[AG-TECH] Audio and Video Syncronization

Rick Stevens stevens at mcs.anl.gov
Wed Jun 27 09:57:48 CDT 2001

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 or 
> other
> >weirdness but others would know more about why that is.  I'd also check 
> things
> >like the rat settings to make sure everyone is at 16khz audio and look 
> at the
> >playout and decode stats for remote sites
>I'd always understood/assumed (I may be wrong) that the sync problem was
>due to the apps being totally independent and sizing their own playout
>buffers (or delays) independently. Hence if jitter and/or loss get larger
>for one stream over another (packet size issues?) the playout buffer is
>"stretched" (or delay increased) and sync goes out the window. I think rat
>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 it
>can also be a function of the content
>So Marty's suggestion to watch the playout delay stats is spot on. You need
>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 reason.
>Has anybody looked at MBUS hooks for vic and rat? Synch is one of the areas
>that it is meant to address.
>Perhaps one dirty option for now is to force vic/rat to drop late packets
>in preference to variable buffers - set the playout delay to a constant...
>Just my two bits worth. However, I can't stress enough how important it is
>to ultimately get this right - we have to put up with enough crap from the
>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 the
>synch perfect. Bundled MPEG streams would be another way we could do it,
>with a little effort...
>          Markus
>P.S. Lip synch can be lost at the *source* if the audio and video encoders
>aren't time synched to each other (and ditto the decoders at the receiving
>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 server
>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 on
>the CPU on the encoding side or NIC on the machine. If they are on
>different machines it may be reflecting network problems somewhere upstream
>(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 the
>other direction and see how it behaves, and then turn on your transmission.
>Markus Buchhorn, Faculty of Engineering and IT,          | Ph: +61 2 61258810
>email: markus.buchhorn at anu.edu.au, mail: CSIT Bldg #108  |Fax: +61 2 61259805
>Australian National University, Canberra 0200, Australia |Mobile: 0417 281429

More information about the ag-tech mailing list