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Old October 13th, 2010, 08:55 AM   #1
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Split-Frame Audio Synchronization

SUMMARY:
When synchronizing DV audio from two clips in a NLE you can only do so in increments of full frames. Sometimes this is not accurate enough. A plug-in or effected is needed to add from zero to 16 milliseconds of delay.

DISCUSSION:
For a multi-cam shoot we place two or more clips on a single timeline and adjust the relative time difference between them such that the audio in each superimposes over the other leaving no echo or other weird sound from slightly mismatched audio. However, the accuracy of the sync can only be adjusted to the frame level. Usually this level of accuracy is sufficient. However, in theory there could still remain up to a half-frame difference between them since NLEs only allow shifting a clip by full frames plus or minus. On occasion when syncing audio on a PPro timeline between multi-cameras I find that I need to shift the audio on a sub-frame level.

Consider a quantitive analysis. At 29.97 fps we have 33.3 milliseconds per frame or 16.68 milliseconds per half-frame. At STP sound travels at 340.29 meters/second. At STP therefore sound travels 5.68 meters or 18.6 feet in the time of a half-frame.

Consider two clips, each taken from a different camera, of the same event using wireless microphones attached to two subjects in the video. The two subjects might be standing within less than a couple of feet of each other but in theory the audio on each clip on the timeline could be separated by up to a half-frame which would create the effect of the two individuals standing 18 feet apart—an annoying but maybe tolerable echo. At 24 fps it would be 23 feet.

Is there a plug-in or audio effect to shift a given audio track from zero to 16 milliseconds or from -8 to +8 milliseconds?
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Old October 13th, 2010, 09:41 AM   #2
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Although only full-frame increments are possible in some time lines, most NLEs have the capacity for subframe adjustments, but they may need to be made in the audio clip window or elsewhere. I have often made subframe adjustments in Final Cut Pro, for example, and these are also possible in Soundtrack Pro, which is part of the Final Cut Suite. You do not need a delay plugin, or any other plugin, to do this. I am not a Premiere user, but I would be very surprised if it does not have the same capability. Have you carefully reviewed the user guide on this point?

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Old October 13th, 2010, 10:16 AM   #3
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Likewise Vegas Pro allows sub-frame movement of audio events on the timeline, for just this purpose. And, yes, absolutely, sub-frame adjustment is needed for best lip sync. I've used it often when doing multicam shoots of musical performers, with a simultaneous 24-track audio recording.

(note that earlier versions required one deselect "quantize to frames", v9 and later does this automatically when only an unlinked audio track is slipped.)

The other approach to achieving this level of lip sync is to genlock the cameras and any audio recorders, but most prosumer cameras don't have this capability. Genlock allows frame-level sync while shooting. Correction for the time delay while sound travels through the air (propagation) might still be needed in the NLE, if you did have subjects eg. 5 meters apart.
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Old October 13th, 2010, 11:04 AM   #4
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Specifically, Vegas' sub-frame level is the project's (global) audio sample-rate setting, usually 48kHz. ( 1/1600 of a frame @ 30fps.)
Additionally, different sample rate clips (& bit depth) can be mis-matched within the same or multiple tracks. For instance, 48k for camera dialog & 44.1k music off a CD.

Vegas Pro 10.0 has just been released. However it still lacks a few audio features of 'Slo-Tools' which I also use on occasion.

Like Seth, I've used Vegas since Ver.1.0 when it was an audio-only 'NLE' designed and sold by Sonic Foundry.
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Old October 14th, 2010, 08:42 AM   #5
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It turns out to be an incredibly simple process. I found that you click on the flyout menu in the upper right corner of the timeline and make an appropriate selection. From that point the audio track can be shifted relative to the video by microseconds.
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Old October 14th, 2010, 10:18 AM   #6
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What NLE are you editing on?
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Old October 14th, 2010, 07:26 PM   #7
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Premiere Pro will also do sub-frame adjustment and has done for years.
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Old October 14th, 2010, 10:43 PM   #8
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Honestly,

I know great boards like this make it tempting to make this the FIRST stop when something gets confusing in our editing lives - but really, Google search lives everywhere and works really well.

i.e. FCP subframe audio editing: Final Cut Pro 1.0: Subframe Audio Editing
Premier Pro subframe audio editing: Adobe Forums: How to edit video to audio with a...
etc.
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