Re: Saving and playing a video conference
- From: "Alessandro Angeli [MVP::DigitalMedia]" <nobody@xxxxxxxxxxxxxxxxxx>
- Date: Mon, 14 Nov 2005 22:03:28 +0100
David wrote:
> An approach is to save both sides of the conversation to
> two different files. If both files get created at the
> same time and the recording process stops for both files
> at the same time, then playing the two files at the same
> time should cause the two files to be close to
> synchronized?
>
> This approach would not seem to be reliable.
>
> Is there a reliable way of doing this? I would like to be
> able to play the two files in synchronization without
> user interaction. Meaning that once a conversation is
> saved the user could go back and choose the saved
> conversation and play the files and they would play
> synchronized.
>
> Is there any information stored in the two files that
> could be used to sync them up? If so, is it possible to
> get to the information and use it?
>
> I'm fine with restrictions like the computers have to be
> time synchronized. My application will be running on a
> local area network.
>
> Is there a way to insert a marker in each stream to be
> used for syncing?
If you can record both sides on the same machine, create an
AVI or WMV with 2 video and 2 audio streams so that the
container will keep the streams in sync for you.
If you need to record each side on different machines, you
can still play the separate files in the same graph so that
the samples will be rendered in sync, as long as the files
have the same rate.
However, there might be a time offset which is not easy to
correct. If you keep the machines in perfect sync using NTP
(Windows 2000/XP/2003 has a built-in client and the servers
also a built-in server), you can save a timestamp either in
the file header or anywhere else (even the file name) that
tells you the exact start time, so that you can correct the
offset during playback. However, NTP may not be precise
enough (NTP can be precise to the millisec, but that may not
be true or enough).
Even harder than the offset will be a difference in rate
caused by a drift in the machines' clocks. You can resync
using NTP often, but that may not be enough.
--
// Alessandro Angeli
// MVP :: Digital Media
// a dot angeli at psynet dot net
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