[FFmpeg-user] Need help understanding framerate conversion!

[Matthias, Thomas]

This worked!  Thank you very much.

On 1/13/17, 8:15 AM, "ffmpeg-user on behalf of Carles Vila" <ffmpeg-user-bounces at ffmpeg.org on behalf of cvilad at gmail.com> wrote:

    The reason that the resulting video is longer is simple: it plays slower!
    which is what you want (pull-down)
    
    By default, ffmpeg does nothing to the audio, so you must stretch it. The
    most simple solution is to resample it.
    Try this command line below: the -r before the input tells ffmpeg to
    interpret the input as 29,97. Assuming that your source audio is at 48.000
    Hz,  the asetrate sets a "fake"  sampling rate to 47952 (stretches the
    audio longer by the same ratio) and after that aresample resamples back to
    48000 Hz.
    
    ffmpeg -r 30000/1001 -i input_30fps.mov –i initial_audio.wav -af
    asetrate=47952,aresample=48000 -c:v rawvideo -c:a pcm_s16le –pix_fmt
    uyvy422 output_2997fps_withresampledaudio.mov
    
    HTH
    
    On 13 January 2017 at 03:54, Matthias, Thomas <Thomas.Matthias at dolby.com>
    wrote:
    
    > Hi All,
    >
    > I need to mux and then framerate convert a rawvideo .mov file, and a PCM
    > 16bit 48Khz wav file.
    >
    > For example,  a .mov file is exactly 10 seconds long, and a .wav file is
    > also exactly 10 seconds.  The initial .mov file is a 30 frames-per-second,
    > but after muxing with the audio, I need to convert it to 29.97.  The issue
    > I’m seeing is that for some reason, the resulting video file is slightly
    > longer than 10 seconds, and the resulting audio is just shy of 10 seconds.
    > This leads to the audio being out-of-sync with the video by the end of the
    > video.  In reality, I’m performing this task with much longer clips, and so
    > the av-sync issue is much more noticeable, but even at 10 seconds the sync
    > offset is noticeable in a video editor.
    >
    > What am I doing wrong?  I’ve tried separating the muxing and framerate
    > conversion into separate FFmpeg calls, but that doesn’t help. I’ve tried
    > numerous setting combinations, but nothing keeps the audio at the correct
    > length.  I’ve tried using atempo to stretch the audio, but that doesn’t
    > even seem to work.  Here is an example command I’ve tried:
    >
    > ffmpeg –y –i initial_video.mov –i initial_audio.wav –c:v  rawvideo –c:a
    > pcm_s16le –pix_fmt uyvy422 –r 30000/1001 output.mov
    >
    > I’m clearly missing something here, but I have no idea why the audio track
    > would end up shorter (in the 10s example, it’s about 9.98 seconds after),
    > and the video longer (about 10.06).  Thanks!
    >
    > ~Thomas
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