[Kdenlive-devel] A/V sync problem

[Dan Dennedy]

On Wed, Aug 4, 2010 at 11:06 AM, Simon Eugster <simon.eu at gmail.com> wrote:
> 2010/8/4 Dan Dennedy <dan at dennedy.org>:
>> On Wed, Aug 4, 2010 at 3:22 AM, Simon Eugster <simon.eu at gmail.com> wrote:
>>> Dear *,
>>>
>>> It seems we've really got a big A/V sync issue.
>>> http://kdenlive.org/forum/dnxhd-av-out-sync
>>> http://kdenlive.org/forum/there-any-way-keep-audio-sync
>>> http://kdenlive.org/forum/audiovideo-out-sync-after-rending-uploading-youtube
>>> http://kdenlive.org/forum/cannot-render-mpeg4xvid4h264flash
>>> etc.
>>>
>>> Seems like something is wrong?
>>
>> You must be new to audio/video and reviewing problem reports of a
>> modestly popular software package. :-) IOW, a/v sync is *always* a
>> problem due to proliferation of standards, formats, and tools. It is
>> easy to feel like chicken little while reviewing a bunch of reports.
>> So, ignore those reports for a moment, and based on your experience
>> what do you think?
>
> As you said: Everyone wants his own format? Frame rates of 29.97? Tons
> of funny ideas ffmpeg would have to consider to decode/sync it
> «correctly»? Kind of OOXML?

What is interesting about the reports above is that one is about the
ffmpeg-based Transcode feature and another occurs just with the
YouTube conversion. I do know that variable framerate clips are a
problem, and it is not typically obvious when a clip has variable
rate. I also know that for some classes of input (e.g. MPEG-2 program
stream, DV) that FFmpeg with MLT maintains good sync in the decoding,
processing, and SDI output stages because I have clients who have
payed me to ensure that. However, that is only critically verified at
certain points in time. Then, there may be major changes that create a
problem, but these become prominent and repaired due to the high
exposure afforded by the open source projects like Kdenlive and
OpenShot. I suppose there could be some things wrong in the encoded
output, but much of that code is based on ffmpeg, the utility, and it
seems to work fine most of the times I use it. With that said, it is
important to qualify problem reports in this area and look for
patterns that pinpoint a specific area to look.

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