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• ffmpeg, how to avoid audio drift when muxing audio and video

  1er mai 2023, par Monkeybus

  I use scrcpy to mirror my Android screen to my PC. I have to capture the audio via bluetooth.

  


  The result is two files. One .mkv and one .aac. They are both of equal length, possibly a second or two out.

  


  I mux them together, but the audio always drifts.

  


  I mux them together with the following command, $show is the bash variable for the video file.

  


  ffmpeg -i $show.mkv -ss -00:00:02 -i $show.aac -c:v copy -c:a copy -shortest $show.muxed.mkv


  


  The -ss sets the audio start time. I generally have to play around with this on a vid by vid basis. I can generally match the audio and video perfectly at the beginning of the file, but the audio will always drift over the course of the video.

  


• Avoiding ffmpeg audio drift for live dash output

  22 mai 2021, par mdale

  Have people experienced playback drift with ffmpeg based dash segment generation ? For example :

  


  ffmpeg -threads 2 -re -fflags +genpts -stream_loop -1 -i AVSyncTest.mp4 \
-af "aresample=async=1:min_hard_comp=0.100000:first_pts=0" \
-c:v:0 libx264 \
-pix_fmt:0 yuv420p \
-preset:0 medium \
-a53cc:0 1 \
-nal-hrd:0 cbr \
-x264opts:0 scenecut=-1:rc_lookahead=0 -b:v:0 5000k -bufsize:0 500k \
-force_key_frames:0 "expr:gte(t,n_forced*2)" \
-bf:0 8 \
-r 30 \
-c:a:0 aac  -ar 48000 \
-b:a:1 96k  \
-f tee -map 0:v \
-map 0:a "[f=dash:media_seg_name='chunk-stream_\$RepresentationID\$-\$Number%05d\$.mp4':init_seg_name='init-stream_\$RepresentationID\$.mp4':seg_duration=2:utc_timing_url=https\\\://time.akamai.com\\?iso:remove_at_exit=0:use_timeline=0:http_user_agent=ffmpeg_encoder.0:streaming=1:index_correction=1:timeout=1:dash_segment_type=mp4:method=PUT:http_persistent=1:adaptation_sets='id=0,streams=v\:0 id=1,streams=a' ]http://localhost:5000/out.mpd "


  


  After a bit of playback it starts audio starts to drift. Analysis of the moof (with tools like mp4box) shows 1024 sample size with 93 Moofs with Moof->traf->default_sample_duration=1024 at 48000 sample rate giving segment duration of 1.984 seconds with every 3rd segment or so with 92 Moofs with duration of 1.962 … While each segment starts aligned ( if you reset playback you get alignment) it drifts against continuous playback.

  


  Other examples like Akamai’s public stream. show 94 Moofs or 2.005 duration with every 3rd segment or so at the 1.984 duration. (what one would want)

  


  Akamai’s (presumably ffmepg based) stream is averaging out to the 2s target where the above ffmpeg command is not …

  


• ffmpeg - pts drift after seek with codec copy

  24 mars 2021, par Andrey Rikunov

  There are two ffmpeg commands. First one is used to seek and copy video chunk. Second one is used to transcode video chunk applying select filter for exact frames match.

  


  Here is how :

  


  

  1. ffmpeg -ss  -to  -copyts -i <input /> -map 0:v:0 -c copy chunk.mp4

  


  2. ffmpeg -copyts -i chunk.mp4 -vf 'select=between(pts\,\,)' transcoded_cunk.mp4

  


  


  It works fine most of the times. But for some inputs there is a little pts drift in downloaded chunk so missing frames is possible. In other words pts of the same packets (compared by hash) are shifted by several points (in my case 0,0002 sec) between input and chunked output.

  


  What is the possible reason for such pts drift ?

  


  UPDATE 1 : That's because ffmpeg set timescale=1000 in mvhd atom so edit list media time to start from looses precision. Is it possible to force mvhd timescale ?

  


  UPDATE 2 : It's not possible to change mvhd timescale because ffmpeg uses constant (MOV_TIMESCALE 1000) :

  


  https://github.com/FFmpeg/FFmpeg/blob/82bd02a2c73bb5e6b7cf5e5eba486e279f1a7358/libavformat/movenc.c#L3498

  


  UPDATE 3 : same issue discussed earlier