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• ffmpeg, /dev/video0, -f decklink

  20 mars 2019, par Camille Goudeseune

  I’m trying to capture video from a PCI card, the Blackmagic DeckLink Mini Recorder, via ffmpeg, on a headless host running Ubuntu 18.04.2 LTS, hopefully with a command like

  ffmpeg -f decklink -i /dev/video0 ...

  How can I make that work ? I have two obstacles.

  No /dev/video0

  ffmpeg -i /dev/video0 ... fails : /dev/video0: No such device or address.
  v4l2-ctl --list-devices fails with the same error message.

  I built /dev/video0, and it looks okay :

  mknod /dev/video0 c 81 0
  
  chown root.video /dev/video0
  
  chmod g+rw /dev/video0

  To compare this file with a working one, I ran strace cat /dev/video0 on this host, and on another host (Ubuntu 14) with a working /dev/video0. The outputs began to differ here (good, then bad) :

  fstat(1, {st_mode=S_IFREG|0644, st_size=0, ...}) = 0
  
  open("/dev/video0", O_RDONLY)           = 3  
  
  fstat(3, {st_mode=S_IFCHR|0660, st_rdev=makedev(81, 0), ...}) = 0
  
  fadvise64(3, 0, 0, POSIX_FADV_SEQUENTIAL) = 0
  
  ----
  
  
  
  fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 0), ...}) = 0
  
  openat(AT_FDCWD, "/dev/video0", O_RDONLY) = -1 ENXIO (No such device or address)

  So /dev/video0 is broken at a level lower than ffmpeg or v4l2 or even cat.

  On Ubuntu 14, man 8 MAKEDEV suggests that the error message means that "the kernel does not have the driver configured or loaded."

  This Ubuntu 18 host lacks that manpage, but it does have a few /snap/core/*/sbin/MAKEDEV, all the same, so I tried

  /snap/core/6350/sbin/MAKEDEV -n -v video

  It would have created over a hundred devices videoXX, radioXX, vtxXX, vbiXX. Those devices didn’t exist yet, so it seemed harmless to try it.

  rm /dev/video0; /snap/core/6350/sbin/MAKEDEV video

  That rebuilt /dev/video0, but "No such device" remains, from cat or ffmpeg.

  No decklink

  ffmpeg -f decklink ... fails with Unknown input format: 'decklink'.

  Neither black nor deck nor link is mentioned by ffmpeg -devices (fbdev, lavfi, oss, v4l2) and ffmpeg -formats (about 350), either for Ubuntu’s own version 3.4.4-0ubuntu0.18.04.1, or for version N-93330-g7ff89574c7 compiled from source on 2019 Mar 13 :

  git clone https://git.ffmpeg.org/ffmpeg.git ffmpeg
  
  cd ffmpeg
  
  ./configure --enable-nonfree --disable-doc --disable-w32threads --enable-pthreads

  (Although ./configure --help mentions --enable-decklink, using that yielded "ERROR : DeckLinkAPI.h not found." updatedb && locate DeckLinkAPI.h finds no file with that name, either.)

  The DeckLink PCI card is recognized by hwinfo and lspci.

  lsmod reports the loaded modules blackmagic and blackmagic_io.

  Maybe the PCI card is installed ok, but ffmpeg just can’t reach it because I can’t configure it for that.

  Edit : Rebooting didn’t fix anything.

• How To Specify FFMPEG Input Volume For Multple Input Files

  30 septembre 2014, par gbd

  I have 6 similar/separate audio input files feeding into ffmpeg - no video. I’m mixing the 6 input channels down to a stereo output using amix and that works fine. But now I need to change the volume of each individual input channel before the mix down - or maybe as part of the mix down. I’ve looked at and tried aeval (which seems very slow) in the form

   'aeval=val(0)*volChg1:c=same|val(1)*volChg2:c=same|val(2)*volChg3:c=same|val(3)*volChg4:c=same|val(4)*volChg5:c=same|val(5)*volChg6:c=same'

  but that only seems to change the volume of the first channel.

  So the whole input part of my ffmpeg expression looks something like this right now :

  -i inFilePath1 -i inFilePath2 -i inFilePath3 -i inFilePath4 -i inFilePath5 -i inFilePath6 
  
  -filter_complex 'aeval=val(0)*$volChg1:c=same|val(1)*$volChg2:c=same|val(2)*$volChg3:c=same|val(3)*$volChg4:c=same|val(4)*$volChg5:c=same|val(5)*$volChg6:c=same','amix=inputs=6:duration=longest'

  $volChg1-6 are php variables containing the individual volume multipliers - 0.5 for example should halve the volume of that individual channel while 1.0 would leave it unaffected. How do I do this ?

• How to show segment lengths of hls file with ffmpeg

  30 mai 2017, par endorphins

  I’m trying to use ffmpeg to retrieve the segment duration of every segment for every stream of an HLS file. It looks like I can do this if I know all of the names of the individual files in advance and call ffprobe on each one. This seems really tedious and I won’t always know the names of all of the individual files, only the main .m3u8. Is there any way I can get all of this information with just one ffmpeg command ?

  Additionally, is the duration retrieved by ffprobe on a .aac file accurate ? I get the message that it’s "Estimating duration from bitrate, this may be inaccurate".