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Old February 10th, 2008, 05:28 PM   #1
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Audio sync problem

Hello everyone! I'm new to this forum, and have done a lot of reading but haven't yet found a resolution to a stubborn stumbling block in my editing job. Any help anyone can offer would be deeply appreciated!! I have 18 hours of footage to capture and edit in a tight timeframe, but this has me stumped and desperate!

Upon capturing, the A/V sync drifts progressively on long clips, no matter what I do... (Long = more than about 15-20min). I'm capturing onto a recently purchased Macbook with plenty of RAM and storage (I'm saving the captured files onto the internal drive), and the software is Final Cut Pro.

Could be a sample rate issue - I'm not sure. I'm capturing footage that was recorded onto mini-DV at a sample rate of 32khz. Even if I want it to be 48, I'm stuck with the master tapes being 32, and FCP and dvd authoring seem to require 48. So I realized I apparently needed to do a sample rate conversion for each clip. I manually tweaked the FCP audio settings to capture at 32, then I tried doing a sample rate conversion of the audio track from 32 into 48, then re-imported the new 48khz audio track into the timeline. Should I have done this? Anyway: same problem - back at square one!!
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Old February 10th, 2008, 05:30 PM   #2
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Oh, and...

Btw, as a resolution to another problem, I also had to manually set FCP to capture the 2nd two tracks rather than the first two, or it wouldn't pick up the audio tracks which were recorded via external mikes on a Canon XL1. (I believe this is also the reason the tapes were recorded at 32 not 48, because of the 2nd two tracks being used to enable use of external mikes - which I think cannot be done at 48 on this camera). Hopefully this isn't throwing anything off? Other clips recorded on the first two tracks are captured fine without any sync issue, but then that may well be because the clips recorded on the first two tracks were recorded at 48khz.

Also, I'm not capturing from the camera the clips were recorded on, as FCP seems to point-blank not recognise the XL1 (no idea why - that's another problem, but one I can live with!) So I'm playing the DV tapes for capturing from a Samsung VP-D371W camcorder I bought especially to use as a deck in this way. I also thought that Final Cut Studio 2 (which includes Final Cut Pro 6, so that's the version I'm using) came with some kind of auto-correct feature to match up any sample rate discrepancies. Anyway, be that as it may, it's not working!! I have no idea why, and have spent already days and days trying different things, to no avail...What could I be doing wrong? I know I must be doing something wrong, but I don't know what it is...
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Old February 10th, 2008, 10:56 PM   #3
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Scroll over in your Browser (I call it a bin) and see if you media file sample rates are matching you sequence sample rate settings. Also check that the frame rates are the same.
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Old February 12th, 2008, 04:54 PM   #4
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frame rate

Brooks - Thanks very much for your suggestions.

Sample rates - I've played around a little more after reading your post, and it seems that possibly I thought it was a sample rate issue when maybe it was a frame rate issue. So I'm beginning to suspect your suggestion was a good lead! But I've not got a definite solution yet:

Frame rates - I checked these, and found the following:
- My sequence in FCP, and DVD authoring settings, and QuickTime Player, all pump out 25 fps for PAL.
- The clips that are in perfect sync (the ones recorded on the camera's first two audio tracks) register a frame rate of either 24.98 or 24.99 fps. This appears to be close enough to 25 that it makes no difference to the viewer at least in my clips of up to an hour.
- In contrast, the clips (the majority) which are out of sync (recorded on the camera's second two audio tracks) register a frame rate of 24.95. For example, in QP the Movie Inspector logs while playing them:

FPS: 24.95.
Playing FPS: 25.

Likewise Final Cut Pro is interpreting them as being 25 and states they have a "vid rate" of 25, yet in reality perhaps they are 24.95...

Could this discrepancy between being 24.95 but being played at 25 account for the desyncronization? Or am I clutching at straws? I wouldn't have thought this small discrepancy could account for it, but maybe it's enough...?

And if so, which part should I tweak to compensate for it? Many thanks again to anyone who has any further suggestions here. . .
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Old February 12th, 2008, 05:33 PM   #5
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I've seen something like this before, but it was the sample rate that was odd.
29.97 media... 29.97 sequence.... and the sample rate which should have been
48K instead was something like 47980... or something really odd.
Shot from a Sony TRV-900 NTSC cam... Only thing I could conclude was... something wrong with camera. I don't know about your situation. What camera are you shooting on?
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Old February 12th, 2008, 06:06 PM   #6
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camera

Thanks for your thoughts.
Hmm... Hope it's not a camera issue!
The shoot was done on a Canon XL1.

