Why revert to dynamic with --keep-loudness-range-target?

[rsholmes]

What does it mean if you get a message like

WARNING: 04 - Tom Of Bedlam.m4a: You specified linear normalization, but the loudnorm filter reverted to dynamic normalization. This may lead to unexpected results. Consider your input settings, e.g. choose a lower target level or higher target loudness range.

without any message explaining why it reverted to dynamic? I was using --keep-loudness-range-target because otherwise I get a lot of reversions to dynamic, but on a few songs I still get dynamic.

Command:

ffmpeg-normalize 04\ -\ Tom\ Of\ Bedlam.m4a --debug -c:a aac -ext m4a --keep-loudness-range-target -f

Debug output:

DEBUG: Running command: /usr/local/bin/ffmpeg -filters
DEBUG: Validating all input files before processing...
DEBUG: Running command: /usr/local/bin/ffmpeg -i '04 - Tom Of Bedlam.m4a' -c copy -t 0 -map 0 -f null /dev/null
DEBUG: Current extension is set from output file, using extension: m4a
DEBUG: Parsing streams of 04 - Tom Of Bedlam.m4a
DEBUG: Running command: /usr/local/bin/ffmpeg -i '04 - Tom Of Bedlam.m4a' -c copy -t 0 -map 0 -f null /dev/null
DEBUG: Stream parsing command output:
DEBUG: ffmpeg version 8.1 Copyright (c) 2000-2026 the FFmpeg developers
  built with gcc 13 (Ubuntu 13.3.0-6ubuntu2~24.04.1)
  configuration: --prefix=/usr/local --disable-static --enable-shared --enable-gpl --enable-libx264 --enable-libx265 --enable-libvpx --enable-libmp3lame --enable-libopus --enable-libaom --enable-libass --enable-libfreetype --enable-libvorbis --enable-libtheora
  libavutil      60. 26.100 / 60. 26.100
  libavcodec     62. 28.100 / 62. 28.100
  libavformat    62. 12.100 / 62. 12.100
  libavdevice    62.  3.100 / 62.  3.100
  libavfilter    11. 14.100 / 11. 14.100
  libswscale      9.  5.100 /  9.  5.100
  libswresample   6.  3.100 /  6.  3.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '04 - Tom Of Bedlam.m4a':
  Metadata:
    major_brand     : mp42
    minor_version   : 0
    compatible_brands: mp42mp41isomiso2
    creation_time   : 2025-04-16T12:13:13.000000Z
    album           : Dark Ships In The Forest
    artist          : John Roberts, Tony Barrand
    genre           : Folk
    title           : Tom Of Bedlam
    encoder         : iTunes 10.0, QuickTime 7.6.6
    track           : 4/12
    compilation     : 0
    gapless_playback: 0
  Duration: 00:03:57.76, start: 0.000000, bitrate: 306 kb/s
  Stream #0:0[0x1](und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 305 kb/s (default)
    Metadata:
      creation_time   : 2025-04-16T12:13:13.000000Z
      handler_name    : SoundHandler
Stream mapping:
  Stream #0:0 -> #0:0 (copy)
Output #0, null, to '/dev/null':
  Metadata:
    major_brand     : mp42
    minor_version   : 0
    compatible_brands: mp42mp41isomiso2
    gapless_playback: 0
    album           : Dark Ships In The Forest
    artist          : John Roberts, Tony Barrand
    genre           : Folk
    title           : Tom Of Bedlam
    compilation     : 0
    track           : 4/12
    encoder         : Lavf62.12.100
  Stream #0:0(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 305 kb/s (default)
    Metadata:
      creation_time   : 2025-04-16T12:13:13.000000Z
      handler_name    : SoundHandler
Press [q] to stop, [?] for help
[out#0/null @ 0x5f72b6ffee40] video:0KiB audio:0KiB subtitle:0KiB other streams:0KiB global headers:0KiB muxing overhead: unknown
[out#0/null @ 0x5f72b6ffee40] Output file is empty, nothing was encoded
size=N/A time=00:00:00.00 bitrate=N/A speed=   0x elapsed=0:00:00.00    

