[wdmaudiodev] Re: Audio Video Sincronization

  • From: Tim Roberts <timr@xxxxxxxxx>
  • To: wdmaudiodev@xxxxxxxxxxxxx
  • Date: Fri, 27 Jan 2006 09:52:01 -0800

Bipin Mistry wrote:

> Hello All,
> Please do help me out,
> Currently I am doing Video Audio recording and facing problems of
> sincronization.
> I am not able to sincronize the Audio with Video after merging.
> I mean to say if recording of 5 minutes is done then  Audio turns out
> to be of 4:52 minutes... means some thing less then Video.

That's a significant difference -- 3%.  Is it possible the audio simply
started late, or is the audio really running faster than the video? 
What is the capture device?  A television capture graph should never
encounter this problem.  Both streams should be timestamped.

I worked on one camera where the designers used the wrong audio crystal
-- 24.000 MHz instead of 24.576.  That resulted in a very similar effect.

> How can I streach the Audio or Shrink the video at merging. without
> loosing any data.

If audio just starts late, that's an easy problem to solve.  If audio is
really running fast, your best bet is to drop a video frame.  The eye
will not notice.

If this is coming from a TV source, you can count the number of video
frames to see if the video is really running too slow.  At the NTSC
frequency, 29.97 Hz, you should see 8,991 frames in exactly 5 minutes.

Tim Roberts, timr@xxxxxxxxx
Providenza & Boekelheide, Inc.


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