Conference facilities for a tdm exchange

Claim 1

An automatic telephone exchange in which communication connections are set up in time division multiplex manner, comprising means for conveying speech by delta-sigma modulation whereby a steady state value repesenting the absence of speech is transmitted as a series of discrete bits alternating between 1 and 0 and a variable signal representing speech is transmitted as a series of bits where each successive bit indicates an increase or a decrease in speech level, a plurality of incoming multiplex highways to and outgoing multiplex highways from the exchange, a conference circuit provided for the establishment of conference calls involving three or more subscriber lines coupled to the exchange and served by the exchange in time division multiplex channels, said conference circuit including storage means for recording signals from each subscriber line at the time received, said conference circuit including a scanner which scans the storage means containing signals from each subscriber line coupled for a conference call in search of a particular line whose user has commenced to talk, said scanner determining which channel is in use by a user who has commenced to talk by detecting when the bits occurring in a chanel are initially a sequence of 1''s indicating the channel is in the talking condition, gating means coupled responsive to the detection of said particular line to switch that line''s channel to the signals in use by the other lines involved iN the conference, and gating means responsive to said detection for transmitting an ''''idle'''' signal formed by an alternating series of 1''s and 0''s from an idle code generator to the channel of said particular line and preventing the transmission of said particular line''s signals from being returned thereto, thereby preventing excessive sidetone in the talking channel.