Recherche avancée

Sur d’autres sites (9770)

• 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 ?

• ffmpeg clip audio interval with starting and end time

  27 novembre 2018, par Raymond Huang

  I am trying to clip an MP3 between two starting points, like starting at 10 seconds and ending at 16 seconds (time interval of 6 seconds).

  I am using this command :

  ffmpeg -ss 10 -i input.mp3 -t 6 output.mp3

  The resulting output.mp3 contains the 6 seconds that I specified followed by 8 or 9 seconds of empty audio. Is there something wrong with my command ?

  Edit :

  ffmpeg -ss 10 -t 6 -i input.mp3 output.mp3 says -t is not an input option, keeping it for the next output; consider fixing your command line. and gives me a file that’s got 8 seconds of audio starting from 10s and then some 9 or 10 seconds of silence.

  ffmpeg -ss 10 -to 16 -i input.mp3 output.mp3 produces a file that is twice the length of the original - basically the same audio file repeated again.

  Testing the output :

  I used Quicktime and it has silent audio at the end. The description of the output file in finder says like 14 seconds. When I use VLC, it plays for the correct 6 seconds and stops, even though its duration in the file browser in VLC says 14. My MPlayer doesn’t work properly. I also did the preview audio in Finder, and it plays the 6 seconds properly and then stops. But the round seeker bar of the MP3 didn’t reach the end. And it also says 14 seconds instead of 6.

  My goal is to stream this 6 second file through a REST API to the front end. I want the user to be able to download this file properly. Ideally it won’t have inconsistent metadata (14 seconds instead of 6).

• ffmpeg clip audio interval with starting and end time

  31 mars 2017, par Raymond Huang

  I am trying to clip an MP3 between two starting points, like starting at 10 seconds and ending at 16 seconds (time interval of 6 seconds).

  I am using this command :

  ffmpeg -ss 10 -i input.mp3 -t 6 output.mp3

  The resulting output.mp3 contains the 6 seconds that I specified followed by 8 or 9 seconds of empty audio. Is there something wrong with my command ?

  Edit :

  ffmpeg -ss 10 -t 6 -i input.mp3 output.mp3 says -t is not an input option, keeping it for the next output; consider fixing your command line. and gives me a file that’s got 8 seconds of audio starting from 10s and then some 9 or 10 seconds of silence.

  ffmpeg -ss 10 -to 16 -i input.mp3 output.mp3 produces a file that is twice the length of the original - basically the same audio file repeated again.

  Testing the output :

  I used Quicktime and it has silent audio at the end. The description of the output file in finder says like 14 seconds. When I use VLC, it plays for the correct 6 seconds and stops, even though its duration in the file browser in VLC says 14. My MPlayer doesn’t work properly. I also did the preview audio in Finder, and it plays the 6 seconds properly and then stops. But the round seeker bar of the MP3 didn’t reach the end. And it also says 14 seconds instead of 6.

  My goal is to stream this 6 second file through a REST API to the front end. I want the user to be able to download this file properly. Ideally it won’t have inconsistent metadata (14 seconds instead of 6).