[AG-TECH] Alternative mechanism for telco bridging
Robert Olson
olson at mcs.anl.gov
Tue Sep 3 16:44:19 CDT 2002
See http://www-unix.mcs.anl.gov/~olson/AG/TelcoBridging.html for this
document. I set this up in the ANL Workshop node last week for a meeting,
and it worked quite well.
--bob
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Alternative Telephone Bridging Solution
The standard mechanism used in the Access Grid for telephone bridging
using the Gentner matrix to configure a three-way link between room audio,
network audio capture PC, and telephone interface has some serious
usability problems. Most serious is that obtaining an ideal balance of
levels between the three audio realms can be quite difficult, and simple
adjustments (say to the local microphones) can have undesired consequences
on telephone audio levels.
An alternative mechanism dedicates an audio card and instance of the rat
audio capture tool to the telephone interface. In this configuration, a
second audio capture PC (this can be the Display Machine in a standard AG
node, if it has an audio card) is connected to the Gentner in the same way
that the standard audio capture PC is. It can use the second audio channel
on the Matchmaker MM100 if the node is not configured for stereo
connections; otherwise, it is recommended that a second MM100 be installed.
The Gentner matrix is then set up as follows. I assume below that the
display computer has been attached to the Gentner for telco bridging purposes.
<Table deleted for email purposes,see the webpage>
In other words, the telco and display computer are interconnected, and the
audio capture and the room audio (speakers and microphones) are
interconnected. The telco is completely independent of room audio and the
audio capture computer. If you have the telco output routed to the echo
cancellation reference output, remove that routing (as that audio will not
directly appear in the room audio and this setting will likely confuse the
echo cancellation).
We install rat on the display computer (I did this by copying from a
separate computer which had the PIG software installed. It is recommeded to
use the rat that comes with the PIG because the stock Windows rat will
introduce a roughly 1-second latency). In my testing I invoked the Windows
rat by hand with the proper address; however, it should be possible to
install the arm-eventlistener from the PIG software suite on the display
machine as well. I have not tested this, however. The command looks like this:
rat -t 127 <ipaddress>/<port>
You can find <ipaddress> and <port> from the arm-eventlistener window on
the audio capture machine if you are running rat by hand.
Once this rat is up and running, you should be able to connect to a
telephone call as usual, and see incoming level on the display machine rat
when there is audio coming from the telephone. When network audio comes in,
you should hear this audio on the telephone line. Note that in this
configuration, the audio from the local room is network audio from the
point of view of the telephone interface. This makes setting telephone
levels much easier the telephone audio you hear in your node is exactly
the telephone audio that others will hear. I advise turning off silence
suppression on the telephone audio rat. Adjust the levels on that rat for
comfortable audio levels to and from the telephone (you will likely have to
get feedback from the people on the telephone as to what an appropriate
level is for them to listen to).
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