11 Replies Latest reply: Mar 22, 2013 2:40 PM by Thien Do
Thien Do Level 1 Level 1 (25 points)

Hello,

 

I'm running into a weird audio synching problem that I haven't seen before.

 

We shot on an Epic at 4K, 24fps. The footages were transcoded to ProRes LT at 2K resolution for offline. The audio was recorded with a combination of boom and lavalier mics. A mix of these tracks were fed into the camera as a guide audio track.

 

What I wind up with are video clips (with stereo audio tracks) synch'ed to 4 mono audio tracks (in .wav format).

 

I'm noticing 2 problems:

 

1. Judging from the slate, the video and the stereo audio tracks (from the audio feed into the camera) are off by 1 frame. How could this have happened? Could it be something that happened in the transcoding? Or maybe there's some kind of latency from the mixer into the camera?

 

2. No matter how I sync the 4 mono tracks to the video (whether by the spike in the guide track's wave form or by the slate), the audio starts to drift. This is very noticeable in takes longer than 2 minutes or so. My audio guy says that this is a known issue with FCP. It has something to do with FCP's internal clock. I've never heard of this before. Has anyone heard of this?

 

I also noticed that the guide audio tracks (from the transcoded clips) are at 48kHz, 16-bit integer, whereas the mono .wav tracks are 48kHz, 24-bit integer. Could this be the cause for the lag? If so, is there a way to correct it?

 

Thanks.

 

Thien


iMac, OS X Mountain Lion (10.8.2), Thunderbolt, FCP7
  • 1. Re: Problem with audio lag
    Jim Cookman Level 7 Level 7 (23,435 points)

    sounds like you have two issues.  First off look up subframe editing here and in the manual.  That might fix the camera audio problem. you would have to perform this at the dailies level.  Don't wait until you have a cut to do this.

     

    Secondly, I'm not sure I understand:  did you also go to an audio recorder?  Did that recorder generate timecode? If not, that might be your problem.  As long as I'm working with double system audio from a recorder that has a timecode generator/reader I've NEVER had a problem in FCP-- been using it since version 1.  The only tracks that drift are those running on a different clock.

     

    As a test, take one of the audio tracks, and resample it to 48k 16 bit and see if that does the trick.

  • 2. Re: Problem with audio lag
    Meg The Dog Level 6 Level 6 (9,665 points)

    Were both the camera and audio recorder set to the same framerate? 24fps, 23.98 fps, 23.976 fps?

     

     

    MtD

  • 3. Re: Problem with audio lag
    Thien Do Level 1 Level 1 (25 points)

    Hi Jim,

     

    Thanks for the reply. Our audio recorder did not generate timecode. Still it seems weird that the audio would lag that much. I don't know the exact amount of lagging but it's very noticeable toward the end of a 2-3min take. Wouldn't a 3min take have the same length of time whether or not you record with timecode?

     

    I resampled an audio track to 48k 16 bit. It didn't make any difference.

  • 4. Re: Problem with audio lag
    Thien Do Level 1 Level 1 (25 points)

    Hi Meg,

     

    We shot at 24fps, but I don't think the audio recorder had a framerate setting.

  • 5. Re: Problem with audio lag
    Michael Grenadier Level 7 Level 7 (20,115 points)

    audio recorders usually do not have a frame rate setting (unless they record timecode) however some people report problems importing audio only files when the easy setup frame rate does not match the frame rate you shot at.

  • 6. Re: Problem with audio lag
    Thien Do Level 1 Level 1 (25 points)

    I didn't see an easy set-up for 2K ProRes LT at 24fps and couldn't figure out how to create one, so I just used the timeline our data wrangler set up, which has a framerate of 24.

  • 7. Re: Problem with audio lag
    Michael Grenadier Level 7 Level 7 (20,115 points)

    First, make sure you shot 24 fps not 24p (23.976 fps).  Once you've determined that, choose any easy setup with the exact same frame rate. 

  • 8. Re: Problem with audio lag
    Jim Cookman Level 7 Level 7 (23,435 points)

    Think about it this way:  you've got two recorders, each with a clock.  One runs just slightly slower or faster than the other.  When you playback a recording made with one of the recorders, ON THAT MACHINE, it sounds great and is the expected length.  However when you play the same recording on the machine that has a different clock, and which clock controls the playback of the recording, it's going to be off.

     

    Am I the only person who understands this?

  • 9. Re: Problem with audio lag
    D Gilmore Level 4 Level 4 (3,525 points)

    Jim Cookman wrote:

    Am I the only person who understands this?

    I understand Jim

  • 10. Re: Problem with audio lag
    Nick Holmes Level 7 Level 7 (29,835 points)

    I understand you both.

     

    Kids these days need to get the heck off my lawn set.

  • 11. Re: Problem with audio lag
    Thien Do Level 1 Level 1 (25 points)

    yes i do understand. thanks.