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• Poweramp Equalizer : how they intercept and process the audio coming from another app ?

  25 mai 2021, par Fabio Fracassi

  I'm spending a lot of time trying to understand how Poweramp Equalizer works. Seems that they have a service that intercepts the audio using the audio session ID and after, they process the audio using the FFmpeg.
My question is : how is possible to intercept and substitute the audio streamed by another app knowing the audio session ID ?

  


  Legenda : They -> Maxim Petrov :-)

  


• Is there a way to get FFprobe to return codec_name from firs first video, audio, and subtitle stream in a video in a single call to FFprobe ?

  26 août 2023, par Chase Westlye

  ffprobe -v error -select_streams v:0 -show_entries stream=codec_name -of default=noprint_wrappers=1:nokey=1 (filename) 


  


  is how to retrieve the codec_name of the first video stream in a file via FFprobe.

  


  Replace v:0 with a:0 to get the first audio stream, and it returns the codec_name of the first audio stream in the file.

  


  And replace v:0 with s:0 to get the codec_name of the first subtitle stream in the file.

  


  Awesome.

  


  What isn't awesome, is having to call FFprobe three times for these operations and having to hit the file three times.

  


  Is there a way to call FFProbe once, and have FFProbe return the codec_name of the first video, audio, and subtitle stream (instead of separate calls) ?

  


• Scene detection and concat makes my video longer (FFMPEG)

  12 avril 2019, par araujo

  I’m encoding videos by scenes. At this moment I got two solutions in order to do so. The first one is using a Python application which gives me a list of frames that represent scenes. Like this :

  285
  
  378
  
  553
  
  1145
  
  ...

  The first scene begins from the frame 1 to 285, the second from 285 to 378 and so on. So, I made a bash script which encodes all this scenes. Basically what it does is to take the current and previous frames, then convert them to time and finally run the ffmpeg command :

  begin=$(awk 'BEGIN{ print "'$previous'"/"'24'" }') 
  
  end=$(awk 'BEGIN{ print "'$current'"/"'24'" }') 
  
  time=$(awk 'BEGIN{ print "'$end'"-"'$begin'" }') 
  
  
  
  ffmpeg -i $video -r 24 -c:v libx265  -f mp4 -c:a aac -strict experimental -b:v 1.5M -ss $begin -t $time "output$count.mp4" -nostdin

  This works perfect. The second method is using ffmpeg itself. I run this commands and gives me a list of times. Like this :

  15.75
  
  23.0417
  
  56.0833
  
  71.2917
  
  ...

  Again I made a bash script that encodes all these times. In this case I don’t have to convert to times because what I got are times :

  time=$(awk 'BEGIN{ print "'$current'"-"'$previous'" }') 
  
  ffmpeg -i $video -r 24 -c:v libx265  -f mp4 -c:a aac -strict experimental -b:v 1.5M -ss $previous -t $time "output$count.mp4" -nostdin

  After all this explained it comes the problem. Once all the scenes are encoded I need to concat them and for that what I do is to create a list with the video names and then run the ffmpeg command.

  list.txt

  file 'output1.mp4'
  
  file 'output2.mp4'
  
  file 'output3.mp4'
  
  file 'output4.mp4'

  command :

  ffmpeg -f concat -i list.txt -c copy big_buck_bunny.mp4

  The problem is that the "concated" video is longer than the original by 2.11 seconds. The original one lasts 596.45 seconds and the encoded lasts 598.56. I added up every video duration and I got 598.56. So, I think the problem is in the encoding process. Both videos have the same frames number. My goal is to get metrics about the encoding process, when I run VQMT to get the PSNR and SSIM I get weird results, I think is for this problem.

  By the way, I’m using the big_buck_bunny video.