I did connect up the tapes directly to a TV, and found there was no sync problem showing up this way. The sync problem appears after capturing onto the computer software.
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Old February 12th, 2008, 06:37 PM   #7
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Quote:
Originally Posted by Simon Rees View Post
I'm capturing footage that was recorded onto mini-DV at a sample rate of 32khz. Even if I want it to be 48, I'm stuck with the master tapes being 32, ...
Recording at 32khz is never a good idea.

But, now that your stuck...you could try Apple's automater to convert it.

http://www.geniusdv.com/weblog/archi...mple_rates.php
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Old February 12th, 2008, 06:45 PM   #8
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ya, that's a good idea.... I'd convert the audio to 48k, and replace the 32k.
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Old February 12th, 2008, 09:02 PM   #9
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Thanks, Jim. It's a good suggestion, but I tried it already... Even after converting the audio sample rate to 48khz then re-importing it back into the timeline, the sync was off the same as prior to doing this. I don't know why. I'll try doing it one more time just in case I performed the operation wrong in some way. But failing that, this is why I was beginning to wonder if something else was afoot, such as the frame rate, camera or I don't know what...

Brooks, regarding the possibility the camera caused a problem, I found mention of something similar at this link which made me wonder if this is at all relevant. However, even if this happens to be the problem (I don't know if it is), the poster Heitzo never actually posted his solution script as far as I can see: http://lists.apple.com/archives/stre.../msg00022.html.

Still desperate to get to the bottom of what's gone wrong...
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Old February 13th, 2008, 08:10 AM   #10
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The mysteries of sync and FCP. I don't have a solution, unfortunately, just another anecdote.

I've shot with a Canon XL2 and double recorded to a Sound Devices 744T. During post, I realized I had the 744T set to 44.1 kHz instead of 48 kHz. A simple sample rate convert and it all worked fine. Held my breath for a long time on that though.

The thought that it might be the camera is provocative. How about the transfer method. I had one of those fairly cheap firewire transfer boxes for analog and firewire transfer. I always had transfer problems. Not sync, mostly what it said were timecode breaks on what were continuous streams of video.

With my new JVC box, so far, none of those problems.

How are you transferring?

Regards,

Ty Ford
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Old February 13th, 2008, 10:20 AM   #11
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Why not try rendering the audio in FCP and not reimporting, or do the sample rate in a different app such as peak which gives you options as to the quality of the conversion. It may be the conversion process which is changing the duration of the audio files.

I would import the video into pro tools and use the elastic audio function in 7.4 to stretch the audio back into sync. That might be an easy option. You can obviously convert the sample rate there too. Yes I know, it's not much help if you don't have pro tools LE.

P.S. for next time the XL1 will record from the input 1 line in phono's with out any problems at line level and at 48k without the front mic, you just select the input 1 which is in the middle between rear and front.
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Old February 13th, 2008, 02:02 PM   #12
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How are you monitoring the audio and viewing the picture? computer monitor?
Computer audio out? FW out?
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Old February 14th, 2008, 07:11 AM   #13
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Thank you all for your suggestions.

Ty Ford, sounds like you had a similar headache at that time till doing the sample rate conversion. I still don't get why converting isn't fixing my problem. I tried again and, same as before, the sync remains out even after converting the audio from 32 to 48. As regards the transferring: I'm doing this via firewire, direct from a camcorder (not the same one that did the recording) onto FCP...

Jimmy, thanks for several helpful tips. Regarding the XL1, that sounds promising for next time so I'll look into that. Where you wrote that the conversion might be changing the audio duration, I didn't quite understand that, because the audio duration is out of sync with the video both before and after conversion and in fact to much the same degree. Audio lags behind video. Why would rendering in FCP or converting with peak make a difference?

I like the 'elastic audio' idea... I don't have pro tools, but I wonder if there's a similar 'elastic audio' function in FCP?

Brooks, as regards how I'm monitoring the audio and picture: So far I've been doing this on the computer monitor. Playing the clips either through FCP or QT, with the audio playing on the inbuilt speakers of the mac...
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Old February 14th, 2008, 07:59 AM   #14
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Quote:
Originally Posted by Simon Rees View Post
I tried again and, same as before, the sync remains out even after converting the audio from 32 to 48. As regards the transferring: I'm doing this via firewire, direct from a camcorder (not the same one that did the recording) onto FCP..
Ahhhh! Can you use the camera that you shot with to transfer?

Regards,

Ty Ford
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Old February 14th, 2008, 09:12 AM   #15
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Yes, exactly. Seen this before more than once. Use the camera that you shot the footage to transfer!!! Right on TY.
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