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• Find out what device to use for capturing with ffmpeg ?

  14 septembre 2013, par wvxvw

  I'm trying to broadcast my screen and I cannot capture audio. There's one complication however. I don't have pulseaudio, and it simply doesn't work on my system, so installing it is out of question. Below is the command I'm trying :

  ffmpeg -f alsa -i ??? -f x11grab -s 1920x1200 -r 15 -i :0.0 \
  
      -acodec pcm_s16le -vcodec libx264 \
  
      -preset fast -pix_fmt yuv420p -s 1280x800 -threads 0 -f flv "$URL"

  Here's the list of my audio devices :

  $ aplay -L
  
  null
  
      Discard all samples (playback) or generate zero samples (capture)
  
  default:CARD=PCH
  
      HDA Intel PCH, ALC269VB Analog
  
      Default Audio Device
  
  sysdefault:CARD=PCH
  
      HDA Intel PCH, ALC269VB Analog
  
      Default Audio Device
  
  front:CARD=PCH,DEV=0
  
      HDA Intel PCH, ALC269VB Analog
  
      Front speakers
  
  surround40:CARD=PCH,DEV=0
  
      HDA Intel PCH, ALC269VB Analog
  
      4.0 Surround output to Front and Rear speakers
  
  surround41:CARD=PCH,DEV=0
  
      HDA Intel PCH, ALC269VB Analog
  
      4.1 Surround output to Front, Rear and Subwoofer speakers
  
  surround50:CARD=PCH,DEV=0
  
      HDA Intel PCH, ALC269VB Analog
  
      5.0 Surround output to Front, Center and Rear speakers
  
  surround51:CARD=PCH,DEV=0
  
      HDA Intel PCH, ALC269VB Analog
  
      5.1 Surround output to Front, Center, Rear and Subwoofer speakers
  
  surround71:CARD=PCH,DEV=0
  
      HDA Intel PCH, ALC269VB Analog
  
      7.1 Surround output to Front, Center, Side, Rear and Woofer speakers
  
  iec958:CARD=PCH,DEV=0
  
      HDA Intel PCH, ALC269VB Digital
  
      IEC958 (S/PDIF) Digital Audio Output
  
  hdmi:CARD=NVidia,DEV=0
  
      HDA NVidia, HDMI 0
  
      HDMI Audio Output
  
  hdmi:CARD=NVidia,DEV=1
  
      HDA NVidia, HDMI 0
  
      HDMI Audio Output
  
  hdmi:CARD=NVidia,DEV=2
  
      HDA NVidia, HDMI 0
  
      HDMI Audio Output
  
  hdmi:CARD=NVidia,DEV=3
  
      HDA NVidia, HDMI 0
  
      HDMI Audio Output

  I know that ??? should be something like hw:X,Y, but maybe it can be something else. I can't find the corresponding entry in the man page.

  As an aside, it would be great if you can suggest another audio codec. I can't find what are the options, and this particular one isn't compatible with FLV because of too high bitrate.

  PS. This is the error I'm getting :

  Could not write header for output file #0 (incorrect codec parameters ?): Invalid data found when processing input

• How to find the actual size occupied by an AVFrame ?

  23 septembre 2013, par jsp99

  The main task at hand is to find the actual size of a decoded AVFrame.

  Here is some background information about the problem -
  I wrote a program which takes as input a media file and displays all the video frames of the file, on the screen. My Code (written in C using SDL and ffmpeg) worked perfectly with .wmv files. When it came to .mkv files, it failed (SIGSEGV) due to the following reason :

  Example : Suppose I have a decoded frame from my video stream of file (filename.mkv) in AVFrame *pFrame. The frame has a pix_fmt (pFrame->pix_fmt) of yuv420p. It has dimensions 640 * 346. The Y-plane has it's linesize (pFrame->linesize[0]) as 672.

  I think you get the point here. When I use avpicture_get_size() to get the size of the frame, it calculates a wrong size. It takes pix_fmt, width and height as arguments. Nowhere does it consider 672 instead of 640.

  avpicture_get_size() : Calculates the size in bytes that a picture of the given width and height would occupy if stored in the given picture format.

  I got around this problem by adding some yuv420p frame specific code, which can find the size of the frame in situations like above.

  • Can someone explain why are most of the decoded frames (from video streams of most of the formats) this way ? And why is it not this way with the wmv format ?

  But if I have to find the actual size occupied by an AVFrame of another format (Eg: yuv410p), I have to write the format specific code again. Coming to the main problem,

  • How can I find the actual size occupied the decoded AVFrame (size in bytes occupied by the data of the AVFrame) ?

  • I tried using av_image_get_buffer_size(), but I couldn't understand how to use the align parameter. Can someone explain the usage of the parameter align ? Tried using many other functions in pixdesc.c, but I am missing something.

• Find and remove multiple audio tracks in video using ffmpeg

  14 février 2016, par Jaimin Patel

  I have 1 video which has multiple audios in it. There are some dialogues and few different soundtracks. But, when i use ffmpeg -i 1.mp4 it shows only 2 streams.

  Stream #0:0(und) : Video : h264 (High) (avc1 / 0x31637661), yuv420p(tv, bt709), 1920x1080 [SAR 1:1 DAR 16:9], 2964 kb/s, 24 fps, 24 tbr, 90k tbn, 48 tbc (default)

  Stream #0:1(und) : Audio : aac (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 125 kb/s (default)

  Its showing only 1 audio stream, If i remove that stream whole video will be mute. But, i only want to remove particular soundtrack from video, and its not showing separate audio stream for different soundtracks and dialogue used in video.