DEBUG: Found duration: 237.076 s
DEBUG: Found audio stream at index 0 (default: True)
DEBUG: Created MediaStream for 04 - Tom Of Bedlam.m4a, audio stream 0
INFO: Normalizing file 04 - Tom Of Bedlam.m4a (1 of 1)
DEBUG: Running normalization for 04 - Tom Of Bedlam.m4a
DEBUG: Parsing normalization info for 04 - Tom Of Bedlam.m4a
INFO: Running first pass loudnorm filter for stream 0
DEBUG: Running command: /usr/local/bin/ffmpeg -hide_banner -y -i '04 - Tom Of Bedlam.m4a' -map 0:0 -filter_complex '[0:0]loudnorm=i=-23.0:lra=7.0:tp=-2.0:offset=0.0:print_format=json' -vn -sn -f null /dev/null
DEBUG: ffmpeg output: Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '04 - Tom Of Bedlam.m4a':
Metadata:
major_brand     : mp42
minor_version   : 0
compatible_brands: mp42mp41isomiso2
creation_time   : 2025-04-16T12:13:13.000000Z
album           : Dark Ships In The Forest
artist          : John Roberts, Tony Barrand
genre           : Folk
title           : Tom Of Bedlam
encoder         : iTunes 10.0, QuickTime 7.6.6
track           : 4/12
compilation     : 0
gapless_playback: 0
Duration: 00:03:57.76, start: 0.000000, bitrate: 306 kb/s
Stream #0:0[0x1](und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 305 kb/s (default)
Metadata:
creation_time   : 2025-04-16T12:13:13.000000Z
handler_name    : SoundHandler
Stream mapping:
Stream #0:0 (aac) -> loudnorm:default
loudnorm:default -> Stream #0:0 (pcm_s16le)
Stream #0:0 -> #0:1 (aac (native) -> pcm_s16le (native))
Press [q] to stop, [?] for help
Output #0, null, to '/dev/null':
Metadata:
major_brand     : mp42
minor_version   : 0
compatible_brands: mp42mp41isomiso2
gapless_playback: 0
album           : Dark Ships In The Forest
artist          : John Roberts, Tony Barrand
genre           : Folk
title           : Tom Of Bedlam
compilation     : 0
track           : 4/12
encoder         : Lavf62.12.100
Stream #0:0: Audio: pcm_s16le, 192000 Hz, stereo, s16, 6144 kb/s
Metadata:
encoder         : Lavc62.28.100 pcm_s16le
Stream #0:1(und): Audio: pcm_s16le, 44100 Hz, stereo, s16, 1411 kb/s (default)
Metadata:
encoder         : Lavc62.28.100 pcm_s16le
creation_time   : 2025-04-16T12:13:13.000000Z
handler_name    : SoundHandler
[Parsed_loudnorm_0 @ 0x74d8d00020c0]
{
"input_i" : "-21.81",
"input_tp" : "2.86",
"input_lra" : "6.80",
"input_thresh" : "-32.14",
"output_i" : "-22.30",
"output_tp" : "-2.00",
"output_lra" : "6.80",
"output_thresh" : "-32.51",
"normalization_type" : "dynamic",
"target_offset" : "-0.70"
}
[out#0/null @ 0x5fee2ddf4dc0] video:0KiB audio:219332KiB subtitle:0KiB other streams:0KiB global headers:0KiB muxing overhead: unknown
DEBUG: Loudnorm first pass command output: Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '04 - Tom Of Bedlam.m4a':
Metadata:
major_brand     : mp42
minor_version   : 0
compatible_brands: mp42mp41isomiso2
creation_time   : 2025-04-16T12:13:13.000000Z
album           : Dark Ships In The Forest
artist          : John Roberts, Tony Barrand
genre           : Folk
title           : Tom Of Bedlam
encoder         : iTunes 10.0, QuickTime 7.6.6
track           : 4/12
compilation     : 0
gapless_playback: 0
Duration: 00:03:57.76, start: 0.000000, bitrate: 306 kb/s
Stream #0:0[0x1](und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 305 kb/s (default)
Metadata:
creation_time   : 2025-04-16T12:13:13.000000Z
handler_name    : SoundHandler
Stream mapping:
Stream #0:0 (aac) -> loudnorm:default
loudnorm:default -> Stream #0:0 (pcm_s16le)
Stream #0:0 -> #0:1 (aac (native) -> pcm_s16le (native))
Press [q] to stop, [?] for help
Output #0, null, to '/dev/null':
Metadata:
major_brand     : mp42
minor_version   : 0
compatible_brands: mp42mp41isomiso2
gapless_playback: 0
album           : Dark Ships In The Forest
artist          : John Roberts, Tony Barrand
genre           : Folk
title           : Tom Of Bedlam
compilation     : 0
track           : 4/12
encoder         : Lavf62.12.100
Stream #0:0: Audio: pcm_s16le, 192000 Hz, stereo, s16, 6144 kb/s
Metadata:
encoder         : Lavc62.28.100 pcm_s16le
Stream #0:1(und): Audio: pcm_s16le, 44100 Hz, stereo, s16, 1411 kb/s (default)
Metadata:
encoder         : Lavc62.28.100 pcm_s16le
creation_time   : 2025-04-16T12:13:13.000000Z
handler_name    : SoundHandler
[Parsed_loudnorm_0 @ 0x74d8d00020c0]
{
"input_i" : "-21.81",
"input_tp" : "2.86",
"input_lra" : "6.80",
"input_thresh" : "-32.14",
"output_i" : "-22.30",
"output_tp" : "-2.00",
"output_lra" : "6.80",
"output_thresh" : "-32.51",
"normalization_type" : "dynamic",
"target_offset" : "-0.70"
}
[out#0/null @ 0x5fee2ddf4dc0] video:0KiB audio:219332KiB subtitle:0KiB other streams:0KiB global headers:0KiB muxing overhead: unknown
