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

[Alexandre Prokoudine]

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:

"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