[ffmpeg] Output out of sync when speeding up

[yetanotherlogin]

Hello,

For some reason the following command works fine to speed up some videos while, with other files, video and audio are out of sync.

#+20%
ffmpeg.exe -i input.mp4 -vf "setpts=(PTS-STARTPTS)/1.2" -af atempo=1.2 -c:a aac -b:a 64k -ac 2 -ar 44100 -threads 2 -f mp4 output.mp4

Any idea why?

Thank you.

PS:

ffmpeg version N-109165-g9f5a9a7499-20221122 Copyright (c) 2000-2022 the FFmpeg developers
  built with gcc 12.2.0 (crosstool-NG 1.25.0.90_cf9beb1)
  configuration: --prefix=/ffbuild/prefix --pkg-config-flags=--static --pkg-config=pkg-config --cross-prefix=i686-w64-mingw32- --arch=i686 --target-os=mingw32 --enable-gpl --enable-version3 --disable-debug --disable-w32threads --enable-pthreads --enable-iconv--enable-libxml2 --enable-zlib --enable-libfreetype --enable-libfribidi --enable-gmp --enable-lzma --enable-fontconfig --enable-libvorbis --enable-opencl --disable-libpulse --enable-libvmaf --disable-libxcb --disable-xlib --enable-amf --enable-libaom --enable-libaribb24 --enable-avisynth --enable-chromaprint --enable-libdav1d --disable-libdavs2 --disable-libfdk-aac --enable-ffnvcodec --enable-cuda-llvm --enable-frei0r --enable-libgme --enable-libkvazaar --enable-libass --enable-libbluray --enable-libjxl --enable-libmp3lame --enable-libopus --enable-librist --enable-libssh --enable-libtheora --enable-libvpx --enable-libwebp --enable-lv2 --disable-libmfx --enable-libvpl --enable-openal --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenh264 --enable-libopenjpeg --enable-libopenmpt --disable-librav1e --enable-librubberband --enable-schannel --enable-sdl2 --enable-libsoxr --enable-libsrt --disable-libsvtav1 --enable-libtwolame --disable-libuavs3d --disable-libdrm --disable-vaapi --enable-libvidstab --enable-vulkan --enable-libshaderc --enable-libplacebo --enable-libx264 --enable-libx265 --disable-libxavs2 --enable-libxvid --enable-libzimg --enable-libzvbi --extra-cflags=-DLIBTWOLAME_STATIC --extra-cxxflags= --extra-ldflags=-pthread --extra-ldexeflags= --extra-libs=-lgomp --extra-version=20221122
  libavutil      57. 43.100 / 57. 43.100
  libavcodec     59. 54.100 / 59. 54.100
  libavformat    59. 34.101 / 59. 34.101
  libavdevice    59.  8.101 / 59.  8.101
  libavfilter     8. 50.101 /  8. 50.101
  libswscale      6.  8.112 /  6.  8.112
  libswresample   4.  9.100 /  4.  9.100
  libpostproc    56.  7.100 / 56.  7.100
Hyper fast Audio and Video encoder

[ProWo]

Retry with

ffmpeg.exe -i input.mp4 -vf "setpts=(PTS-STARTPTS)/1.2" -af atempo=1.2 -threads 2 -f mp4 output.mp4

[yetanotherlogin]

No change. Still out of sync.

But then, why would removing that part make a difference?

-c:a aac -b:a 64k -ac 2 -ar 44100