audio / clock drift

[Olivia Sammons]

Hi Lindsay,

If you have access to it, another option is to use Final Cut Pro to align your video and audio tracks. The latest version has a new feature (Synchronize Clips) which allows you to do this automatically. There are several tutorials online that can show you how to do this. Here’s a quick one: 

https://www.youtube.com/watch?v=j0hyfX-aRys

This can work pretty well, depending on your recording device. If you find that the audio and video tracks are still slightly off (which is often the case for me depending on the camera), you can use Final Cut’s retime feature to sync the clips manually.

To do this, mark the same speech event (such as a clap) on both the audio and video tracks somewhere towards the beginning of the recording. Then adjust the tracks so the markers line up. After that, go to the end of the recording and mark the same speech event on both tracks. You’ll probably have to zoom in quite a bit, but once you do you can see how many frames the tracks are off by. Then select ‘Retime’ and drag the video file from the end until the markers line up.  It may take some practice, but once you get the hang of it, it can be really useful.

Hope that helps!

Best,
Olivia


On Sep 19, 2014, at 4:40 PM, Evan Gardner <evan at whereareyourkeys.org> wrote:

> I am in the full habit of 3 quick claps or finger snaps at the beginning and end of any sessions video or audio. I have found that gives all the participants a firm "go" marker and a "we made it to the end!" marker. 
> 
> I like 3 because then it gives a good visual to look for on the audio strip, and it gives three points to line up or match to a video file and a visual reference if needed. Having just one point is just that much harder.
> 
> I also like to practice having the participants (either informants or students or skit actors) pausing and freezing for one second after the quick 3 snap in. This gives us some room to fade.
> 
> Thanks for breaking down your solution. I'm sure I'll refer to it someday!
> 
> 
> On Sep 19, 2014, at 3:17 PM, Lindsay Marean <lmarean at bensay.org> wrote:
> 
>> 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> 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
>>> 
>> 

Olivia N. Sammons
PhD Candidate
Department of Linguistics
4-32 Assiniboia Hall, University of Alberta
Edmonton, Alberta, Canada
sammons at ualberta.ca

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