This IS the Avid L2: Re: [Avid-L2] importing .wav audio 29.97 into 23.976 project audio drift?

Thursday, October 23, 2014

Re: [Avid-L2] importing .wav audio 29.97 into 23.976 project audio drift?

I think you are thinking of video pulldown when I am talking about audio pull-up and pull-down.

Audio pull-up and pull-down is a configuration of many pro audio recorders. It allows compatibility with 24.0 frame shooting. When film is shot 24.0 fps, audio rolls at 48 kHz. That causes a problem when you go to post, and video gets pulled down to 23.976 fps, which means audio must be pulled down to 47952 Hz. Many DAWs wanted only 48 kHz audio, so location recorders developed pull-up and pull-down for the audio. The audio is recorded in the location recorder at 48048 Hz but marked as 48 kHz in metadata. Then when it's brought in to sync, it's already pulled down, so it can sync with 23.976 accurately. Or something like that. So the sample rate is stamped to play back at a slightly different speed than what it was recorded.

This is all old stuff - most recorders don't need to do this anymore because we have 23.976 fps. This was mostly to combat a 24.0/48kHz shoot going to post at 29.97/47952Hz. But some old farts and novices think they still need to record with pull-up/pull-down technique, so they do, and stick you with it.

On Thu, Oct 23, 2014 at 3:57 PM, bigfish@pacbell.net [Avid-L2] <Avid-L2@yahoogroups.com> wrote:

I doubt they added pull up or down on purpose in the field but who knows. The wave files were imported directly into the 23.976 project and they display 24 frame code even thought the original files have 29.97 drop frame code. I'm beginning to think that what is being seen as drift might be offset but that's just a guess as our sequences seem okay but the editor working off site said "drift". I'm wondering if the reinterpretation of the .wav time code to 24 frame code on import might be causing some offset depending on where the files time code starts. On some clips when I choice 23.98 code when importing there is a 1 frame discrepancy between the (24) code and file start code (30). I'm thinking depending on what the AE used for sync he might have been fooled matching this time code to the video time code. Just a guess. They are also telling me the files were brought in with the polyphonic wav box checked when they aren't poly files. Don't know if that even factors in.

---In avid-l2@yahoogroups.com, <cutandcover@...> wrote :

If there's a drift, they may have recorded audio with pull-up or pull-down. Try bringing the audio files in (import) with the "Do not convert sources with pull-up and down rates" checkbox checked. This will force MC to pull the sample rate to the project rate, speeding up or slowing down the file. It usually works.

On Thu, Oct 23, 2014 at 2:46 PM, John Moore bigfish@... [Avid-L2] <Avid-L2@yahoogroups.com> wrote:

Got a multicamera Arri Alexa shoot done at 23.976 but the audio was recorded at 29.97. The offline editor says the audio is drifting. I'm of the impression that audio depends on samples not frame rate so if they shot 23.976 video and recorded 29.97 audio at 48K if we import the audio directly into the 23.976 project it should come in at 48K and run the same clock time. I have no idea why production did this mismatch in frame rate but that's what we've been handed. Fortunately it's a very simple stage performance with multiple cameras.

What might be happening that would cause a drift of the audio?

John Moore
Barking Trout Productions
Studio City, CA
bigfish@...