Subject: [AVT] Mixers, and echo cancellation

From: Badri Natarajan <[email-masked]>
Date: Thu, 4 Dec 2003 00:10:15 -0500

All,

  Have a query about RTP mixing, and echo
cancellation/prevention in the mixed stream during playback.

Am currently considering a problem of payload
identification/separation in conferences, where multiple
speakers are active at an instant of time.

 To elaborate, the output of a mixer generally does not have any
input stream specific information(apart from the SSRC that it
carries as the CSRC).It appears to me that the only means of
preventing an echo during playback would be: 
a.) The input stream(s) are buffered at the mixer and 
subtracted out from the mixed stream, thereby creating N+1
streams, where N is the number of active speakers. 
b.) The input streams are buffered at the endpoints (or at a
different mixer in the case of cascaded mixers) and are
subtracted out from the mixed stream that comes back.In the
event of using cascaded mixers, we "might" generate lesser
number of streams than option (a).

 However,in case (b), it would be necessary to know which
stream, from the set of mixed streams  that comes back for
playback is to be used for the subtraction. Again it appears to
me that, the only way it could be done, is by tagging a header
extension to the stream, buffering it at the endpoint, create a
mixed stream with the same header extension and then subtracting
it out before playback, by comparing the extensions. Needless to
say, the scheme is complex and has slim chances of working ,
considering network delays, the fact that all endpoints (or
mixers in the case of cascaded mixers )might need to be sync'ed,
and that the header extension would have a high overhead.

Just wanted to know if anybody else has considered this problem,
and if you would you be aware of any other mechanism that would
identify a stream in a mixed stream?
(--- Or is it that I am missing something ??)

Also this brings me to my next question -- is it possible that
in some fashion, a set of cascaded mixers could be synchronized
so that there would be just one timing trigger that could
signal
the start of mixing operation at all mixers?

Would be glad if someone could comment on my queries,

Thanks in advance,
B.

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