[AG-TECH] Audio and Video Syncronization
stevens at mcs.anl.gov
Wed Jun 27 21:42:48 CDT 2001
I would be concerned that it might confuse the echo cancellation too have
you noticed any problems with that ?
At 11:12 AM 6/27/2001 -0400, Will, Rodger (R.) wrote:
>The existing room already had this installed. We can disable.
>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
> >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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