ffmpeg doesn't output lame vbr header correctly

[spotter]

Summary of the bug: lame includes the xing header that defines the VBR quality level that was used. ffmpeg inserts the header, but seems to include incorrect values.

one can see this by inspecting the file with mediainfo a file encoded with lame -V0 shows

Audio
Format : MPEG Audio
Format version : Version 1
Format profile : Layer 3
Format settings : Joint stereo
Duration : 3 min 56 s
Bit rate mode : Variable
Bit rate : 280 kb/s
Minimum bit rate : 32.0 kb/s
Channel(s) : 2 channels
Sampling rate : 44.1 kHz
Frame rate : 38.281 FPS (1152 SPF)
Compression mode : Lossy
Stream size : 7.89 MiB (100%)
Writing library : LAME3.100
Encoding settings : -m j -V 0 -q 0 -lowpass 22.1 --vbr-new -b 32

while a file encoded with ffmpeg -q:a 0 (per ​https://trac.ffmpeg.org/wiki/Encode/MP3)

shows

Format : MPEG Audio
Format version : Version 1
Format profile : Layer 3
Format settings : Joint stereo
Duration : 3 min 56 s
Bit rate mode : Variable
Bit rate : 280 kb/s
Channel(s) : 2 channels
Sampling rate : 44.1 kHz
Frame rate : 38.281 FPS (1152 SPF)
Compression mode : Lossy
Stream size : 7.89 MiB (100%)
Writing library : LAME3.100

so first thing we note is data seems to be the same, but ffmpeg doesn't include the Encoder settings. so what happens if we use ffmpeg to copy the lame encoded file (above) into an mp3 (in this case, getting the id3 tags from another file)

then we see

Audio
Format : MPEG Audio
Format version : Version 1
Format profile : Layer 3
Format settings : Joint stereo
Duration : 3 min 56 s
Bit rate mode : Variable
Bit rate : 280 kb/s
Channel(s) : 2 channels
Sampling rate : 44.1 kHz
Frame rate : 38.281 FPS (1152 SPF)
Compression mode : Lossy
Stream size : 7.89 MiB (100%)
Writing library : LAME3.100
Encoding settings : -m m -V 10 -q 0

that's weird. it now output the encoding settings, but messed with them.

inspecting the all 3 files with Mpeg Audio Info (which can decode the xing header, haven't found a linux utility that can do it, but it runs in wine without a problem, ​https://www.codeproject.com/Articles/8295/MPEG-Audio-Frame-Header ). It shows the lame encoded file as having quality 100, but the ffmpeg copied and ffmpeg encoded files as having Quality 0.

version of ffmpeg: ffmpeg version 4.2.2-1ubuntu1

[Carl Eugen Hoyos]

Please test current FFmpeg git head and provide the command line you tested together with the complete, uncut console output to make this a valid ticket.

[spotter]

built with ./configure --enable-libmp3lame && make (configure lists mp3 as a decoder, but not as encoder)

ffmpeg version N-98341-gcca982ee01

./ffmpeg -i input.flac -codec:a libmp3lame -q:a 0 /tmp/test2.mp3

while they don't return the same exact bits (md5sums differ), the ffmpeg files are the same exact sizes and it has the same xing header issue.

[spotter]

./ffmpeg -i input.flac -codec:a libmp3lame -q:a 0 /tmp/test2.mp3
ffmpeg version N-98341-gcca982ee01 Copyright (c) 2000-2020 the FFmpeg developers
  built with gcc 9 (Ubuntu 9.3.0-10ubuntu2)
  configuration: --enable-libmp3lame --enable-encoder=mp3
  libavutil      56. 55.100 / 56. 55.100
  libavcodec     58. 93.100 / 58. 93.100
  libavformat    58. 47.100 / 58. 47.100
  libavdevice    58. 11.100 / 58. 11.100
  libavfilter     7. 86.100 /  7. 86.100
  libswscale      5.  8.100 /  5.  8.100
  libswresample   3.  8.100 /  3.  8.100
Input #0, flac, from 'input.flac':
  Metadata:
    disc            : 1
    TOTALDISCS      : 2
    TOTALTRACKS     : 18
  Duration: 00:03:56.29, start: 0.000000, bitrate: 977 kb/s
    Stream #0:0: Audio: flac, 44100 Hz, stereo, s16
File '/tmp/test2.mp3' already exists. Overwrite? [y/N] y
Stream mapping:
  Stream #0:0 -> #0:0 (flac (native) -> mp3 (libmp3lame))
Press [q] to stop, [?] for help
Output #0, mp3, to '/tmp/test2.mp3':
  Metadata:
    TPOS            : 1
    TOTALDISCS      : 2
    TOTALTRACKS     : 18
    TSSE            : Lavf58.47.100
    Stream #0:0: Audio: mp3 (libmp3lame), 44100 Hz, stereo, s16p
    Metadata:
      encoder         : Lavc58.93.100 libmp3lame
size=    8084kB time=00:03:56.30 bitrate= 280.3kbits/s speed=41.6x    
video:0kB audio:8084kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.005883%