audio / clock drift

[Doug Whalen]

  Dear Lindsay,
  It seems likely that your video recorder was not digital; hopefully the audio recorder was.  If so, do the following:  Open the two audio files in Praat.  Get the (exact) ratio of the from-audio wav file to the from-video wav file (dur_from_audio/dur_from_video).  Select the from-video sound object.  From the Convert menu, choose "Lengthen (overlap-add)".  This will use the PSOLA algorithm to change the duration but maintain the original F0.  Put your ratio in the "Factor" field and click OK.  This should create a new sound object with the same duration as the from-audio file.  Check to make sure it all seems right.  
  Don't forget to save the new sound object!  It's not a file until you save it.
  Hope this works, Doug DhW


On Sep 15, 2014, at 4:16 PM, Lindsay Marean <lmarean at bensay.org> wrote:

> I'm hoping that someone here on ILAT has run into this problem and can suggest a way to deal with it:
> 
> We're recording fluent speakers talking, with both audio recorders and video recorders.  Recently I recorded a session in which an audio recorder picked up one speaker really well, and a video recorder picked up another speaker really well.  I can use Audacity to combine the audio (one on each stereo channel) into a single WAV file that I can then use with ELAN for transcription.
> 
> The problem is that the two recorders don't record at exactly the same speed.  In a long session, this difference becomes very noticeable - the two tracks may be perfectly synchronized at the beginning, but they will be out of sync by the end.
> 
> Here are a few things I've found on the Internet that I think discuss the same issue: http://www.acoustica.com/mixcraft/v4/help/hs1550.htm and http://forum.audacityteam.org/viewtopic.php?f=46&t=75868.
> 
> I've been trying to use the change tempo function of Audacity to adjust one track, but it's been trial-and-error - change by a small value, see if it works, and then when it doesn't, undo the change, and change by a different small value.  So far my approach is really time-consuming but still not really successful for making a transcribable track.
> 
> Has anyone else dealt with this problem?  How?  Does anyone know of a better way to get both tracks moving at the same speed, beginning to end?
> 
> Thank you!
> Lindsay Marean



Douglas H. Whalen, President 
Endangered Language Fund 
300 George St., Suite 900 
New Haven, CT 06511 
USA 
+1-203-865-6163, ext. 265 (or 234 for Whalen) 
elf at endangeredlanguagefund.org 
 www.endangeredlanguagefund.org

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