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Old December 17th, 2007, 04:37 AM   #1
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avoiding flicker when time-stretching

Hello all...

this is my first post here, so please be gentle with me... As will become immediately clear, I'm very much a hobbyist with limited experience, skills and equipment.

I'm putting together a concert recording that was taped from two angles with a Canon XM1 and a small domestic Sony DV recorder.

I dumped both sources into Vegas alongside a separate soundboard recording of the audio. I then time stretched the audio to match the XM1 recording and then stretched the Sony footage to match the soundboard and XM1 audio. So far so good, seemingly.

I did the multicam editing using Ultimate S3 to create a composite master video track which I then rendered (alongside the audio) to a PAL compliant MPEG. But when I play this file back -either on the PC or after authoring a DVD- the Sony footage is intermitently horrendously flickery. Other times it seems fine.

I'm guessing that this has been caused by the timestretching process but I would have thought that that would lead to a constant flicker, plus the problem does not seem evident (or as pronounced, certainly) when watching the footage prior to rendering to MPEG.

So my question is: if this is a timestretching issue how does one put together this sort of project without introducing such artifacts?

Or is there something else I've done wrong here?

Many thanks if you've made it through to the end of this post, I apologies for any misuse of terminology - I'm far more at home working with audio and am well out of my depth here... Any help would be much appreciated!

Last edited by Yousef Sheikh; December 17th, 2007 at 05:50 AM.
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Old December 17th, 2007, 05:18 AM   #2
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Yousef, first of all, welcome to DV Info.
Since we were all beginners at one point, we're always gentle with newcomers.

I'm curious as to why you felt it necessary to time-stretch the Sony footage to match the audio.
The usual method is to time-stretch any audio to match the video.
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Old December 17th, 2007, 05:48 AM   #3
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Thanks for the reply.

I initially time stretched the soundboard recording to match the footage from the XM1. But when I added in the Sony footage, I had to stretch it to match the synced XM1 and soundboard audio.

So, in effect, I've used the XM1's audio track as the guide that the two other sources have been stretched to fit.

I hope that makes sense - the more I write, the less sense it makes to me...
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Old December 17th, 2007, 06:23 AM   #4
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The way I've done this in the past is to first find a visual reference that was seen by all 3 cameras. This could be a flash going off (keep this in mind for future shoots) or something else that's distinctive, short and easy to sync to.
Once that's done, pick one camera as your master shot and sync your audio to that camera.
As long as it remains in sync for the master (and you didn't pause any of them during the shoot), the other cameras will remain in sync as well.
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Old December 17th, 2007, 06:36 AM   #5
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But isn't this more or less what I've done here? As I understand it, I've used the XM1 footage as the master and then matched up the Sony camera to it using an audio cue (possibly a drum hit, I think).

But by the end of the one hour set, the two cameras drift out of sync with each other - hence the need to time stretch.

Does this mean that something may have gone wrong during the capture process? No dropped frames were reported.

It's similar to what I've come across when trying to combine two digital audio recordings, but I would have thought that with a set rate of frames-per-second, there wouldn't be a problem when matching up video from different cameras.

Another thing that bothers me is that the flicker doesn't seem to be a problem until I convert to MPEG - is this just a less forgiving format?
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Old December 17th, 2007, 07:08 AM   #6
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In theory, the two cameras shouldn't drift as video is referenced to a master oscillator.
However, as you've found out, it seems that it does.
Rather than time stretching, what I'd do is edit the footage from the Sony before I started any editing.
By that, I mean place both cameras on the timeline and check the video sync (not the audio as that's a fixed reference) periodically.
At points where I discovered a drift, I'd do an edit on the Sony footage to compensate.
Hopefully you'll be able to cover any edits by switching to the Canon during these cuts.
I hope this makes sense.

To answer your other question, yes, MPEG is a a less-forgiving format, simply due to the compression.
Have you burned this to a DVD and played it back on a TV yet to see if the flickering is only on your computer monitor?

Last edited by Mike Kujbida; December 17th, 2007 at 07:10 AM. Reason: added questions
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Old December 17th, 2007, 07:22 AM   #7
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Thanks for all your help, Mike.

Looks like just as I thought DVD 1 of this project was finished, I've got to go back in and get my hands dirty again... Still, I'm sure it'll be worth the effort.

I did burn to DVD and checked it on a TV and the problem was much, much worse. Not sure if it was just the screen size difference but what was slightly annoying on the computer monitor became horribly jarring on the TV screen.

As a last resort before re-editing, I've re-checked all my menu options and found that my MPEG render setting had 'variable bit rate' ticked. I can't imagine rendering again at the highest possible constant rate will make much difference, but I'm giving it a try.

Thanks again for your time and advice.
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Old December 18th, 2007, 12:56 AM   #8
 
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Yousef,
First, be sure you're not in an NTSC template. This alone would cause 'flicker."
Second, you might attempt to put the project template in a Progressive mode vs interlaced. There is nothing wrong with deinterlacing the entire project. Since it's a concert, the "Blend" setting is likely the better of the two project settings. Rendering to 25p or 50p might fix your problem. Without seeing footage, it's entirely possible that I'm shooting ducks in the dark, but this often can help deal with problems.
Another problem is it might be frequency of lighting causing the flicker, and without a high enough framerate, it may well be that you'll need to live with the flicker.
As far as the flicker showing up in the MPEG only, I'd suggest you set playback of the preview monitor to Best/Full and render a short section to RAM (SHIFT + B) and see if it shows up there. It may be that it is, but of course, compressing to MPEG will only serve to make it more visible/noticable.
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Old December 18th, 2007, 06:56 AM   #9
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Thanks for that.

As it happens, I have good news and bad...

I imported the Sony footage again and lined it up and although the sync drifts enough to cause a slight doubling of the audio, when the camera's audio is muted (as per my original intention to use the soundboard audio only), there doesn't seem to be enough loss of sync between sound and vision to be noticable. So I didn't timestretch and the flicker disappeared.

The bad news: I burned it to DVD and in my excitement getting finished disc to player, I knocked over my external hard drive and killed it.

So not only do I have to recapture and re-edit everything, but I have a DVD of the original version which the new one will almost ineveitably seem inferior to...

Needless to say, backup lesson well learned here.

But many thanks to both of you for all your advice, it's all been duly noted.
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