Abstract: A method and circuit for nulling each talker's speech in a conference call having a plurality of parties who are selectively talkers and listeners. The parties are interconnected on PCM telephone carrier lines (T1 or CEPT) which are connected to a digitized conference bridge. The circuit of the present invention has a summation circuit for adding together all of the speech data on the carrier lines occurring in the conference call during a particular time slot. A circuit is used for delaying the speech of each individual talker. Finally, the circuit of the present invention utilizes a subtraction circuit for subtracting the delayed speech of each individual talker from the summation of all speech for delivery back to the ears of the individual talker. The delaying circuit precisely delays the speech of each individual talker so as to corespond in timing with the output of the summation circuit.

Abstract: A circuit enabling whisper conferencing between two whisper conferees in a conference call. The conference call may contain any number of parties including the two whisper conferees and is interconnected to a digitized conference bridge over PCM telephone carrier lines. The present invention includes a cross-point switch connected to the PCM telephone carrier lines for interchanging the PCM speech data occurring in the time slots for the two whisper conferees so that the PCM speech data for the first of the two whisper conferees is placed in the time slot for the second of the two whisper conferees and vice versa. A summation circuit is utilized to sum the telephone conversations of all remaining conferees and the two interchanged telephone conversations of the whisper conferees are then selectively added to the summed signals.

Abstract: An automatic gain control for an inputted multiplex PCM data stream in a conferencing bridge in a telephone system. The inputted multiplexed data stream has a predetermined number of time slots wherein each time slot carries PCM speech data corresponding to an individual party in the conference call. The control of the present invention is capable of operating in .mu.-Law or A-Law companded format at a frequency of 192-256 kilobytes per second. The automatic gain control incorporates a digital root-mean-square meter, a COUNT sequence to change GAIN at predetermined intervals, and a set of GAIN rules to selectively increase or decrease GAIN based upon the output of the digital root-mean-square meter.