Re: Audio/Video Sync problems

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Thanks, that's pretty helpful. I've not been able to identify a known
(systematic) source of lag from basic research.

One thing which strikes me is a possible mismatch between audio clock
and the video frame rate clock. NTSC comes in at 29.97fps which is a
fractional multiple of your actual video frame rate. I wonder if this
might be an influencing factor in the audio lag.



If you had to quantify it, what's the relationship of the 100 -> 500ms
lag compared to the encoded length of the video ?

Plotting that graph may be instructive - try it as a straight through
capture if you can reproduce it there, then try it with a pause in the
encoding to see if the lag is consistent with the video duration.

I'll have a think and a read around, to see if I can identify other
possible causes - as I say, it's not something I've specifically hear
of with WME, in general pulldown issues are more common when
converting PAL <-> NTSC <-> Film 24fps.


Something's nagging about audio lag which mght come back to me later :
What specific type of camera is this - a DV cam ? Or webcam ?

DV cam may be provided with locked audio on better equipment, but many
consumer models are likely to have unlocked audio which may not
precisely sync with the video rate. There's a good read on this here :
http://www.adamwilt.com/DV-FAQ-tech.html#LockedAudio

The only way that could influence the encode is if the camera and
encoder combination is consistently and systematically moving backward
relative to the video frames. Any chance of swapping camera models ?

HTH
Cheers - Neil

On Sun, 05 Aug 2007 21:57:18 -0500, Chris Richards
<gizmo@xxxxxxxxxxxxx> wrote:

I'm encoding straight from the camera, using the IWMEncoder2 interface.
I'm loading my profile using the IWMEncProfile2 object, and then setting
that to my source group.

I'm then loading the first audience in the profile so that I can get the
size of the video buffer stored in the profile. I then add 1 second
to that and store it internally. (I'm doing this because I've found
that if I don't wait at least this long after the user presses 'Stop'
before I actually stop the archive file, the video will be chopped off.)
I'm then calling encoder->archive to stop the archive (this is actually
how I start/stop/pause encoding; the encoder runs all the time, and I
just flip the archive on or off as necessary; this is necessary for the
next part).

Now, as for the profile itself. It is currently set for a 56K stream only:
Audio: 10 Kbps, 16 KHz, mono CBR
Video: 240x180, 15 fps, 8 second key frame interval, 40Kbps, 5 second
buffer, video smoothness at 75, and decoder complexity at auto. Both
audio and video are CBR.

Audio Codec: Windows Media Audio 9.1
Video Codec: Windows Media Video 9

Video format: NTSC

If there is any other info that you need, please let me know and I'll
try to provide it.

Thanks in advance.

Later,
Chris




Neil Smith [MVP Digital Media] wrote:
Nobody knows anything about the encoding profile, or the source files
you're encoding. Start there and somebody can probably help ;-)

Cheers - Neil

On Sun, 05 Aug 2007 14:16:16 -0500, Chris Richards
<gizmo@xxxxxxxxxxxxx> wrote:

I've an application using WMEncoder 9. It works on most computers just
fine, but I'm increasingly running into people who are using laptops,
and experiencing issues with the synchronization of audio and video. In
almost all cases, the video is lagging the audio. On a few of the
systems, I have seen situations where improving the lighting of the
video would reduce or eliminate this problem (i.e. the video would be
very dark, and the audio and video would be way out of sync (like 100 or
more mS, up to as much as 500 mS; increasing the lighting on the subject
would improve this.)

Has anyone else experienced this? I know there are issues relating to
CPU and memory requirements, and MS recommends a P4 3.0 GHz or better
with at least 1 gig of RAM. However, I have desktops running
comfortably on 2 GHz Celerons and 256 Meg of RAM, while I have laptops
with 2.4 GHZ Core2Duos with 2 Gig of RAM that are having issues?

This is a real problem for me, because more and more people are using
laptops, and the laptops seem to be the ones having the majority of the
problems.

Thanks in advance.

Later,
Chris
------------------------------------------------
Digital Media MVP : 2004-2007
http://mvp.support.microsoft.com/mvpfaqs
------------------------------------------------
Digital Media MVP : 2004-2007
http://mvp.support.microsoft.com/mvpfaqs
.

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