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• Will ffmpeg transcoding change the media's duration?

  6 July 2012, by MaiTiano

  For example, I have a example flv media: 1775818137_1775828481_10344.flv. Only audio data has contained in it. (No video data)

  The major properties of this flv are:

  _____________________________________________________________________
  
  Format                                   : Flash Video
  
  
  
  File size                                : 87.4 KiB
  
  
  
  Duration                                 : 494 hours
  
  
  
  Overall bit rate mode                    : CBR
  
  
  
  Overall bit rate                         : 0 bps
  
  
  
  Writing application                      : Lavf53.4.0
  
  
  
  Audio
  
  
  
  Format                                   : MPEG Audio
  
  
  
  Format version                           : Version 1
  
  
  
  Format profile                           : Layer 3
  
  
  
  Mode                                     : Joint stereo
  
  
  
  Mode extension                           : MS Stereo
  
  
  
  Codec ID                                 : 2
  
  
  
  Codec ID/Hint                            : MP3
  
  
  
  Duration                                 : 49hours
  
  
  
  Bit rate mode                            : CBR
  
  
  
  Bit rate                                 : 64.0 Kbps
  
  
  
  Channel(s)                               : 2 channels
  
  
  
  Sampling rate                            : 44.1 KHz
  
  
  
  Compression mode                         : Lossy
  
  
  
  Stream size                              : 13.3 GiB
  
  
  
  _____________________________________________________________________

  Because this is the flv piece generated from the Suse10.0 server, the timestamp may be not correctly wrote into the flv metadata. Therefore, the Duration item listed above is not the real time of it.
  I has record the time in its file name.
  So, the beginning time is 1775818137 ms
  the ending time of this flv is 1775828481 ms
  The actual duation of this flv is 10344 ms which is about 10 seconds.

  **HERE IS MY QUESTION -.- **

  When I use ffmpeg to transcode flv fromat into ts format, like this,

  ./ffmpeg -i 1775818137_1775828481_10344.flv -f mpegts -vn -acodec libfaac -ar 44100 -ab 48k 1775818137_1775828481_10344.ts

  Is there possibility that the duration of final gotten ts file is not equal to the duation of original flv file? In other words, the ts file's duration is not 10344ms.

• Trouble with the ffmpeg -ss flag when capturing one frame from a Macbook iSight webcam

  31 August 2011, by Andy

  For about five years I've used ffmpeg in a shell script to grab one frame from my linux'd-up Macbook's iSight:

  ffmpeg -f video4linux2 -s 640x480 -r 15 -i /dev/video0 -an -vframes 1 -vcodec mjpeg -y -sameq -ss 1.5 snapshot.jpg

  I just upgraded my Ubuntu distro from Lucid to Natty (ffmpeg ver. 0.6.2-4:0.6.2-1ubuntu1).
  Now that syntax turns the iSight on but hangs indefinitely.

  [output snipped, ending with:]
  
  frame=    0 fps=  0 q=0.0 Lsize=      -0kB time=10000000000.00 bitrate=  -0.0kbits/s    
  
  video:0kB audio:0kB global headers:0kB muxing overhead -inf%

  Without the '-ss' flag it seems to successfully grab the first frame and exit immediately - the only difference in output being:

  frame=    1 fps=  0 q=0.0 Lsize=      -0kB time=0.07 bitrate=  -2.6kbits/s    
  
  video:16kB audio:0kB global headers:0kB muxing overhead -100.132730%

  However, the '-ss 1.5' was necessary to delay the frame capture by 1.5 seconds to allow the cam sufficient time to adjust the exposure.

  The -itsoffset flag seemed promising, but doesn't seem to change ffmpeg's behavior (ie doesn't hang, but no delay).

  Any ideas?

• Get TS packets into buffer from libavformat

  31 May 2012, by Nioreh

  I would like to capture video, compress it and mux it as an mpeg2 ts. All of this is quite simple with the ffmpeg libraries, but what I would like to do, instead of writing it to a file, is capture the ts packets in a buffer and using this stream in other ways. Is there a simple way to do this?

  To write the encoded frames to file right now, I use av_interleaved_write_frame(oc, &pkt). OC is an output context with a filename and format defined. pkt is an AVPacket containing the compressed frame. The av_interlaced_write_frame is doing the muxing and writing the file.

  What I would like to ultimately achieve is to write the same ts stream to multiple files, and to start and stop writing the different files independent of each other. Starting a new muxer for each stream seems inefficient, as a TS stream should not need any header or correct ending to be played back correctly (If what I've read is correct)

  I'm kind of lost trying to find good examples in the ffmpeg code, and the libraries are not very well documented. Any input on this subject is appreciated. Thank you!