ffmpeg "-shortest" yielding a clip longer than audio file

Question

I'm working on adding .mlp audio / TrueHD w/ Atmos content into an .mkv container (certain playback programs require a video track to bitstream audio). I'm synthesizing a color background for the duration of the clip to serve as video track. Strangely, the command below worked for a time, but now I'm getting a clip that is 12 minutes and 2 seconds, 2 minutes longer than the audio file. The command is as follows:

ffmpeg -i test.mlp -f lavfi -i color=c=blue:s=640x480:r=15 -acodec copy -shortest output.mkv

ffprobe on the .mlp audio is as follows:

ffprobe version 2022-01-27-git-3c831847a8-full_build-www.gyan.dev Copyright (c) 2007-2022 the FFmpeg developers
  built with gcc 11.2.0 (Rev7, Built by MSYS2 project)
  configuration: --enable-gpl --enable-version3 --enable-static --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-libdav1d --enable-libdavs2 --enable-libuavs3d --enable-libzvbi --enable-librav1e --enable-libsvtav1 --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxavs2 --enable-libxvid --enable-libaom --enable-libopenjpeg --enable-libvpx --enable-mediafoundation --enable-libass --enable-frei0r --enable-libfreetype --enable-libfribidi --enable-libvidstab --enable-libvmaf --enable-libzimg --enable-amf --enable-cuda-llvm --enable-cuvid --enable-ffnvcodec --enable-nvdec --enable-nvenc --enable-d3d11va --enable-dxva2 --enable-libmfx --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-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      57. 19.100 / 57. 19.100
  libavcodec     59. 20.100 / 59. 20.100
  libavformat    59. 17.101 / 59. 17.101
  libavdevice    59.  5.100 / 59.  5.100
  libavfilter     8. 26.101 /  8. 26.101
  libswscale      6.  5.100 /  6.  5.100
  libswresample   4.  4.100 /  4.  4.100
  libpostproc    56.  4.100 / 56.  4.100
Input #0, truehd, from 'C:\[]\[]\[]\test.mlp':
  Duration: N/A, start: 0.000000, bitrate: N/A
  Stream #0:0: Audio: truehd, 48000 Hz, 7.1, s32 (24 bit)

any idea how to force a true "shortest" output? thanks!

Answer 1

From your ffprobe output:

Input #0, truehd, from 'C:\[]\[]\[]\test.mlp':
  Duration: N/A, start: 0.000000, bitrate: N/A

No duration, no bitrate in your .mlp file. MLP format (container) is raw, it doesn't contain metadata (as a duration and bitrate).

Moreover, -acodec copy in your ffmpeg command simply copies the mlp audio as a stream of bytes, obtaining no info beyond number of bytes.

So FFmpeg has hard time to estimate the duration of your audio

• no direct info about duration,
• no bitrate info for a fair estimation,
• it has some info from your audio stream (as the sample rate and the bit depth),
• it may know the size (in bytes) of your audio stream.

It means that the duration of your audio is guessed roughly enough, which results in such a difference (the -shortest option is based only on such an approximation).

One possibility how to fix it is using the -t option (for total time) instead of -shortest, for example

ffmpeg -i test.mlp -f lavfi -i color=c=blue:s=640x480:r=15 -acodec copy -t "10:02" output.mkv

(you may use apostrophes instead of quotes – it depends on your operating system, or simply express the time in seconds: -t 602).