[AG-TECH] Audio and Video Syncronization

Will, Rodger (R.) rwill1 at ford.com
Wed Jun 27 10:12:43 CDT 2001

The existing room already had this installed.  We can disable.

-----Original Message-----
From: Rick Stevens
To: Will, Rodger (R.); Ag-Tech (E-mail)
Cc: 'Markus Buchhorn'
Sent: 6/27/01 10:57 AM
Subject: RE: [AG-TECH] Audio and Video Syncronization

At 06:01 AM 6/27/2001 -0400, Will, Rodger (R.) wrote:
>I am very new to this technology, but I assumed that with vic and rat 
>being independent programs, that sync would be a challenge.
>One of our nodes employs an automatic camera/microphone system that 
>switches vic's input to the active speaker's camera and microphone. I 
>would assume that this effect would be akin to high-motion.

I should point out that we dont recommend this type of camera/microphone

coupling.  What is the problem
you are trying to solve with it ?

>I will watch the stats today, and pass along the information.
>I agree that someone urgently needs to solve this problem in software.
>-----Original Message-----
>From: Markus Buchhorn [mailto:Markus.Buchhorn at anu.edu.au]
>Sent: Tuesday, June 26, 2001 11:22 PM
>To: ag-tech at mcs.anl.gov
>Subject: Re: [AG-TECH] Audio and Video Syncronization
>At 08:21 PM 26/06/2001 -0500, Marty Hoag wrote:
> >However, I'm not sure how much
> >effect [ntp] will have on the actual audio/video syncronization.
I've had a
> >feeling that rat or vic can drift, especially if there is packet loss
> other
> >weirdness but others would know more about why that is.  I'd also
> things
> >like the rat settings to make sure everyone is at 16khz audio and
> at the
> >playout and decode stats for remote sites
>I'd always understood/assumed (I may be wrong) that the sync problem
>due to the apps being totally independent and sizing their own playout
>buffers (or delays) independently. Hence if jitter and/or loss get
>for one stream over another (packet size issues?) the playout buffer is
>"stretched" (or delay increased) and sync goes out the window. I think
>and vic try to keep this to a reasonably constant level, and drop the
>playout delay whenever they can - but they are doing it independently.
>Audio can catch up during silence, video during "static" periods - so
>can also be a function of the content
>So Marty's suggestion to watch the playout delay stats is spot on. You
>to watch both audio and video and how they relate to each other.
>"Professional"/commercial software does audio and video in the same
app, or
>tighly linked apps, and I'd understood it to be (partly) for that
>Has anybody looked at MBUS hooks for vic and rat? Synch is one of the
>that it is meant to address.
>Perhaps one dirty option for now is to force vic/rat to drop late
>in preference to variable buffers - set the playout delay to a
>Just my two bits worth. However, I can't stress enough how important it
>to ultimately get this right - we have to put up with enough crap from
>ISDN VC guys without giving them more ammunition :-) (not that ISDN is
>always fully synched either...). There's no reason why we couldn't get
>synch perfect. Bundled MPEG streams would be another way we could do
>with a little effort...
>          Markus
>P.S. Lip synch can be lost at the *source* if the audio and video
>aren't time synched to each other (and ditto the decoders at the
>end). So it is worth doing ntp or similar. Tying it to a specific ntp
>server on the Net I don't believe is necessary though, a local ntp
>should suffice.
>P.P.S. I'm not sure why Rodger leaving the room should make it better
>(presuming it is not a personal reflection :-) ). If the encoding and
>decoding is on the same machine it may be that it reduces the pressure
>the CPU on the encoding side or NIC on the machine. If they are on
>different machines it may be reflecting network problems somewhere
>(sending less/smaller packets if the video is static and the audio is
>quiet). You could try watching a one-way stream, with nothing sent in
>other direction and see how it behaves, and then turn on your
>Markus Buchhorn, Faculty of Engineering and IT,          | Ph: +61 2
>email: markus.buchhorn at anu.edu.au, mail: CSIT Bldg #108  |Fax: +61 2
>Australian National University, Canberra 0200, Australia |Mobile: 0417

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