[AG-TECH] Audio and Video Syncronization

[Marty Hoag]

   I always edit or ntp.conf to add a couple local servers because I figure the
lower delay and variance is better and things should sync even if the remote
server is unreachable (more on that later).  However, I'm not sure how much
effect that 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 (but again, others may have more exact
reasons as to why the drift).

   One other thing about ntp.  There is an "ntpdate" command that is invoked on
boot of the AGiB versions of the system that also points to
ntp-1.accessgrid.org.  This command initially sets the clock for the video
capture and audio capture machines.  Once in a while (maybe 1 in 10?  Haven't
really measured) this fails because of a dns lookup error or something like
this.  Then, since we are on daylight time, xntpdc (which points to the local
servers) conks out because the difference in time is greater than 3600 (like
3600.3 or something).  I suspect that the cmos clock is set to CST which is an
hour off from CDT.  Anyway, when this happens we are all of a sudden running an
hour off.  I'm not sure if there is any solution (customizing the ntpdate or
running it more than once each for a different server? ;-).   

   Also, you can sync your display machine using w2k display machine (I think
Bill Nickless first pointed this out).  From command line (may need to have
admin privs - not sure) you enter something like 
   net time /setsntp:"ntp.host1.foo ntp.host2.foo ..."
where the ntp.host1.foo and ntp.host2.foo are local ntp servers (if just one you
don't need the quote marks).  Then just go to the Administrative "Services"
entries, make sure the Windows Time Service is set to auto and start it.  I also
found a description of this at 
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnw2kmag01/html/TimeWin2K.asp