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• FFMPEG : audio unsynchronized, losing framerate and strange behavior when pavucontrol is open

  10 mars 2021, par Rik Ferreira

  I'm trying to record my desktop screen with internal audio and mic, but I'm noticing some issues with the output, according to this empirical scale :

  


  

  • If I record only the desktop, the output is GOOD.

  


  • If I record the desktop + mic, the output is GOOD.

  


  • If I record the desktop + internal audio only, the output is BAD.

  


  • If I record the desktop + mic + internal audio, the output is BAD.

  


  • If I record the desktop + mic + internal audio + open pavucontrol, the output is GOOD.

  


  


  Good means the two audio sources and the video are acceptably synchronized and bad means not synchronized at all.

  


  I recorded some samples to show what is going on :

  


  

  • Screen + Mic + Internal audio

  


  • Screen + Mic

  


  • Screen + Mic + Internal audio + pavucontrol

  


  


  I'm snapping my fingers near the microphone when the Beep! message appears along with the sound.

  


  I'm using a simple script to play a sound and show a message in the terminal instead of using pavucontrol because it masks the problem.

  


  FFMPEG call

  


  Although the entire script is visible in the videos, here is the ffmpeg call in plain text :

  


  Dual audio

  


  ffmpeg -y \
    -f x11grab -s 1920x1080 -i :0.0 \
    -f pulse -i 0 \
    -f pulse -i 1 \
    -filter_complex amix=inputs=2 \
    output.mp4


  


  Only microphone

  


  ffmpeg -y \
    -f x11grab -s 1920x1080 -i :0.0 \
    -f pulse -i 1 \
    output.mp4


  


  Other info

  


  SO : Arch Linux

  


  When pavucontrol isn't open, pacmd list-source-outputs returns : 0 source output(s) available., but when pavucontrol is open :

  


  pacmd list-source-outputs | grep source

2 source output(s) available.
        source: 0 
                module-stream-restore.id = "source-output-by-application-id:org.PulseAudio.pavucontrol"
        source: 1 
                module-stream-restore.id = "source-output-by-application-id:org.PulseAudio.pavucontrol"


  


  What am I missing ? Why the ffmpeg works fine when pavucontrol is open ?

  


• ffmpeg : recommended bitrate vs resolution [duplicate]

  15 octobre 2016, par Santhosh Yedidi

  This question already has an answer here :

  • FFMPEG sensible defaults
    
    2 answers
    

  I have high resolution video

    Duration: 00:06:28.80, start: 0.000000, bitrate: 15968 kb/s
  
      Stream #0:0(eng): Video: h264 (High) (avc1 / 0x31637661), yuv420p(tv, bt709), 1920x1080 [SAR 1:1 DAR 16:9], 15809 kb/s, 25 fps, 25 tbr, 25k tbn, 50 tbc (default)
  
      Metadata:
  
        creation_time   : 2016-10-11 05:35:02
  
        handler_name    : Alias Data Handler
  
        encoder         : AVC Coding
  
      Stream #0:1(eng): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, mono, fltp, 157 kb/s (default)
  
      Metadata:
  
        creation_time   : 2016-10-11 05:35:02
  
        handler_name    : Alias Data Handler

  Its 6+min video.

  I am ok with resolution of 240p (because i want to send it on whatsapp)

  In order my video to look good quality what is the recommended bitrate for 240p. Also is what is the minimum bitrate below which the chances of pixelating will be there in the video.

  I dont want to go for high bitrate also. Because ultimately i want the size to be not more than 240p.

  I use mpv to see the video. I scale the original video to 240p, the quality after conversion should match the quality visible in mpv. That will give me first hand idea of how good is the conversion.

  I expect a good amount of reduction in size(MB : where original is 740mb) of the file when reduced from 1920x1080 to 240p

  I have found some information regarding this.

  

  How much is this true

• How to record from webcam using ffmpeg with precise time information of when it was taken ?

  27 février 2021, par Trac3rZ

  I need to record from my 2 webcams and output each frame recorded into images. An additional requirement is I need the time when the image is captured (it has to be precise up to around 10 milliseconds)

  


  Things I've tried :

  


  ffmpeg -y -f video4linux2 -r $frame_rate -i /dev/video0 -vframes 100 -strftime 1 cam0_"%H-%M-%S".png &
ffmpeg -y -f video4linux2 -r $frame_rate -i /dev/video1 -vframes 100 -strftime 1 cam1_"%H-%M-%S".png


  


  This is only good up to a second, so if 2 images are taken within 1 second, 1 is overwritten.

  


  for i in [1..100]
    ffmpeg -y -f video4linux2 -r $frame_rate -i /dev/video0 -vframes 1 -strftime 1 cam0_%5d.png
    date +%s%3N >> time.txt


  


  Then 2 scripts are run in parallel. The timing is now precise up to a millisecond but this degrades the performance a lot since ffmpeg has to be initialized again each time I call the script.

  


  I also tried this

  


  ffmpeg -f video4linux2 -pixel_format yuv420p -timestamps abs -copyts \
   -i /dev/video0 -vf setpts=PTS-STARTPTS -vsync 0 -vframes 10 \
   camera0-%5d.jpeg -c copy -vsync 0 -vframes 10 -f mkvtimestamp_v2 timings.txt


  


  And here's timings.txt

  


  1614427227518
1614427227718
1614427227818
1614427227918
1614427228018
1614427228118
1614427228218
1614427228318
1614427228418
1614427228518


  


  This is good up to 100ms.

  


  Is there any better way to do this ?