beware! Quicktime Bug creates 1 frame delay in vid

CDM wrote on 11/14/2007, 8:39 AM

Hi Everyone -
I've always been slightly bothered by the sync in quicktime movies that people give me when working on a film. They output from FCP and I import in Vegas and convert to AVI. It always looked "not quite right" to me.

yesterday I did a TON of tests and discovered that in QT files exported from FCP, there is a repeated frame at Frame 90 in Vegas. So, starting at fame 90, the audio becomes 1 frame ahead of the video.

So, just an FYI for those of you doing ADR, you need to offset your QT audio by 1 frame before you start.

Vegas people say they will hopefully address this problem in a future release.

thanks

Comments

Chienworks wrote on 11/14/2007, 8:56 AM

I suspect it's actually an FCP problem, where FCP is using an incorrect frame rate of 29.97. Vegas uses the correct rate of 29.97002997002997002997... This leads to a mismatched frame about once every 9 hours 11 minutes or so. You're just happening to hit that point about 3 seconds in to the file.

rmack350 wrote on 11/14/2007, 9:06 AM

I remember this being a problem with files output from after FX too, but hadn't heard anything about it for quite a few years.

It's an odd sort of problem where Vegas is being more exact about its math than most other NLEs. Unfortunately, Sony needs to accomodate the other, more sloppy systems.

Rob Mack

rs170a wrote on 11/14/2007, 9:14 AM

I remember that discussion.
The solution was an ingenious app called SpeedMangler

From the ReadMe:
SpeedMangler attempts to mangle the framerate of an AVI file
to the exact framerate of NTSC video, which is actually (30000/1001)fps,
or to any framerate you want.

Some other apps mark their NTSC AVIs as 29.97 even, which causes a
speed mismatch in other apps that use the more precise framerate
calculation and can cause frames to drop or repeat. For example, Adobe's
After Effects writes it's movies as 2997/100, whereas Sonic Foundry's
Vegas captures and operates at 30000/1001.

SpeedMangler needs to be run from a command prompt (Start Menu ->
Programs -> Accessories -> Command Prompt, or use the keyboard
Windows Key+R and type "cmd")

Usage: SpeedMangler.exe [filename] [rate] [scale]

[rate] and [scale] are optional arguments to set the rate and
scale arbitrarily; If not specified, they default to 30000
and 1001 respectively.

NOTE: Altering the framerate with SpeedMangler does not actually
resample the video frames; Drastically changing the framerate will
cause audio in the AVI file to no longer be in sync.

Mike

Coursedesign wrote on 11/14/2007, 9:40 AM

Adobe's After Effects writes it's movies as 2997/100

That was true ye longe time ago, not in recent versions.

Chienworks wrote on 11/14/2007, 9:44 AM

Yep, but a sync error of 1/30th of a second over 9 hours isn't much to worry about. In a typical 2 hour program that's under 1/4 of a frame. Since physical audio/video sync drifts back and forth an average of 1/2 of a frame every frame (think about it!) i would say this is negligible. It's just when the sync suddenly jumps a full frame all at once that it becomes noticeable.

I've suggested that Vegas allow the option of setting an override frame rate that the user can enter in these cases. The reponse was that it would mess with audio sync. OK, so we know that. But why not let us users take the risk when we know it's warranted and workable?