Why do so many videos on YouTube have horribly unsynchronized video / audio?
April 17, 2007 3:49 PM   Subscribe

Why do so many videos on YouTube have horribly unsynchronized video / audio?

You know, like the old movie dubbing thing where someone's lips move and then three seconds later you hear what they said.
Someone told me vaguely that it was because the Flash Video player was horrible. Perhaps, but it isn't consistent at all, as some videos are on and others are way off.
I've never posted anything to YouTube. The videos that come from my camera always synch together correctly, and it doesn't require any action on my part.
posted by Bokononist to Computers & Internet (10 answers total) 1 user marked this as a favorite
It may have come from the original recording - keeping audio and video in sync is surprisingly difficult, particularly if you're using poorly written software to take the video off DVD, or to convert to a new file type. Issues with framerates is one cause of the problems - if the conversion involves moving from one framerate to another (say 30fps to 25fps), the conversion software may not correctly "stretch" the audio to reflect the difference.

Read this and you may be more enlightened.
posted by Jimbob at 4:06 PM on April 17, 2007

(Just to expand on this, on how video files work)...

Video files store two completely different types of data. Video, a series of images at, say, 12 or 25 or 29.9997 or 30 images per second. Audio, a series of samples, at 22500 or 44100 or 48000 samples per second. Then (particularly in the case of streaming video) these have to be "muxed" together, so you have a little section of audio data in the file, followed by a little section of the video data, followed by some more audio, followed by some more video.

Changing things - editing the video, cutting sections, adding sections, changing the framerate to suit different playback formats, combined with the fact that the framerate of the video and the samplerate of the audio probably don't divide into each other evenly, means that keeping all this stuff in sync is surprisingly difficult.
posted by Jimbob at 4:11 PM on April 17, 2007

I consistently experience audio/video lag, and the degree of lag is consistent as well—I think it's likely either (a) something in youtube's upload/transcode software or (b) something in client buffering/playback.

I'm favoring (b). I also suspect it's browser/platform dependent, but I haven't tested that yet.
posted by cortex at 4:24 PM on April 17, 2007

It's almost certainly their encoding and not the original source file. I have heard rumors that they use some sort of open source encoding filters, but that is unsubstatiated.

Anyway, I have uploaded the same file to different video sites on multiple occasions and the YouTube version is often out of sync.

The difference is slight, but if you've done editing work before, you'll definitely notice the sync issues. (full disclosure: both of these are self-links and I work for iFilm):



posted by dhammond at 4:36 PM on April 17, 2007

Hey Kevin.

It would have been handy if you had linked to some videos that were out of sync.
posted by aubilenon at 5:19 PM on April 17, 2007

Flash 7 has terrible sync, at least on Linux. Flash 9 is so much better it's like night and day. Try upgrading your player.
posted by DU at 5:21 PM on April 17, 2007

It would have been handy if you had linked to some videos that were out of sync.

The youtube video seems horribly out of sync to me, while the ifilm video isn't.
posted by melvix at 6:00 PM on April 17, 2007

It would have been handy if you had linked to some videos that were out of sync.

Yeah. dhammond did it for me.

Here's another example that isn't as far off as some, but the subject makes it more obvious. Music and sports being the best at making any discrepancy obvious and/or annoying. Actually it seems like conversations and mouths have to be really bad to catch attention.
posted by Bokononist at 6:21 PM on April 17, 2007

I've had this problem occur with the codec. for example, I posted a 30sec video that was encoded with H.264 and after YT had converted it to Flash and uploaded it, it was out of sync by about a second.

I exported the same vdeo again (same framerate) but used Sorenson 3 and it was in sync.

Slightly more than you asked for, I know, but Ken Stone has written a good article on exporting from Final Cut to Youtube:

posted by rocco at 7:01 PM on April 17, 2007

Also check the version of your flash plugin, flash 7 (for linux at least) was nearly never able to get the audio in sync, flash 9 is a lot better.
posted by fvw at 9:09 PM on April 17, 2007

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