BABAR Note 371
Univ. of Texas at Dallas
July 15, 1997
A phone patch is a circuit that takes a telephone line, a twisted pair which carries information in both directions, and provides separated, line level, audio input and output connections which can be used by a computer and CU-SeeMe. Predictably, hybrid CU-SeeMe/POTS (Plain Old Telephone System) conferences don't work well when microphones are simply dangled in from of speakers. There are feedback loops which interfere with full duplex communications.
As configured at SLAC, the CU-SeeMe Mac/Phone Patch requires its own telephone line which calls into the telephone conference. The Mac should not have its speakers or microphone enabled; this would produce a feedback loop as the Mac sits in the same room as a telephone speakerphone used in the conference. The Mac should do all of its 'audio I/O' electronically through its audio input and output connectors. Participants in the SLAC conference speak and listen only through the telephone speaker phone. A pleasant value added feature that comes from having the CU-SeeMe Mac/Phone Patch physically present in the conference room at SLAC is that telephone participants in the room can see the CU-SeeMe participants on the Mac screen, and CU-SeeMe participants can see the telephone participants at SLAC when the SLAC V-Tel system cameras are used to send a video feed to the CU-SeeMe Mac/Phone Patch
Plugging in the Phone Patch
The front panel of the phone patch is shown in Figure 1. The
patch requires +12V power which is provided with a Radio Shack
power supply. The patch accepts a standard RJ-11 telephone connector.
A real telephone should be located on the same line for dialing
and testing purposes. The toggle switch can take the telephone
line off hook (the equivalent of picking up the receiver on a
normal telephone). Three LED's are used to indicate when the
patch has Power, when it is off Hook, and when the
line is Ringing. The trapezoidal connector is not used.
Audio connections are made from the side panel using RCA connector
cables such as used in home audio systems. The audio output (input)
of the patch should be connected to one of the audio inputs (outputs)
of the Mac. Note that the Mac is equipped for stereo, but a telephone
is monaural. Should this set-up be duplicated with a Mac without
separate RCA audio connectors, audio I/O to the Mac is possible
via the microphone and headphone 1/8 inch mini jacks in a less
Establishing a CU-SeeMe/POTS Connection
To establish the CU-SeeMe conference, connect the Mac CU-SeeMe
client to the CU-SeeMe reflector being used for the conference.
CU-SeeMe participants would connect to the same conference, either
on the reflector or on a reflector which has been unicast-peered
to that reflector. The phone patch/CU-SeeMe client should be
set to transmit 100 ms sound packets using the Intel DVI (32 kb/s)
audio codec (Figure 3) to maintain compatibility with all combinations
of White Pine, Cornell, and vat clients on Windows, NT, Mac and
The microphone threshold settings in the Audio window shown in
Figure 4 is important. These are the two horizontal, sliding
bars to the right of the microphone volume. The effect of the
very low setting of the lower threshold is to transmit all sounds
received from the telephone conference when none is talking on
the CU-SeeMe side. The very high threshold for the telephone
conference takes effect when someone is talking on the CU-SeeMe
side. A little of the CU-SeeMe sound which is relayed by the
Phone Patch to the telephone side is presented back to the CU-SeeMe
client by the patch due to cross talk. The high threshold setting
blocks the crosstalked signal, at the cost of blocking the telephone
conversation on the CU-SeeMe side when someone speaks on the CU-SeeMe
side. This isn't a serious problem, since when everyone talks
at once, information transfer is poor. Care should be taken to
prevent a rogue CU-SeeMe client from transmitting continuous noise.
The little diamonds beside the mike and speaker volume indicators
are volume controls. To change the volume of CU-SeeMe participants
as heard on the telephone, use the speaker volume control. To
change the volume of telephone participants as heard by CU-SeeMe
clients, use the microphone volume control. This should not need
to be tinkered with once it has been set up, however speaker phone
participants in particular should pay attention to speaking loud
enough that all remote participants, telephone and CU-SeeMe can
hear them clearly.
To establish the telephone side of the conference, use the telephone attached to the phone patch to dial into the telephone conference. Once the phone connection is established, flip the toggle to Off Hook and hang up the telephone. The telephone and CU-SeeMe connections can be established in either order.
When a conference is finished, the CU-SeeMe client should be disconnected from the reflector and the Phone Patch should be place on hook.
A BABAR hybrid conference meeting is shown in Figure 5.
The board inside the Phone Patch is an autopatch interface for
an RLC3 controller from Link Communications, a company that supplies
the amateur radio community. Note that this is only the autopatch
interface ($125 + $15 shipping as of early 1997) and not the controller
itself which costs over $1000.
Link Communications Inc.
115 2nd Ave NE
PO Box 1071
Sidney, MT 59270
Figure 6 shows the physical layout of the Link Communication board,
and Figure 7 is a circuit diagram. Figures 6 and 7 are reproduced
with permission, courtesy of Link Communications. To take the
circuit Off Hook, the toggle connects pin 3 to ground. Audio
In is pin 4, and Audio Out is pin 5. The LED's on the board have
been replaced with LED's visible from the outside of the circuit
Launch the CU-SeeMe application on the Mac and turn on the V-Tel
unit if you will be using its cameras for a CU-SeeMe video feed.
Connect Mac CU-SeeMe client to the reflector.
With the phone patch toggle 'on hook', i.e. toggle set away from 'off hook', pick up the receiver on the attached phone and dial into the telephone conference. After the connection is established, flip the toggle to 'on hook' and place the telephone back in its cradle. The phone patch will keep the line active. The CU-SeeMe and telephone portions of the conference have now been connected.
When your conference is completed, flip the phone patch toggle
to 'on hook' to release the phone line, and disconnected the CU-SeeMe
program from the reflector.
Note: this 'Cheat Sheet' assumes the phone patch and Mac have been properly connected, and some prior experience with CU-SeeMe. If there is a problem, please consult the videoconferencing support staff at SLAC. BABAR Note 371, "A Phone Patch for CU-SeeMe Packet Video" may also be useful.