CRR Room Combine – Why traditional room combiners cannot work for teleconferencing

The Room Combine objects provided by most DSP manufacturers are incapable of correctly combining two or more teleconference rooms. This is because all signals sent to a far end such as a phone or video conference device must be mix minus signals.

How they work
A traditional room combiner can be used to combine meeting rooms by simply mixing the speaker signals together as needed. Each room has one speaker output. When the rooms are divided, Speakers-A will receive Room-A Microphones and Program while Speakers-B will receive Room-B Microphones and Program. If a room combiner simply mixes those two speaker signals together and sends that mix to the speakers in both rooms, Speakers-A and Speakers-B will both receive Room-A Microphones and Program plus Room-B Microphones and Program. This is a perfect implementation of a traditional room combiner.

How they fail
If a phone is added to each room, each room now has a speaker output and a phone output. The speaker output for each room receives the Microphones, Program, and phone signals from that room. The phone output receives only the Microphones and Program signals from that room. The phone input is not sent back to the phone output because that would cause the far end participants to hear their own voices echoed back to them. When the rooms are combined using a traditional room combiner, the combined speaker output will be receiving the Microphones, Program, and phone signals from both rooms. Sending that combined signal to the speakers in both rooms will correctly allow both rooms to hear everything. However, when the phone output signals are combined, the resulting signal will be only the Microphones and Program from each room. The phone output for each room is a type of mix-minus. The phone input signal is not sent to the phone output, so the phone output for each room contains everything except the phone input from that room. Combining the two phone outputs together with a traditional room combiner and sending the combined signal to both phone outputs will mean that each phone is unable to hear the other phone. Far end participants will hear the Microphones and Program sources from both rooms, but they will not be able to hear the participants on the other phone line.

How they fail..again
Traditional room combiners also fail for the same reasons when working with mix-minus speaker zones. If two rooms each have two mix-minus speaker zones, connecting the speaker outputs of those two rooms to a traditional room combiner will result in each room only hearing some of the microphones from the other room.

Room Combining that works
In the Bose ControlSpace Conference Room Router (CRR), all phone and video codec inputs are referred to as “Far End” inputs and outputs. When uncombined, each Far End output receives the Microphones, Program, and any other Far End inputs from that CRR. A mix-minus is created for each Far End output that includes everything except its own Far End input.
When two or more CRR are added to the same Room Combine Group (RCGroup), each CRR sends a Mic Mix and a Non-Mic Mix to all other CRR in the same RCGroup. The Mic Mix contains the Post-AEC microphone mix for that room and the Non-Mic Mix contains Program input signals and all Far End input signals for that room. When two CRR are combined, each Far End output will still be receiving the local Microphones, Program, and any other local Far End signals, but will also receive the Mic Mix and Non-Mic Mix from the other CRR. Since the Non-Mic Mix contains all the Program and Far End signals from the other CRR, each Far End output will receive a perfect mix-minus and will be able to hear all other participants in the teleconference.