Transcoding to mp2 within an mp4 container will be recognized as mp3

[Olaf Ahrens]

Summary of the bug:
When transcoding to an mp4 container with mp2 audio codec, the audio codec is marked non-conclusively. This results in ffmpeg itself incorrectly recognizing the audio codec in the transcoded video as mp3, while other programs recognize it as the intended mp2:

ffmpeg -i TargetAsMp4.mp4

[...]Stream #0:1[...]Audio: mp3 (mp3float) (mp4a / 0x6134706D)[...]

Mediainfo

Format - MPEG audio
Format version - Version 1
Format profile - Layer 2
Codec ID - mp4a-6b

VLC Media Player

Codec - MPEG Audio layer 1/2 (mpga)

However, if another container format is used, for example mkv, the audio codec will be recognized correctly by all programs:

ffmpeg -i TargetAsMkv.mkv

[...]Stream #0:1[...]Audio: mp2[...]

Mediainfo

Format - MPEG audio
Format version - Version 1
Format profile - layer 2
Codec ID - A_MPEG/L2
Codec ID/hint - MP2

VLC Media Player

Codec - MPEG Audio layer 1/2 (mpga)

What is noticeable is the audio codec ID 0x6134706D shown within the mp4 container, which as far as I know is associated with the aac codec, but neither with mp2 nor with mp3.

How to reproduce:

% ffmpeg -i Source.mp4 -c:v h264 -c:a mp2 TargetAsMp4.mp4
ffmpeg version 2024-03-28-git-5d71f97e0e-full_build-www.gyan.dev Copyright (c) 2000-2024 the FFmpeg developers
  built with gcc 13.2.0 (Rev5, Built by MSYS2 project)
  configuration: --enable-gpl --enable-version3 --enable-static --pkg-config=pkgconf --disable-w32threads --disable-autodetect --enable-fontconfig --enable-iconv --enable-gnutls --enable-libxml2 --enable-gmp --enable-bzlib --enable-lzma --enable-libsnappy --enable-zlib --enable-librist --enable-libsrt --enable-libssh --enable-libzmq --enable-avisynth --enable-libbluray --enable-libcaca --enable-sdl2 --enable-libaribb24 --enable-libaribcaption --enable-libdav1d --enable-libdavs2 --enable-libuavs3d --enable-libzvbi --enable-librav1e --enable-libsvtav1 --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxavs2 --enable-libxvid --enable-libaom --enable-libjxl --enable-libopenjpeg --enable-libvpx --enable-mediafoundation --enable-libass --enable-frei0r --enable-libfreetype --enable-libfribidi --enable-libharfbuzz --enable-liblensfun --enable-libvidstab --enable-libvmaf --enable-libzimg --enable-amf --enable-cuda-llvm --enable-cuvid --enable-dxva2 --enable-d3d11va --enable-d3d12va --enable-ffnvcodec --enable-libvpl --enable-nvdec --enable-nvenc --enable-vaapi --enable-libshaderc --enable-vulkan --enable-libplacebo --enable-opencl --enable-libcdio --enable-libgme --enable-libmodplug --enable-libopenmpt --enable-libopencore-amrwb --enable-libmp3lame --enable-libshine --enable-libtheora --enable-libtwolame --enable-libvo-amrwbenc --enable-libcodec2 --enable-libilbc --enable-libgsm --enable-libopencore-amrnb --enable-libopus --enable-libspeex --enable-libvorbis --enable-ladspa --enable-libbs2b --enable-libflite --enable-libmysofa --enable-librubberband --enable-libsoxr --enable-chromaprint
  libavutil      59. 10.100 / 59. 10.100
  libavcodec     61.  4.100 / 61.  4.100
  libavformat    61.  2.100 / 61.  2.100
  libavdevice    61.  2.100 / 61.  2.100
  libavfilter    10.  2.100 / 10.  2.100
  libswscale      8.  2.100 /  8.  2.100
  libswresample   5.  2.100 /  5.  2.100
  libpostproc    58.  2.100 / 58.  2.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'Source.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    creation_time   : 2024-03-30T09:24:24.000000Z
    encoder         : Lavf60.23.100
  Duration: 00:00:05.75, start: 0.000000, bitrate: 1531 kb/s
  Stream #0:0[0x1](und): Video: h264 (Main) (avc1 / 0x31637661), yuv420p(progressive), 720x406 [SAR 406:405 DAR 16:9], 1373 kb/s, 30 fps, 30 tbr, 15360 tbn (default)
      Metadata:
        creation_time   : 2024-03-30T09:24:24.000000Z
        handler_name    : VideoHandler
        vendor_id       : [0][0][0][0]
        encoder         : Lavc60.41.100 libx264
  Stream #0:1[0x2](eng): Audio: mp3 (mp3float) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 160 kb/s (default)
      Metadata:
        creation_time   : 2024-03-30T09:24:24.000000Z
        handler_name    : SoundHandler
        vendor_id       : [0][0][0][0]
Stream mapping:
  Stream #0:0 -> #0:0 (h264 (native) -> h264 (libx264))
  Stream #0:1 -> #0:1 (mp3 (mp3float) -> mp2 (native))
Press [q] to stop, [?] for help
[libx264 @ 000002ae3902bc00] using SAR=406/405
[libx264 @ 000002ae3902bc00] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX FMA3 BMI2 AVX2
[libx264 @ 000002ae3902bc00] profile High, level 3.0, 4:2:0, 8-bit
[libx264 @ 000002ae3902bc00] 264 - core 164 r3190 7ed753b - H.264/MPEG-4 AVC codec - Copyleft 2003-2024 - http://www.videolan.org/x264.html - options: cabac=1 ref=3 deblock=1:0:0 analyse=0x3:0x113 me=hex subme=7 psy=1 psy_rd=1.00:0.00 mixed_ref=1 me_range=16 chroma_me=1 trellis=1 8x8dct=1 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=-2 threads=13 lookahead_threads=2 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=3 b_pyramid=2 b_adapt=1 b_bias=0 direct=1 weightb=1 open_gop=0 weightp=2 keyint=250 keyint_min=25 scenecut=40 intra_refresh=0 rc_lookahead=40 rc=crf mbtree=1 crf=23.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
Output #0, mp4, to 'TargetAsMp4.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf61.2.100
  Stream #0:0(und): Video: h264 (avc1 / 0x31637661), yuv420p(progressive), 720x406 [SAR 406:405 DAR 16:9], q=2-31, 30 fps, 15360 tbn (default)
      Metadata:
        creation_time   : 2024-03-30T09:24:24.000000Z
        handler_name    : VideoHandler
        vendor_id       : [0][0][0][0]
        encoder         : Lavc61.4.100 libx264
      Side data:
        cpb: bitrate max/min/avg: 0/0/0 buffer size: 0 vbv_delay: N/A
  Stream #0:1(eng): Audio: mp2 (mp4a / 0x6134706D), 44100 Hz, stereo, s16, 384 kb/s (default)
      Metadata:
        creation_time   : 2024-03-30T09:24:24.000000Z
        handler_name    : SoundHandler
        vendor_id       : [0][0][0][0]
        encoder         : Lavc61.4.100 mp2
[out#0/mp4 @ 000002ae39007cc0] video:819KiB audio:271KiB subtitle:0KiB other streams:0KiB global headers:0KiB muxing overhead: 0.608235%
frame=  171 fps= 81 q=-1.0 Lsize=    1097KiB time=00:00:05.63 bitrate=1594.7kbits/s speed=2.66x
[libx264 @ 000002ae3902bc00] frame I:1     Avg QP:24.65  size: 83042
[libx264 @ 000002ae3902bc00] frame P:43    Avg QP:25.31  size: 14442
[libx264 @ 000002ae3902bc00] frame B:127   Avg QP:30.79  size:  1058
[libx264 @ 000002ae3902bc00] consecutive B-frames:  0.6%  0.0%  3.5% 95.9%
[libx264 @ 000002ae3902bc00] mb I  I16..4:  0.7%  5.3% 94.0%
[libx264 @ 000002ae3902bc00] mb P  I16..4:  0.1%  0.4%  0.6%  P16..4: 56.6% 17.3% 18.2%  0.0%  0.0%    skip: 6.9%
[libx264 @ 000002ae3902bc00] mb B  I16..4:  0.0%  0.0%  0.0%  B16..8: 49.2%  4.7%  1.0%  direct: 0.7%  skip:44.4%  L0:44.2% L1:46.1% BI: 9.7%
[libx264 @ 000002ae3902bc00] 8x8 transform intra:14.8% inter:28.0%
[libx264 @ 000002ae3902bc00] coded y,uvDC,uvAC intra: 88.1% 96.4% 80.8% inter: 16.1% 12.5% 2.9%
[libx264 @ 000002ae3902bc00] i16 v,h,dc,p: 43% 10% 38%  9%
[libx264 @ 000002ae3902bc00] i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 37% 15% 18%  3%  4%  4%  4%  4% 10%
[libx264 @ 000002ae3902bc00] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 20% 19% 11%  7%  7%  7%  8%  7% 13%
[libx264 @ 000002ae3902bc00] i8c dc,h,v,p: 44% 25% 19% 12%
[libx264 @ 000002ae3902bc00] Weighted P-Frames: Y:9.3% UV:0.0%
[libx264 @ 000002ae3902bc00] ref P L0: 59.9% 19.3%  7.2% 12.6%  1.1%
[libx264 @ 000002ae3902bc00] ref B L0: 78.0% 19.7%  2.3%
[libx264 @ 000002ae3902bc00] ref B L1: 84.1% 15.9%
[libx264 @ 000002ae3902bc00] kb/s:1176.68

ffmpeg version 2024-03-28-git-5d71f97e0e-full_build-www.gyan.dev
built for Windows 7 ff

OS on which the tests were run: Windows 11 Pro Version 10.0.22631 Build 22631

[Olaf Ahrens]

source video

[Olaf Ahrens]

target video: container mp4, codecs h264 and mp2

[Olaf Ahrens]

target video: container mkv, codecs h264 and mp2

[quinkblack]

It's a known issue, and caused by the specification since it defined the same config for MP2 and MP3. So with an mp4 container, we don't know whether it's MP2 or MP3 until decoding.