DEBUG: Parsing loudnorm stats from output
DEBUG: Loudnorm stats for stream 0 parsed: {
"input_i" : "-21.81",
"input_tp" : "2.86",
"input_lra" : "6.80",
"input_thresh" : "-32.14",
"output_i" : "-22.30",
"output_tp" : "-2.00",
"output_lra" : "6.80",
"output_thresh" : "-32.51",
"normalization_type" : "dynamic",
"target_offset" : "-0.70"
}
INFO: Running second pass for 04 - Tom Of Bedlam.m4a
DEBUG: Keeping target loudness range in second pass loudnorm filter
DEBUG: Running command: /usr/local/bin/ffmpeg -hide_banner -y -i '04 - Tom Of Bedlam.m4a' -filter_complex '[0:0]loudnorm=i=-23.0:lra=6.8:tp=-2.0:offset=-0.7:measured_i=-21.81:measured_lra=6.8:measured_tp=2.86:measured_thresh=-32.14:linear=true:print_format=json[norm0]' -map_metadata 0 -map_metadata:s:a:0 0:s:a:0 -map_chapters 0 -map '[norm0]' -c:a:0 aac -c:s copy /tmp/tmpm52jac52/out.m4a
DEBUG: ffmpeg output: Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '04 - Tom Of Bedlam.m4a':
Metadata:
major_brand     : mp42
minor_version   : 0
compatible_brands: mp42mp41isomiso2
creation_time   : 2025-04-16T12:13:13.000000Z
album           : Dark Ships In The Forest
artist          : John Roberts, Tony Barrand
genre           : Folk
title           : Tom Of Bedlam
encoder         : iTunes 10.0, QuickTime 7.6.6
track           : 4/12
compilation     : 0
gapless_playback: 0
Duration: 00:03:57.76, start: 0.000000, bitrate: 306 kb/s
Stream #0:0[0x1](und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 305 kb/s (default)
Metadata:
creation_time   : 2025-04-16T12:13:13.000000Z
handler_name    : SoundHandler
Stream mapping:
Stream #0:0 (aac) -> loudnorm:default
loudnorm:default -> Stream #0:0 (aac)
Press [q] to stop, [?] for help
Output #0, ipod, to '/tmp/tmpm52jac52/out.m4a':
Metadata:
major_brand     : mp42
minor_version   : 0
compatible_brands: mp42mp41isomiso2
creation_time   : 2025-04-16T12:13:13.000000Z
album           : Dark Ships In The Forest
artist          : John Roberts, Tony Barrand
genre           : Folk
title           : Tom Of Bedlam
gapless_playback: 0
track           : 4/12
compilation     : 0
encoder         : Lavf62.12.100
Stream #0:0(und): Audio: aac (LC) (mp4a / 0x6134706D), 96000 Hz, stereo, fltp, 128 kb/s
Metadata:
encoder         : Lavc62.28.100 aac
creation_time   : 2025-04-16T12:13:13.000000Z
handler_name    : SoundHandler
[aac @ 0x6407d78e6780] Queue input is backward in time
[aost#0:0/aac @ 0x6407d78d1480] Non-monotonic DTS; previous: 287744, current: 284171; changing to 287745. This may result in incorrect timestamps in the output file.
[aost#0:0/aac @ 0x6407d78d1480] Non-monotonic DTS; previous: 287745, current: 285195; changing to 287746. This may result in incorrect timestamps in the output file.
[aost#0:0/aac @ 0x6407d78d1480] Non-monotonic DTS; previous: 287746, current: 286219; changing to 287747. This may result in incorrect timestamps in the output file.
[aost#0:0/aac @ 0x6407d78d1480] Non-monotonic DTS; previous: 287747, current: 287243; changing to 287748. This may result in incorrect timestamps in the output file.
[Parsed_loudnorm_0 @ 0x749728002640]
{
"input_i" : "-21.81",
"input_tp" : "2.86",
"input_lra" : "6.80",
"input_thresh" : "-32.14",
"output_i" : "-22.99",
"output_tp" : "-2.00",
"output_lra" : "6.60",
"output_thresh" : "-33.21",
"normalization_type" : "dynamic",
"target_offset" : "-0.01"
}
[out#0/ipod @ 0x6407d78d23c0] video:0KiB audio:3741KiB subtitle:0KiB other streams:0KiB global headers:0KiB muxing overhead: 2.357815%
[aac @ 0x6407d78e6780] Qavg: 375.665
DEBUG: Moving temporary file from /tmp/tmpm52jac52/out.m4a to normalized/04 - Tom Of Bedlam.m4a
DEBUG: Parsing loudnorm stats from output
DEBUG: Loudnorm stats for stream 0 parsed: {
"input_i" : "-21.81",
"input_tp" : "2.86",
"input_lra" : "6.80",
"input_thresh" : "-32.14",
"output_i" : "-22.99",
"output_tp" : "-2.00",
"output_lra" : "6.60",
"output_thresh" : "-33.21",
"normalization_type" : "dynamic",
"target_offset" : "-0.01"
}
DEBUG: Setting second pass stats for stream 0 from {'input_i': -21.81, 'input_tp': 2.86, 'input_lra': 6.8, 'input_thresh': -32.14, 'output_i': -22.99, 'output_tp': -2.0, 'output_lra': 6.6, 'output_thresh': -33.21, 'normalization_type': 'dynamic', 'target_offset': -0.01}
WARNING: 04 - Tom Of Bedlam.m4a: You specified linear normalization, but the loudnorm filter reverted to dynamic normalization. This may lead to unexpected results. Consider your input settings, e.g. choose a lower target level or higher target loudness range.
DEBUG: Normalization finished
DEBUG: Stripping ReplayGain tags from normalized/04 - Tom Of Bedlam.m4a
INFO: Normalized file written to normalized/04 - Tom Of Bedlam.m4a

