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• How can I mux a video and audio from separate sources that go out of sync over time ?

  25 août 2017, par Rapta

  I’m having trouble muxing the video from one source with the audio from another as the audio drifts over time. Said sources are at different framerates, the video’s at 23.976, and the audio’s at 29.97.

  My code so far :

  ffmpeg -y -i source1.mkv -itsoffset 0.85 -i source2.mp4 -map 0:0 -map 1:1 -c:v libx264 -crf 23 -c:a aac -b:a 112k output.mp4

  I’m using nightly build 20170724-03a9e6f.

  Cheers for any help.

• How to convert video to gif and mp3 that sync using ffmpeg ?

  12 août 2019, par johan

  I want to convert video to gif with audio. The two should match when played at the same time.

  The command I use somehow generates results that’s a bit off.

  To create gif :

  ffmpeg -ss 00:00:10 -i input.mp4 -t 4 -pix_fmt rgb24 -r 10 -filter:v "scale=-1:300" out.gif

  To create mp3 :

  ffmpeg -ss 00:00:10 -i input.mp4 -t 4 out.mp3

  I’m guessing this has something to do with the slicing.

• ffmpeg audio and video gradually go out of sync when recorded with shell script and make

  6 août 2020, par cgmil

  I wrote the following shell script for making streamcasts for recording streams using ffmpeg :

  


  [ -z $1 ] && { echo "No recording device selected!" 2>&1 && exit ; } || DEV=$1
VIDFILE=video/bvid.mkv
AUDFILE=audio/baud.wav
DIM=$(xdpyinfo | grep dimensions: | awk '{print $2;}')
(sleep 10 && echo "Streaming...") &
ffmpeg -nostdin -loglevel panic -f x11grab -s $DIM -i $DISPLAY.0 -c:v libx264 "$VIDFILE" &
ffmpeg -nostdin -loglevel panic -f alsa -i hw:$DEV -c:a libmp3lame  "$AUDFILE" &
sleep 1
[ -f $VIDFILE ] && (echo "Created video" 1>&2) || (echo "No video!" 1>&2)
sleep 1
[ -f $AUDFILE ] && (echo "Created audio" 1>&2) || (echo "No audio!" 1>&2)

wait
trap "killall background -s 2" SIGINT


  


  This produces a video and audio file that can then be used to make a stream via make using the command make stream. Here is the Makefile (note that there is a 3-minute long audio file and a file with images that is used to make an opening splash screen) :

  


  FULLAUDIO=audio/full.wav
AUDSTART=84
TITLE="Stats Aside:\\nInterpreting\\nProbability"
SIZEW=1920
SIZEH=1080

audio/clip.wav : $(FULLAUDIO)
        ffmpeg -ss $(AUDSTART) -i $< -t 6 -af "afade=t=in:st=0:d=3:curve=qsin,afade=t=out:st=3:d=3:curve=qsin" -c:a libmp3lame $@

img/thumb.png : img/bg.png img/ico.png
        convert -stroke white -fill white -background black -transparent "#000000" \
                 -font "URWBookman-Light" -gravity Center -size 928x1080 \
                label:$(TITLE) png:- | composite -gravity west -geometry +64+0 - \
                img/bg.png $@
        convert img/ico.png -transparent "#ffffff" png:- | composite \
                 -gravity southeast -geometry +32+32 - $@ $@
        convert $@ -resize $(SIZEW)x$(SIZEH) $@

video/intro.mp4 : img/thumb.png audio/clip.wav
        ffmpeg -i audio/clip.wav -loop 1 -t 6 -i img/thumb.png -vf "fade=t=in:st=0:d=1,fade=t=out:st=5:d=1:c=white,scale=$(SIZEW):$(SIZEH)" -c:v libx264 -c:a libmp3lame $@

ProbabilityInterpretation.mp4 : video/bvid.mkv video/intro.mp4 audio/baud.wav
        ffmpeg -i video/intro.mp4 -ss 11 -i video/bvid.mkv \
                 -ss 10 -i audio/baud.wav \
                 -filter_complex \
                "[1]fade=t=in:st=0:d=1:c=white,scale=$(SIZEW):$(SIZEH)[bvid];\
                 [2]afftdn=nr=20[baud];\
                 [0:v][0:a][bvid][baud]concat=n=2:v=1:a=1[v][a]" -map "[v]" -map "[a]" \
                 -c:v libx264 -c:a libmp3lame $@

stream : ProbabilityInterpretation.mp4

clean:
        -rm ProbabilityInterpretation.mp4
        -rm video/intro.mp4
        -rm audio/clip.wav
        -rm img/thumb.png

veryclean:
        make clean
        -rm video/bvid.mkv
        -rm audio/baud.wav



  


  Because of lag between my webcam and what appears on screen, I do intentionally start the video and audio at different times so that they can eventually become in sync. However, the end result is a video that starts with the video and audio synced together, but eventually become out of sync, with the video lagging well behind the audio. Here is a link to the video with the end result ; it's long, but notice that the video starts in sync and ends out-of-sync. I should probably also note that the operating system is living in a virtual machine (VirtualBox).

  


  Why is this happening and what can I do to prevent or correct ?