2 LUFS

Quote from the specs: "Basically EBU R128 recommends to normalize audio
at -23 LUFS +/- 1 LU, measured with a relative gate at -10 LU."

My video had an Integrated loudness of -11.9 LUFS, far away from the
desired value.

I had to apply a volume=0.27 to get near the desired -23 LUFS:

[Parsed_ebur128_1 @ 00000000043a0b20] Summary:
  Integrated loudness:
    I:         -23.3 LUFS
    Threshold: -33.8 LUFS
  Loudness range:
    LRA:         8.0 LU
    Threshold: -43.8 LUFS
    LRA low:   -28.6 LUFS
    LRA high:  -20.6 LUFS

This was a volume=0.27 on a second pass. It will be a delicate operation
to change the volume=1 to a volume=0.27 during a one pass encoding.

Jan