Additional context

• Your operating system: Linux Mint 22.3
• Your Python version / distribution (python3 --version or python --version): Python 3.12.3
• Your ffmpeg version (ffmpeg -version):

ffmpeg version 8.1 Copyright (c) 2000-2026 the FFmpeg developers
built with gcc 13 (Ubuntu 13.3.0-6ubuntu2~24.04.1)
configuration: --prefix=/usr/local --disable-static --enable-shared --enable-gpl --enable-libx264 --enable-libx265 --enable-libvpx --enable-libmp3lame --enable-libopus --enable-libaom --enable-libass --enable-libfreetype --enable-libvorbis --enable-libtheora
libavutil      60. 26.100 / 60. 26.100
libavcodec     62. 28.100 / 62. 28.100
libavformat    62. 12.100 / 62. 12.100
libavdevice    62.  3.100 / 62.  3.100
libavfilter    11. 14.100 / 11. 14.100
libswscale      9.  5.100 /  9.  5.100
libswresample   6.  3.100 /  6.  3.100

04 - Tom Of Bedlam.m4a.zip

[slhck]

It's a bit complicated to cover all cases. For EBU normalization, ffmpeg applies a single uniform gain. To go from -21.81 (your input) to -23.0 LUFS (default target), it needs about -1.19 dB of gain. But then the true peak would be: 2.86 (original true peak) - 1.19 = ~1.67 dBTP. Your target true peak is -2.0 dBTP (default).

The filter can't achieve the loudness target and the true peak limit with a uniform gain. So ffmpeg's loudnorm filter silently reverts to dynamic mode.

But I can check if the message could be improved to calculate that automatically.

[slhck]

I improved the error message to give you a better explanation and a concrete suggestion on what to change:

WARNING: /Users/werner/Downloads/tmp/04 - Tom Of Bedlam.m4a: Linear normalization would result in an estimated true peak of 1.67 dBTP (input true peak 2.86 dBTP + gain of -1.19 dB), which exceeds the target true peak of -2.0 dBTP. The loudnorm filter will likely use dynamic mode instead. To avoid this, raise --true-peak (-tp) to at least 1.7, or lower the target level (-t) to at most -26.7.

This is in a new release already.