Correcting Audio Drift with sync video clips

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Correcting Audio Drift with sync video clips

Permanent link to this post Posted by jimmar76 » Mon Jul 24, 2017 8:03 pm

I am using OS Sierra 10.12.6
I am looking for ways to correct sync sound audio drift in long video clips ( problem happens when shooting video and audio with DSLR's) without affecting pitch. Are there any features in Audacity that might help with making these corrections? I saw mention of Audacity in this context but I don't see any thing at first glance, when looking at the program that would help anymore than what's built into Final Cut Pro. The only other program I've encountered is Plural Eyes which is expensive.

Thanks,
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Re: Correcting Audio Drift with sync video clips

Permanent link to this post Posted by billw58 » Tue Jul 25, 2017 1:24 am

The Change Tempo might be what you're looking for. It can change the length of a clip without changing its pitch.

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Re: Correcting Audio Drift with sync video clips

Permanent link to this post Posted by Trebor » Tue Jul 25, 2017 1:32 am

jimmar76 wrote:I am looking for ways to correct sync sound audio drift in long video clips ( problem happens when shooting video and audio with DSLR's) without affecting pitch. Are there any features in Audacity that might help with making these corrections?

Audacity can stretch audio using change tempo so the audio has the same run-time as the video.
But that doesn't necessarily mean it will then be in-sync: as the drifting may not occur at a constant-rate.
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Re: Correcting Audio Drift with sync video clips

Permanent link to this post Posted by kozikowski » Tue Jul 25, 2017 1:52 am

I am looking for ways to correct sync sound audio drift in long video clips ... without affecting pitch.

I wouldn't have guessed that. My first choice would be straight speed change and see how it worked out.

It's rough to believe a camera went through the effort to resample a performance to make the pitch correct at at the wrong speed. However, ordinary speed problems have plagued television for decades (said the resident of an NTSC country).

If you shot something on a good quality video camera and a separate recorder and they didn't match later, then the probability is huge that the separate recorder's sample rate was off and the editing system is playing the track at the wrong speed.

If it's two cameras, I bet the video frames don't line up, either.

A sync error that only appears after an hour of playback is not going to appear as a pitch change, but there is one.

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Re: Correcting Audio Drift with sync video clips

Permanent link to this post Posted by steve » Tue Jul 25, 2017 12:34 pm

kozikowski wrote: My first choice would be straight speed change and see how it worked out.

Same here. Recordings from multiple devices will usually drift apart due to the devices running at slightly different speeds. When listening to the recordings together, the timing differences are much more evident than pitch differences. The pitch difference is not likely to be noticeable at all. In my experience it is always a "speed" change affecting both tempo and pitch, even when the pitch change is too small to hear - like playing a tape at the wrong speed, but very much less severe.

Differences in tempo without change in pitch is possible if one of the devices is dropping data (not able to store the data fast enough), but that's a fault with the device, and very likely to be accompanied by clicks or crackles (not mentioned in the original post).
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Re: Correcting Audio Drift with sync video clips

Permanent link to this post Posted by jimmar76 » Wed Jul 26, 2017 4:03 pm

Thanks everyone for your comments and replies.

The audio and video went from a Canon 7d Mk II via HDMI to an external device (Atomos Ninja Star) recorder. This allows ProRes to be recorded. I used an external microphone into the camera. The audio drifts in under 4 minutes and lip sync is lost. I did record some takes in the camera on it's flash drive so I'm going to check to see if there was any drift there. I have a feeling the camera recordings will be okay.

I will try a couple of the suggestions to correct with Audacity.

Ultimately like to know what caused this problem. I suspect it was the external recording device.
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Re: Correcting Audio Drift with sync video clips

Permanent link to this post Posted by jimmar76 » Wed Jul 26, 2017 8:03 pm

kozikowski wrote:If you shot something on a good quality video camera and a separate recorder and they didn't match later, then the probability is huge that the separate recorder's sample rate was off and the editing system is playing the track at the wrong speed.


Footage recorded in the camera is fine; picture and audio in sync. So I agree it is likely the sample rate of the external recorder, picking up the feed from the HDMI out of the camera, may be off.

When you say that the editing system is playing the track at the wrong speed, I believe the fix would be to correct the speed of the playback and make it permanent for the tracks in question.

Thanks
Last edited by steve on Wed Jul 26, 2017 8:11 pm, edited 2 times in total.
Reason: fix quote tags
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Re: Correcting Audio Drift with sync video clips

Permanent link to this post Posted by kozikowski » Thu Jul 27, 2017 4:24 am

Unless told otherwise, a video editor may assume all the cameras and recorders recorded tracks with a 48000 sample rate (for example). If one of the recorders was recording at 48000.01 instead, that track will have a certain percentage of constant timing and pitch error when played at 48000-even.

Once you figure out the percentage or ratio error—and make sure it doesn't drift—then it should be simple to apply Effect > Change Speed to the track you think is wrong. There's lots of options.

Screen Shot 2017-07-26 at 21.13.04.png
Screen Shot 2017-07-26 at 21.13.04.png (39.88 KiB) Viewed 526 times


And Effect > Change Speed is the one least likely to mess up the sound quality.

As a side note. Most editors "know" what 44100 CD quality is and just work around it in the background.

=======================

I can sense a dripping miasma rising up from the distant past. Could this be the difference between straight mechanical film and film for video? They're not the same and they drift off from each other about a frame per minute.

Koz
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Re: Correcting Audio Drift with sync video clips

Permanent link to this post Posted by kozikowski » Thu Jul 27, 2017 4:31 am

video film rate 23.976

Ewwww. I need to go take a shower....

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Re: Correcting Audio Drift with sync video clips

Permanent link to this post Posted by steve » Thu Jul 27, 2017 7:35 am

kozikowski wrote:Once you figure out the percentage or ratio error

Easiest way to do that is to locate a distinct point close to the start of the track, and another close to the end ("Clapper board" claps are ideal), then use the "Start" and "End" times to determine the amount of stretch.
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