audio / clock drift

[Lindsay Marean]

Thank you, Doug!

This didn't exactly work, but your overall approach gave the the 
solution I've been looking for.

First, to clarify, all our recordings are digital; even digital devices 
often run at slightly different speeds, according to the Internet.

I also had to reverse the ratio (video/audio) and changed the audio 
speed rather than the video speed, because I want the resulting audio 
track to be in sync with the actual video in ELAN.

One minor complication is that my audio and video files aren't exactly 
the same length; of course I can trim the beginning or end off of either 
file, but for the total duration ratio to work I will need to trim very 
precisely.

The reason it didn't work to use Praat is that I couldn't find any 
Convert menu.  I'm using Praat version 5.3.85.  I found Convert here and 
there a few places, but nowhere with a "Lenthen..." option.

However, the basic idea of figuring out the ratio of length between the 
two files did work.  Here's what I ended up doing (in case anyone else 
also struggles with this):

1)  Open both files in Audacity (the original audio file and the audio 
file extracted from video).
2)  Find a salient sound event as close to the beginning of both 
recordings as possible, and a salient sound event as near the end of 
both.  I'm trying to get into the habit of clapping my hands at the 
beginning and at the end of a session.
3)  Carefully measure the distance between the salient sounds on both 
recordings.
4)  Determine the ratio of audio / video.  Convert to a percent by 
shifting the decimal 2 places to the left.
5)  Under the Effects menu in Audacity, change tempo for the audio track 
by the indicated percent.  If the original audio file is longer, it 
needs to be a negative percent.
6)  Voila!  Both files should be at the same speed now.  Now just align 
them, and I'm good to go!

Thanks, and I hope this helps someone else.

Lindsay

On 9/17/14, 10:20 AM, Doug Whalen wrote:
>   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 
> <mailto: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 <mailto:elf at endangeredlanguagefund.org>
> www.endangeredlanguagefund.org <http://www.endangeredlanguagefund.org>
>

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