Abstract: A teleconference system comprises a plurality of microphones and a plurality of expanders connected respectively thereto for expanding the outputs of the microphones. A summing ammplifier produces a sum of the output signals of the expanders and feeds a compressor to compress the sum for transmission to a distant conference location. A desired signal from a given microphone has a higher level than undesired signals which may be detected by adjacent microphones, so it is emphasized by the associated expander with respect to the undesired signals. The compressor provides compression of the high- and low-level components of the sum in equal proportions so that the former is converted to a replica of the desired signal and the latter is suppressed to acceptable levels.

Abstract: A tone disabler is designed for use in a full duplex data transmission system adapted to send a tone before each transmission of data and to reverse the phase of such tone during a later half of a full duplex data transmission. Both the tone and the phase reversal are detected. Responsive to a coincidence of such detections, a control signal is given to disable an echo canceller. The control signal is fed back to latch the control signal as long as the tone is present.

Abstract: In order to prevent tap coefficient values from drifting toward excessively large values or divergence, an adaptive echo canceller is provided with a correction signal controller which (a) reduces the tap coefficient values upon the polarities of the tap coefficient signal and the tap coefficient correction signal coinciding and (b) increases the tap coefficient values upon the polarities differing.