[Kdenlive-devel] Feature request: Sync External Audio Automatically

[Simon A. Eugster]

On 02/08/2012 09:08 AM, Alexandre Prokoudine wrote:
> On Wed, Feb 8, 2012 at 11:32 AM, Simon A. Eugster wrote:
>
>>> http://jeff.ecchi.ca/blog/2011/07/25/automated-multicamera-clip-syncing/
>>
>> I have no idea how you manage to have a link to a solution for nearly
>> every problem. Thanks for the link!
>
> YW :)
>
>> How accurate can we position audio streams? Just by full frames, or is
>> it possible to have a finer granularity? When I synced audio/video I
>> often had the problem that the audio was too early and after moving it
>> by one frame it was too late.
>
> Admittedly, I haven't had a chance to test it myself yet. However
> http://bemasc.net/wordpress/2011/07/26/an-auto-aligner-for-pitivi/
> states:

Already read! :)
I rather meant kdenlive/MLT here. Can we move an audio clip by just a 
few samples or only by full frames?

Simon

> "The algorithm I settled on resembles the method a human uses when
> looking at the waveform view. First, it breaks each input audio stream
> into 40 ms blocks and computes the mean absolute value of each block.
> The resulting 25 Hz signal is the “volume envelope”. The code
> subtracts the mean volume from each track’s envelope, then performs a
> cross-correlation between tracks and looks for the peak, which
> identifies the relative shift. To avoid performing N^2
> cross-correlations, one clip is selected as the fixed reference, and
> all others are compared to it. The peak position is quantized to the
> block duration (creating an error of +/- 20ms), so to improve accuracy
> a parabolic fit is used to interpolate the true maximum. I don’t know
> the exact residual error, but I expect it’s typically less than 5 ms,
> which should be plenty good enough, seeing as sound travels about 1
> foot per ms."
>
> Alexandre Prokoudine
> http://libregraphicsworld.org
>
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