[AG-TECH] Single Machine Node

Osland, CD (Chris) C.D.Osland at rl.ac.uk
Mon Jun 24 03:01:14 CDT 2002

-----Original Message-----
From: Gurcharan.S.Khanna at Dartmouth.EDU

<start extract>
...big snip re cameras - BTW, we use a video switcher so we can
have up to 8 video sources (say 5 cameras, visualizer, laptop,
VTR) into four capture cards, so agree that 8 into 4 does go!!

2. Is your Polycom Soundstation echo cancelling mic/spkr combo compatible
other AG nodes (using Gentner systems)? Have you noticed echo problems in
sessions with multiple remote nodes?

>>>Short Answer: yes, it is compatible. and we haven't noticed any echo
<end extract>

In principle, it should never matter what echo canceller is used
as long as it (individually) works properly.  The reason is that
at each site the echo canceller is cancelling out the bit of the
incoming signal that is heard by the local mics, and not resending
it in the outgoing signal.

Naturally this is likely to be imperfect, but as long as the
amount that is fed back out is way down on the original (say
10 dB below), the next time it will be 20 dB below and so will
die away.  The complexity is that the room's acoustics will vary
as people move, doors open/close, etc.., so the echo canceller
has to track the changes and adjust its idea of the way the
incoming signal is transformed by the amps, speakers, room,
and mics - the "transfer function".  Major changes (like picking
the mic up and dropping it on the table! - don't try this at home,
folks) can throw it out for 30 seconds or more.

In a nutshell, echo cancellers do not interact with each other,
but just try to ensure that no incoming signal is retransmitted.



Chris Osland                         Office tel: +44 (0) 1235 446565
Digital Media and Access Grid      Medialab tel: +44 (0) 1235 446459
BIT Department             Access Grid room tel: +44 (0) 1235 445666
e-mail:   C.D.Osland at rl.ac.uk               Fax: +44 (0) 1235 445597

CLRC Rutherford Appleton Laboratory (Bldg. R18)
Chilton, DIDCOT, Oxon OX11 0QX, UK

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