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• Batch mixing audio, given timestamps. Multiple offsets, only two sounds. How to do it efficiently ?

  3 août 2021, par Evil

  I have two stereo sounds, 1.wav and 2.wav, these sounds are less than 1 second long and list of timestamps (miliseconds from start of recording). Recording of pure video (recording.mp4) is several hours long and there are thousands (20 000 - 30 000) of timestamps per sounds.

  


  I want to convert list of timestamps and sounds into one recording, merging it with video. The part of merging audio with video is easy with ffmpeg, so this is not part of the question.

  


  The list of timestamps is tsv, for example :

  


  


  1201\t1.wav
  
1501\t2.wav
1603\t1.wav
  
and so on, up to 50 000

  


  


  I can convert it to anything, I am generating this file.

  


  I have seen mixing sound with padding and mixing audio to existing video, but I have to batch process a lots of samples, running sox that many times is not feasible. Mere constructing input for ffmpeg or sox is a cumbersome task.

  


  


  sox -M f2.wav f3.wav f1.wav out.wav delay 4 4 8 8 remix 1,3,5 2,4,6
  
(assuming stereo), or

  


  


  


  sox -m f1.wav "|sox f2.wav -p pad 4" "|sox f3.wav -p pad 8" out.wav

  


  


  Cool for three files. Not feasible for 50 000+. First one needs to read file multiple times (even if it is the same one) and remix channels. Second executes 50 000 sox invocations, also reading the same two files (1.wav, 2.wav) over and over.

  


  I do not use any effects on sounds. There is no explicit support in sox to take one input and play it multiple times (echo / echos destroys the material). Also creating padding or delay takes a lot of time. FFMPEG also needs long query to make it happen.

  


  Since muxing two files is easy, I have tried to record two sounds separately, but still it takes a lot of time to process.

  


  Is there simpler / faster way ?

  


  Taking advice from fdcpp, since wav is PCM coded I also consider writing C program to parse it. I will update code, when I am done.
  
This extends question : is there way to encode offsets in wav format ?

  


• How to decode video in ffmpeg in real time ?

  18 juillet 2021, par Guerlando OCs

  Basic ffmpeg decoding consists of avcodec_send_packet to send encoded packets and avcodec_receive_frame to receive the decoded frames.

  


  avcodec_send_packet returns EAGAIN in case it's full. That is, in case no one is calling avcodec_receive_frame.

  


  Suppose I'm reading an mp4 file and rendering on the screen. That is, I want real time playback of the video. One way to do that would be to tell the renderer the exact fps the file has, for example :

  


  If the file should play at 30 fps, then the renderer calls avcodec_receive_frame 30 times per second, and the avcodec_send_packet only sends new packets when it can. If it receives EAGAIN, it waits and sends again in 1 millisecond, instead of discarding the packet.

  


  Is this the right way to do ? Because getting the fps of the file is not trivial. At least I've never seen this in ffmpeg. Maybe a .mp4 file has this information, but what about a raw .h264 file ? VLC player can play raw .h264 in the right time, but I don't know how it does that

  


• Running multiple audio for one Twitch stream ? [closed]

  25 avril 2021, par Mangaku

  Find this : https://blog.twitch.tv/en/2017/10/10/live-video-transmuxing-transcoding-f-fmpeg-vs-twitch-transcoder-part-i-489c1c125f28/
Explaining that we can force our inputs to Twitch regarding quality.

  


  Thing is, can we force a different sound, on different quality.

  


  Like, i'm running a video loop on my stream with an audio loop at the same time. Let say song A.

  


  My question and idea is : can we force ffmpeg to abuse twitch and get :
1080p : video loop with song A
720p : video loop with song B
480p : video loop with song C
etc ??

  


  Guess its an odd question but... =)

  


  Thanks all