Tuesday, August 24, 2010

What's that clicking? Mismatched audio sample rates

One of the things you're warned about early on in digital audio editing is to be careful about mismatched audio sample rates; it can cause strangeness. Of course, knowing what you're supposed to do, doesn't mean that you always do it...

For example, a recent project in Final Cut Pro, had a nice stirring sound track set against some video that was shot outside in the rain. The rain was noticeably audible in places, which was why I didn't notice - for days - that there was a clicking sound in the music track; I just wrote it off to other sounds in the audio.

But I was finishing up the video and the clicking was starting to annoy me, so I was dropping down the audio level on the ambient track....and the clicking wasn't going away!

That's when I realized that the clicking was in the music track, not the ambient track. And it wasn't part of the original music track either.

I had imported a 44.1 kHz MP3 track into a Project that was set up for 48 kHz sound (as that was the audio format of the audio that the camera captured.) And even though the audio was being exported to a 44.1 kHz MP4 movie, the clicking was in the 44.1 kHz track.

Converting the MP3 track to a 48 kHz AIFF track (using iTunes) solved the problem (see clip below.)

Important lessons learned:
  • Know the sample rates of your audio tracks
  • Convert them all to the same rate prior to omporting them into your movie
  • Repeated listening to your audio track reveals problems you overlook when doing a rush edit!

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