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• De-quantising audio with ffmpeg

  11 mai 2016, par Henry W.

  I am using FFmpeg library to decode and (potentially) modify some audio.

  I managed to use the following functions to iterate through all frames of the audio file :

  avformat_open_input // Obtains formatContext
  
  avformat_find_stream_info
  
  av_find_best_stream // The argument AVMEDIA_TYPE_AUDIO is fed in to find the audio stream
  
  avcodec_open2 // Obtains codecContext
  
  av_init_packet
  
  
  
  // The following is used to loop through the frames
  
  av_read_frame
  
  avcodec_decode_audio4

  In the end, I have these three values available on each iteration

  int dataSize; // return value of avcodec_decode_audio4
  
  AVFrame* frame;
  
  AVCodecContext* codecContext; // Codec context of the best stream

  I supposed that a loop like this can be used to iterate over all samples :

  for (int i = 0; i < frame->nb_samples; ++i)
  
  {
  
      // Bytes/Sample is known to be 4
  
      // Extracts audio from Channel 1. There are in total 2 channels.
  
      int* sample = (int*)frame->data[0] + dataSize * i;
  
      // Now *sample is accessible
  
  }

  However, when I plotted the data using gnuplot, I did not get a waveform as expected, and some of the values reached the the limit of 32 bits integers : (The audio stream is not silent in the first few seconds)

  I suppose that some form of quantisation is going on to prevent the data from being interpreted mathematically. What should I do to de-quantise this ?

• How to overlay a png to piped video source with audio mixed in via ffmpeg ?

  5 janvier 2018, par Christopher Stevens

  I’m successfully streaming silent video with music added from my Raspberry Pi (Raspbian) to YouTube via ffmpeg, with the help of this GitHub gist and this post :

  raspivid -o - -t 0 -vf -hf -w 1280 -h 720 -fps 25 -b 4000000 | \
  
  ffmpeg -i music.wav \
  
  -f h264 -i - -vcodec copy -acodec aac -ab 128k -g 50 -strict experimental \
  
  -f flv rtmp://a.rtmp.youtube.com/live2/STREAMKEY

  The last step of my project to add a transparent, full width/height png overlay to the video (1280x720 size in my case). I’ve seen a few related answers such as this one and this one.

  With the added complexity of piping in a camera feed, mixing in an audio source and outputting to a video stream, I haven’t succeeded in adding the image overlay. Where/how would I add a transparent image overlay in the example above ?

• ffmpeg - add 2 seconds pause or silence after an audio clip [closed]

  29 janvier 2023, par Sudesh

  I've got the following audio clip and want to add 2 seconds of pause or silence after it, without compromising audio quality. I tried filter_complex with concat but it is decreasing quality.

  


  -filter_complex \"[0:0] [1:0] concat=n=2:v=0:a=1[out]\"


  


  Do I need to reencode the input audio file & silent file to the same sample rate and channel ?
I tried that too but it is still having quality issues.

  


  These are the clips
Audio Clip - https://d9f35555a8b3e9044c8d-95c21efaab8093d23d4124e599a618ee.ssl.cf5.rackcdn.com/065e8247-3df8-4880-b4c6-7c11a5a6d8fb.mp3

  


  Silence/Pause Clip - https://d9f35555a8b3e9044c8d-95c21efaab8093d23d4124e599a618ee.ssl.cf5.rackcdn.com/silence_2.0_sec_1.mp3

  


  Please help me to get the output without losing quality.