Jonathan,
Thank you for taking the time to write out a detailed response. My responses
are below.
scott
>>I'll have to take your word for it; the waveforms as presented are rather
>>difficult to use to verify sync.
The drummer's snare hit should be clear enough to see.
>>Hmm, I assume you mean 48 kHz.
Sorry..too long working with CDs (44.1kHz)
>>One question before we start. Was your multitrack and camera synced to the
>>same clock during the recording session (be it word clock, smtpe etc)?
No.
>>the end. The only way to resolve this with unlocked sources is to
>>resample the audio from the multitrack in such a way that you correct for
>>the subtle clock differences of the gear used.
Wouldn't this difference show up in Cinelerra?
>> http://www.physics.adelaide.edu.au/~jwoithe/audiosync-0.4.tgz
I will try your program out..definetly. Thank you for the detailed explanation!
>>As far as the software you need to work out the sample number of these
>>events I use audacity.
That is a good idea..I will load up both audio files and see about syncing them
there.
>>This seems rather strange. If there's a sync problem you should hear it in
>>cinelerra unless for some odd reason the tracks aren't always being mixed.
>>I must admit that I do *all* my audio mixing in ardour (I export the camera
>>audio to WAV, import that to ardour and then sync off that), so have no
>>experience doing multitrack audio mixes in cinelerra.
Strange indeed..I would think we should hear it in Cin as well.
>>>> sync.mov: Xine shows sync problems, mplayer no problems
>>>> sync.mpg: Xine shows no sync problems, mplayer shows problems
>>That sounds bizarre. While you can get sync problems in certain situations
>>with different AV container formats this would cause the entire audio track
>>to drift out of sync. As I understand it from your opening statements,
>>"sync problem" here refers to an audible echo indicating one source track
>>being out of sync with the other.
I should clarify that when I say sync, I mean the synchronization between the
audio from the multitrack recording to the video. In my case, the sync looks
good in Cinelerra, but not in the rendered output.
I will let you know how I fare after work tonight.
scott
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> I'm having a problem with the audio sync on a rendered video. This is a
> video of a rock band playing. I have used the original audio from the
> cam (720P HDV, MPEGTS, 48.1khz audio) to sync up a multi-track recording
> of the same session (WAV, 48.1khz audio). When reviewing the audio
> waveform in Cinelerra, the new and old audio tracks align perfectly:
> http://content.serveftp.net/video/sync_audioalignment.png
I'll have to take your word for it; the waveforms as presented are rather
difficult to use to verify sync. The other thing to note that at least at
the scale in the above screen grab, two tracks can *look* in sync but
be far enough out to make the sync mismatch audible.
Hmm, I assume you mean 48 kHz.
One question before we start. Was your multitrack and camera synced to the
same clock during the recording session (be it word clock, smtpe etc)? If
not, the reality is that the sampling of your multitrack and that of
the camera audio will differ slightly. The difference might not be large,
but even over 4 minutes I would expect a detectable sync problem towards
the end. The only way to resolve this with unlocked sources is to
resample the audio from the multitrack in such a way that you correct for
the subtle clock differences of the gear used.
I routinely do this kind of thing and wrote a program to do this resampling.
You can check it out at
http://www.physics.adelaide.edu.au/~jwoithe/audiosync-0.4.tgz
It's quick and dirty but does the job for me. In the context of your
job you would pick to points in the recordings which are easily identifiable
in both waveforms - preferably at the start and end of the audio. You then
work out the exact sample number that this event corresponds to. While this
can be easy for direct audio (from the multitrack in your case) room reverb
can make picking events difficult in the camera audio (unless you piped
the direct sound into the camera as well). Usually I pick things like the
snare hits, for obvious reasons. These sample numbers are then used by
audiosync to resample the multitrack recordings so those events line up
exactly. So long as you've chosen accurate samples you will end up with
very good sync over the long term (for example, using this method I
often achieve synchronisation to within 10 samples over a period of 90
minutes).
As far as the software you need to work out the sample number of these
events I use audacity. I set it to work directly with the audio files since
it loads the files much quicker (although with only 4 minutes of audio
you are unlikely to have problems either way). You can set the location
display to be in samples using an item about half-way down the "View" menu.
The zoom buttons are then your friend.
> Also, when playing back the video in the compositor, I hear no evidence
> of a sync problem. I can turn on both cam and externally recorded
> stereo tracks and listen the whole way through the four minute recording
> without any reverb or noticeable sync issues. However, after rendering
> the files to a final format, I get inconsistent results in xine and
> mplayer.
This seems rather strange. If there's a sync problem you should hear it in
cinelerra unless for some odd reason the tracks aren't always being mixed.
I must admit that I do *all* my audio mixing in ardour (I export the camera
audio to WAV, import that to ardour and then sync off that), so have no
experience doing multitrack audio mixes in cinelerra.
> sync.mov: Xine shows sync problems, mplayer no problems
> sync.mpg: Xine shows no sync problems, mplayer shows problems
That sounds bizarre. While you can get sync problems in certain situations
with different AV container formats this would cause the entire audio track
to drift out of sync. As I understand it from your opening statements,
"sync problem" here refers to an audible echo indicating one source track
being out of sync with the other.
Regards
jonathan
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