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  1. Member
    Join Date: May 2014
    Location: Brussels
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    Hi,

    First, let met explain why the "???" in the title of my post.
    I am a real newbie regarding multimedia and don't know the "term" to be used regarding my purpose:

    I am a video in French which is 640x352 pixels, 1072Kbps, 25.000 fps according to MediaInfo. It's an .avi (Xvid).
    I have the same video in English, which is 720p (1280x720), 3124 Kbps, 29.970 fps. It's a .mkv (mpeg4/iso/avc).
    This mkv includes also subtitles. Those are perfectly synced (this is important IMO for the next)


    I found how to extract the audio from the avi (french video) using "Pazera Free Audio Extractor".
    I also found how to add this one as a new track audio into the mkv.

    But the problem is that the avi is 44mm 15s and the mkv is 44mn 30s.


    I may not simply "shift" (with a delay) the french audio track. I did listen the complete audio track and it comes slowly out of sync with the subtitles. Also, it ends exactly 15s before the end of the mkv.

    I would like to "increase the speed" of the video (<= is there a term for that "???").
    Is there a tool (for newbie) that could be asked to change the fps* of the mkv in such a way that it plays in exactly 44mm 15s (like my French audio track) ? I.e.: also compute the new fps.

    * I am guessing this is what must be done for my purpose, right ?

    V.
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  2. Member
    Join Date: May 2014
    Location: Brussels
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    LOL !!! Posting my question I thought that the term I was looking for could be "étirer" in french = Stretch... and I found a solution googling with that keyword

    I can "stretch" easily subtitles and video by a ratio (44mm 15s)/(44mm 30s) (=2556/2672) with mkvmerge (which does also add my audio track into the mkv)

    Sorry for posting too fast :/

    V.
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  3. Member Cornucopia's Avatar
    Join Date: Oct 2001
    Location: Deep in the Heart of Texas
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    No, I follow.
    I recommend that you should NOT stretch the video/subs, you should only stretch the AUDIO. Audio (by the nature of its finer time sampling) allows for more FLUID control of this adjustment, without excessive artifacts. Video would require you to either use a non-standard framerate or to blend/interpolate/duplicate frames, all of which are noticeable.

    This technique falls under various names: time stretch/shrink, time compress/expand, pitch/tempo adjust. Something like Audacity should be able to do it just fine. You want to expand your length by 2672/2655 = ~1.005649. This would expand the length from 2655 seconds to 2672 seconds, making it match your 720p video. Then re-multiplex that into the video (swapping out the English audio for your French).

    Only problem I might see, which may not even be a problem for you (if you are using a computer, or a PAL CE device which allows NTSC playback), would be viewing NTSC framerates on your display.

    Scott
    Last edited by Cornucopia; 1st May 2014 at 17:38.
    "When will the rhetorical questions end?!" - George Carlin
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  4. Member
    Join Date: May 2014
    Location: Brussels
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    The documentation of mkvmergeGui tells that it works best on video and subtitle tracks ?!?
    But I will do a you suggest has I indeed notice some weird "behavior" in the video once stretched (I can't describe but it's kind of temporally out of sync with the audio, from time to time)..

    Thx again !

    V.
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  5. Member Cornucopia's Avatar
    Join Date: Oct 2001
    Location: Deep in the Heart of Texas
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    Yes, since video's timebase is only down to the closest FRAME/FIELD, the increment that one has to work with is 1/50th or 1/60th of a second (or broader), whereas the increment for audio is 1/44100th or 1/48000th of a second. Allows for finer adjustment without noticeable "stairstepping" or "judder". And blending/interpolation is better for the same reason: it is less noticeable to blend and fit in 816,000 new samples into 127,440,000 than it is to modify ~13000 frames out of 80160 (because there is a duration change AND a framerate change).

    Scott
    "When will the rhetorical questions end?!" - George Carlin
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  6. Mod Neophyte Super Moderator redwudz's Avatar
    Join Date: Sep 2002
    Location: AZ, USA
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    vletroye , I changed your title to a more descriptive subject title to allow others to find it in a search for similar topics. Try to choose a subject title that describes your topic.

    Thanks,

    Moderator redwudz

    And welcome to our forums.
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