XPost: alt.comp.software.firefox   
   From: mariasophia@comprehension.com   
      
   Maria Sophia wrote:   
   > The "mp4" seems to be the entire video, but sans sound.   
      
   Apparently what was created were these three files:   
    a. a video-only file (f399.mp4)   
    b. an audio-only file (f251.webm)   
    c. a tiny .temp.webm merge stub   
   But no final merged output, because FFmpeg apparently had failed   
    [Merger] Merging formats into "...webm"   
    ERROR: Postprocessing: Conversion failed!   
      
   The ffmpeg executable has been in the same direcgtory for years.   
      
    C:\> ffmpeg -version   
         ffmpeg version N-86755-g0780ad9 Copyright (c) 2000-2017 the FFmpeg   
   developers   
         built with gcc 7.1.0 (GCC)   
      
   That's an old ffmeg for sure, so I ran this test just now:   
    ffmpeg -i video.f399.mp4 -i audio.f251.webm -c copy out.webm   
   which gave the same failure that the 2017 FFmpeg cannot mux AV1 video into   
   WebM, because at that time WebM only supported VP8/VP9 video, not AV1.   
      
   AV1-in-WebM did not exist in FFmpeg in 2017.   
      
   Possibly YouTube recently switched many formats (especially 1080p+) to   
   AV1-only.   
   So my old FFmpeg cannot mux AV1 -> WebM, so yt-dlp's merge step fails.   
      
   It appears to me that Occam's Razor tells us that yt-dlp is fine.   
   I just need a modern FFMPEG (such as ffmpeg-git-full or ffmpeg-release-full).   
       
      
    C:\> yt-dlp -f "bestvideo[ext=mp4]+bestaudio[ext=m4a]" --merge-output-format   
   mp4 URL   
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