AAC length not detected properly

[superbonaci]

Summary of the bug: Audio runtime is 00:13:20.35 but detected as 00:12:36.41
How to reproduce:

% ffplay fools-the-north-wind.aac
ffplay version 6.0 Copyright (c) 2003-2023 the FFmpeg developers
  built with Apple clang version 14.0.3 (clang-1403.0.22.14.1)
  configuration: --prefix=/opt/homebrew/Cellar/ffmpeg/6.0 --enable-shared --enable-pthreads --enable-version3 --cc=clang --host-cflags= --host-ldflags= --enable-ffplay --enable-gnutls --enable-gpl --enable-libaom --enable-libaribb24 --enable-libbluray --enable-libdav1d --enable-libmp3lame --enable-libopus --enable-librav1e --enable-librist --enable-librubberband --enable-libsnappy --enable-libsrt --enable-libsvtav1 --enable-libtesseract --enable-libtheora --enable-libvidstab --enable-libvmaf --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libxvid --enable-lzma --enable-libfontconfig --enable-libfreetype --enable-frei0r --enable-libass --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libspeex --enable-libsoxr --enable-libzmq --enable-libzimg --disable-libjack --disable-indev=jack --enable-videotoolbox --enable-neon
  libavutil      58.  2.100 / 58.  2.100
  libavcodec     60.  3.100 / 60.  3.100
  libavformat    60.  3.100 / 60.  3.100
  libavdevice    60.  1.100 / 60.  1.100
  libavfilter     9.  3.100 /  9.  3.100
  libswscale      7.  1.100 /  7.  1.100
  libswresample   4. 10.100 /  4. 10.100
  libpostproc    57.  1.100 / 57.  1.100
[aac @ 0x12f638ee0] Estimating duration from bitrate, this may be inaccurate
Input #0, aac, from 'fools-the-north-wind.aac':
  Duration: 00:12:36.41, bitrate: 18 kb/s
  Stream #0:0: Audio: aac (LC), 22050 Hz, mono, fltp, 18 kb/s
   1.47 M-A:  0.000 fd=   0 aq=    2KB vq=    0KB sq=    0B f=0/0

[Balling]

Same happens in ffmpeg -i fools-the-north-wind.aac -f null -

so the bug is in main application ffmpeg.

[superbonaci]

In this case ffmpeg/ffplay/ffprobe should not display any duration, since the bitrate is variable and there's no file container, so can't be known until the full file is processed. Issue reported to other projects:

​https://github.com/mpv-player/mpv/issues/11697
​https://code.videolan.org/videolan/vlc/-/issues/28157
​https://github.com/MediaArea/MediaInfo/issues/908

[Balling]

It is not that simple at all. Estimation is required for stuff to work properly. Even just muxing to M2TS. Or how much resources take here and there. Besides it is accurate on CBR, though FFmpeg until recently could not produce true constant bitrate aac.