Video/Audio sync issue - sampling rates?

[EliasMooseblaster]

I've noticed, when I try to film and record separately, I end up with a weird syncing issue: I can get the video to sync with the start of the audio track, but they seem to get more out-of-sync as the video goes on.

 

Am I just two or three frames out when I line them up, and that lag snowballs through the video? Or could it be something to do with sampling rates (my phone camera typically does 25fps; DAW is recording at 44.1 or 48 kHz)?

 

Anyone else found this problem?

 

(If it helps diagnose: I record in Ardour or Audacity; audio and video are edited together in KdenLive.)

[BigRedX]

If the audio and video slip more out of sync the longer the video runs, then it will probably be a combination of both frame rate and sampling frequency, as they are completely separate attributes and not dependant on each other.

 

File Info/Properties should show you the frame rate and audio sampling frequency for your video and audio. What platform are you on? Windows? Mac OS? Linux?

 

What are you using to record the video and audio? How are you getting the initial sync point? There's a reason why film makes use a clapper board at the start of each take.

 

IME video tends to default to 48kHz sampling frequency whilst many audio applications use 44.1kHz. So start off by making sure any new recordings are done with everything running at the same sampling and frame rate. For existing recordings you may have to use a conversion tool to get them the same, as many video and audio editors will set the project to the attributes of the first Imported file and then impose those attributes on anything imported subsequently.

 

 

[EliasMooseblaster]

Thank you, this is all useful to know. To answer your questions, all the audio and video processing are being done on Linux (Ubuntu Studio, specifically). Video is typically shot on an Android phone, audio is going through a Focusrite into the DAW.

 

To sync, I usually hit a muted chord at the beginning - something which will go through to the DAW, but also make a loud enough 'click' to show up on the camera's audio track. Makes me wonder if I should do one afterwards as well, to make sure things are in sync at both ends!

 

Popping open a recent video edit, I've checked the sampling frequencies, and both the audio and video tracks are 48kHz...so no problem there. The video has a frame rate of 28.7 fps (as does the project) - should there be a similar metric for the audio?

[BigRedX]

28.7fps is a somewhat unusual frame rate. Can your phone be set to use something more conventional like 25fps or 30fps?

 

Audio generally doesn't have a frame rate until it is linked to a video track when it should take on the frame rate of the video.

 

Check the sampling frequency of the video and audio files before they have been imported into your video editing program. It may be imposing new frequencies on one or the other.

 

Otherwise the only thing I can think of is that the reference clock on one of your recording devices is off. I would tend to suspect the phone over the Focusrite interface.

 

Recording a reference point at both the beginning and end of the video is a good idea. If you could do a 1 minute long clip and let me know how far out the end points are in terms of fractions of a second?

 

Also can you run both audio tracks together in your video editing program and check that they sound in tune with each other?

[Leonard Smalls]

I suspect it may be something to do with the frame rate; as BigRed says, 28.7 is a very non-standard rate.

Can you set the recording video frame rate to be 25fps?

And have you tried putting the audio and video together in Audacity? IIRC there's a video option...

[EliasMooseblaster]

OK, so it looks like the phone camera may be the culprit: I popped open the metadata for the video file, and it looks like it's nominally "supposed to be" 30fps, but the decoders on my computer are reporting this weird 28.7fps. I wonder if this is some kind of space-saving shenanigans in the Google Camera app. Might be time to try some different software which offers more control over the video settings...