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Match external audio to 96fps video?

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Ryan Atkins
Match external audio to 96fps video?
on May 27, 2015 at 5:13:49 pm
Last Edited By Ryan Atkins on May 27, 2015 at 7:32:55 pm

I'll be filming a scene this weekend on the Panasonic GH4 at 96fps. Is there anything special I need to do with audio to have it sync for a 23.98 output?


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Ty Ford
Re: Match external audio to 96fps video?
on May 27, 2015 at 7:48:01 pm

Hello Ryan and welcome to the Cow Audio Forum.

I've always been guided by the "use the same frame rate on the recorder that you'll be using on the editing timeline" argument, but I haven't worked at 96fps so I don't know.

Hopefully others will chime in.

Maybe do a test run in the field to see how well it works????

As you may know,there's no connection between 96 fps and 96 kHz audio sample rate.

Regards,

Ty Ford
Cow Audio Forum Leader

Want better production audio?: Ty Ford's Audio Bootcamp Field Guide
Ty Ford Blog: Ty Ford's Blog


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Ryan Atkins
Re: Match external audio to 96fps video?
on May 27, 2015 at 7:50:35 pm

Thanks. I have not posted much here.

When I've used 60p on the Canon 7D, audio is handled automatically. It's slowed down and synced with video. Wasn't sure how to accomplish via an external recorder - if anything is needed at all.


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Blaise Douros
Re: Match external audio to 96fps video?
on May 27, 2015 at 9:42:18 pm

You'll just have to slow down the audio in post. You could record it at a higher sample rate, but unless you are actually performing a resampling operation on it versus a slowdown, it won't make a difference. Audio isn't recorded in FPS, so there won't be a setting on your recorder that will perfectly match.

The key is that when your clips are synced in your timeline, you need to link them so that the slowdown is performed on both at once. You don't want to try to manually slow the audio down separately from the video--that's a recipe for a major headache. Get it synced, and let your NLE do the rest.


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Ryan Atkins
Re: Match external audio to 96fps video?
on May 27, 2015 at 9:45:25 pm

Got it. Good to know. Thanks a lot!


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Bruce Watson
Re: Match external audio to 96fps video?
on May 27, 2015 at 9:52:44 pm

[Ryan Atkins] "I'll be filming a scene this weekend on the Panasonic GH4 at 96fps. Is there anything special I need to do with audio to have it sync for a 23.98 output?"

No. The audio recorder doesn't have any idea what "frame rate" is, it merely records it's audio at the sample frequency (not at all the same thing as frame rate) and bit depth that you specify.

If for example you record half an hour of video at 96 fps, and at the same time half an hour of audio at 48kHz/24bits, what you'll end up with is half an hour of video and audio. When you take your 96 fps video and play it back at 24 fps, you'll end up with two hours (96 / 24 = 4, and 0.5 x 4 = 2 hours) of video.

Clearly, if you sync them both to a clapper, then start playing both at the sync point, after half and hour you'll run out of audio. What you'll do about that is editor's choice, but one would think you'll be editing down your two hours of video footage to half an hour to match up with the playing time of your audio.

If that doesn't answer your question, I'll need more of an explanation of what you're actually trying to accomplish.


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Ryan Atkins
Re: Match external audio to 96fps video?
on May 27, 2015 at 9:54:46 pm

That answers it perfectly. Thanks for the help.


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Ty Ford
Re: Match external audio to 96fps video?
on May 28, 2015 at 12:15:22 pm

Ryan,

Do you expect to slow down the footage for a special effect?

If so, I'm guessing you audio is going to be whacked.

Do you know how much slow down you will use?

Maybe you could explain what you want to do.

Regards,

Ty Ford
Cow Audio Forum Leader

Want better production audio?: Ty Ford's Audio Bootcamp Field Guide
Ty Ford Blog: Ty Ford's Blog


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Ryan Atkins
Re: Match external audio to 96fps video?
on May 28, 2015 at 12:45:52 pm

I plan on filming at 96fps and later slowing footage/audio down to 24fps (23.98) to get a slow motion effect. This is 25% of 96fps. The camera I'm using will automatically slow footage down internally. I would then want to stretch audio so it matches the length of slowed footage.

To briefly explain the scene: a man is spraying a boy with a water hose, as punishment for the boy. The event is meant to be painful and seemingly never ending. I want footage and sound to be long and drawn out to push this idea.

The way I understand from other explanations is to stretch audio in post, as you don't really have a way to select variable frame rates with audio (because it doesn't display in frames). The movie "Inception" does this effect on the first 'kick' with Leo slowly being dunked in the bathtub.

Would stretching be the best method?


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Ty Ford
Re: Match external audio to 96fps video?
on May 28, 2015 at 12:10:37 pm

HI Bruce,

Audio recorders used to double record audio DO have a frame rate setting. My Sound Devices 664 audio recorder has 24, 25, 29.97 drop, 29.97 non-drop, 30 drop and 30 non-drop.

Regards,

Ty Ford
Cow Audio Forum Leader

Want better production audio?: Ty Ford's Audio Bootcamp Field Guide
Ty Ford Blog: Ty Ford's Blog


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Bruce Watson
Re: Match external audio to 96fps video?
on May 28, 2015 at 12:47:45 pm

This is an SMPTE timecode function? This is just metadata in parallel with the audio for video sync purposes? But does it have anything to do with the actual audio that gets recorded? IOW, does it effect the audio sample rate? I don't think it does, but I'm more than willing to be corrected.


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Ryan Atkins
Re: Match external audio to 96fps video?
on May 28, 2015 at 12:54:22 pm

I'm not really sure. The Canon 7D and other DSLR's that record 60p will also record audio in "real time". When you slow down footage 40% to 24p (23.98), the audio is stretched and matches the video track. In this case, I'll be recording audio externally.


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John Fishback
Re: Match external audio to 96fps video?
on May 30, 2015 at 7:00:00 pm

Slowing/stretching the audio to four times its original length will likely introduce artifacts. You may want to consider augmenting/replacing some of the audio. Think about it from a sound design point of view.


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