It doesn't matter which software is used to convert, FFMPEG or CoolEdit 2000, it makes always the same effect.
The original file is WEBM. When I convert it to WAV PCM or MP3 192k using one of following commands
ffmpeg.exe -i "%1" -acodec libmp3lame -ar 44100 -ac 1 -ab 192k -af "dynaudnorm=f=50:p=0.65" "%1.dan.mp3"
the waveform is then even
If I convert either the original WEBM or WAV PCM or MP3 192k to MP3 64k,
ffmpeg.exe -i "%1" -acodec libmp3lame -ar 44100 -ac 1 -ab 64k -af "dynaudnorm=f=50:p=0.65" "%1.dan.mp3"
waveform becomes this:
It doesn't even matter whether the
dynaudnorm effect is used or it is a simple conversion - the effect is always the same.
Entire file contains a voice stream. The protruding regions contain voice with distortion effect.
Update - the whole story
The original file is a WEBM file inputfile.webm and cannot be opened in my audio editor, so all screenshots are here after conversion.
Duration of the whole audio file is 01:52:08
My audio editor can't handle variable bitrate, so I use constant bitrate.
1. Simple conversions
The result of the following two conversions
ffmpeg -i inputfile.webm outputfile.wav ffmpeg -i inputfile.webm -acodec libmp3lame -ab 192k outputfile_192k_stereo.mp3
looks identical (here is stereo and not 2 files next to each other):
Here, the larger amplitude areas contain undistorted main audio that will be processed normally in the future. The image on the right contains a zoom of such an area.
Lower amplitude areas are from a mixed-in audio source and are already distorted. They do not become distorted during my FFMPEG processing! These are areas that will protrude after FFMPEG processing.
2. Dynamic audio normalization
The target is a 192 kbit/s MP3 file, dynamically normalized to level 0.9. The number of audio channels remains unchanged (2).
ffmpeg -i inputfile.webm -acodec libmp3lame -ab 192k -af "dynaudnorm=f=50:p=0.9" 2outputfile_192k_stereo_dan090.mp3
Everything looks fine, but ... I need a monophonic file!
3. Simple conversion to mono
ffmpeg -i inputfile.webm -acodec libmp3lame -ab 192k -ac 1 outputfile_192k_mono.mp3
Oh no! What ist this?! The waveform becomes clipped.
4. Dynamic audio normalization to mono
192 kbit/s, level 0.9
ffmpeg -i inputfile.webm -acodec libmp3lame -ab 192k -ac 1 -af "dynaudnorm=f=50:p=0.9" 4outputfile_192k_mono_dan090.mp3
It gets even worse. Now even more oscillations are cut. So FFMPEG promises to normalize to level 0.9 but it doesn't. (Note: if we convert a monophonic file to a monophonic file or stereophonic to stereophonic, it does its job fine. This only happens when stereo is downsampled to mono).
5. So, we'll try to change normalization level to 0.65
ffmpeg -i inputfile.webm -acodec libmp3lame -ab 192k -ac 1 -af "dynaudnorm=f=50:p=0.65" 5outputfile_192k_mono_dan065.mp3
Perfect! Is there anything left to be desired? Actually yes. The file with 192 kbit/s rate takes 154 MB of disk space.
6. I'll try to get it down to 51MB using a 64kbps rate - the lowest bitrate at which I haven't noticed a significant drop in quality yet.
ffmpeg -i inputfile.webm -acodec libmp3lame -ab 64k -ac 1 -af "dynaudnorm=f=50:p=0.65" 6outputfile_64k_mono_dan065.mp3
Now the non-distorted areas with main audio still look fine, but distorted areas get protruded.
Although when I zoom the protruded areas in, it is not so bad as I thought (it just not obey the normalization level parameter):
Zoom level 1
Zoom level 2
Zoom level 3
7. Final test
Now I'll take the already normalized MP3 192 kbit/s mono file from step 5:
and just convert it to a 64 kbit/s file without changing anything else:
ffmpeg -i 5outputfile_192k_mono_dan065.mp3 -acodec libmp3lame -ab 64k 7outputfile_192k-to-64k.mp3
Now everything looks as in step 6, including zoomed views.
This very section of original file looks like this:
ffmpeg -i 5outputfile_192k_mono_dan065.mp3 7outputfile.wav
produces a WAV output that looks just like the input file.
- Either FFMPEG is buggy and doesn't do what it promises,
- or I do not understand the FFMPEG parameters right,
- or the produced MP3 is fine and my audio editor cannot cope with all MP3 capabilities.