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Thread: A/V synching

  1. #1
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    A/V synching

    I frequently download retro-music video’s from website’s, and it’s no secret to say that most download’s end up with the audio/video out of sync, while others are perfect.
    Can anyone clarify why some video’s are (presumably) in perfect sync when they get posted onto the web, but end up out of sync when downloaded?

    And does anyone know of a utility which can fix this sync issue?

  2. #2
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    This thread may help with programs.

    cheers, Paul

  3. #3
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    Audio stream sync-ing with video stream is an on going problem for digital formats.
    The first sync method is by film. The sides on the film roll are the audio tracks. Simple and clever automatic sync this way, as the film contains both video (pictures) and audio, run by a common motor mechanism (aka same 'clocking'.)

    There is still a delay by the audio electronics (to the speakers and including the speaker delay. Speaker is a linear motor.)
    Light is light speed. But the audio delay is vastly small compared to human audio perception. Normally, human will not feel it when the delay is 50ms or less. The electronics delay and the speaker mechanical delay together are vastly less than that.

    Fast forward to digital age.
    Digital video and digital audio are progressing in parallel tracks but totally independent. Over time the disconnect becomes wider.

    Time to make 'movie' files.
    All of a sudden, you cannot play the video and audio together and expect them to sync. There is no sync signal in the video for audio to 'clock' to. [Side track: even old OLD audio tape systems has a sync spike embedded in the audio!)

    You have different clocks.
    If you align the two at the start, they start to 'drift' apart. Worse, independent development also creates variable bit rates (variable clock) for both video and audio. So you cannot expect to clock them with a common clock.
    Even if the video has a sync pulse, you still have problems today. Film starts either frame A or frame B first. If start playing frame A first but the audio expects to sync to frame B ... "Houston, we have a problem".
    Then, even if the 2 clocks are about the same speed ...
    Video may finish later/earlier. Even if you sync at the start, out-of-sync is slowly increasing, at the end being worst. As long as the delay is 50ms or more, human hears the out-of-sync. A 2-hour movie is very long. Clock difference accumulates, and plenty of time!

    OK, how about stretch or compress the audio so that it ends at the same time as the video.
    If it is more than 50ms stretch/compress, you can hear the difference:
    woman speaks with lower pitch; man speaks in higher pitch tone, etc. If the audio is variable bit rate, the result is worse.

    [My trick: Copy/paste no-sound noise audio to silent portions to stretch the audio time. Or cut out silent portions. Do not just add total silence. It is unnatural! Audiophile: music must have background noise. Without it, it sounds artificial. Background noise is part of the music! Surely don't go overboard!]

    OK, I finally sync them!
    But your friend plays it through a 3:2-pull down TV!
    3:2 pull-down is needed because film is 24 frames/sec and digital is either 50 or 60 cycle per second. They do not play nice (not dividable by integer). So the TV adds 'pictures / frames' to compensate, aka 3:2 pull-down. Suddenly the video 'clocking' is changed! You great sync effort is out the door!

    Enough mess! Ways to solve it?
    1. You can add a delay in the audio of the movie file.
    2. When you play it back in a soft player, add +delay at the start, or 'add' -delay (play the audio earlier than the video stream).

    It is not 100% effective. At the end of the movie the sync maybe intolerable. Adjust the delay for best result.

    By adding a specific delay to the file itself, rather than through the soft player, you can lesson the sync problem over a wide playback schemes.

    For web broadcast, the Internet delay is large. Add the proper delay to the file makes it seem 'sync' on web broadcast.
    However, when you download the file to your PC and play it, the added delay would be too much. You can go into the file and adjust the delay.
    There are software, codecs, soft players, etc., for that. Too complex to explain it here. Please google it.

    Finally, there is one problem for audiophile.
    The video file stretches the audio (to lower frequencies/bandwidth) because of the dynamic 3:2 pull-down. To correct this, you need to 'reclock' the audio. Install Reclock software. Google Reclock. I bet audiophiles can easily hear the improvement.

    Hope my explanation do not put you to sleep.

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  5. #4
    Super Moderator RetiredGeek's Avatar
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    Excellent explanation!

    I actually understood what you were saying even though I have zero background in A/V.
    May the Forces of good computing be with you!


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