Subscriber automatic telephone exchange - permits phone conferences between internal phones and public network by use of constant line impedance

摘要

The subscriber automatic telephone network includes a number of supplementary parts (6-10) connected through respective switched input and output lines (61, 63;71, 72;) 81, 82;91, 92;) to a common bus (3) to which a public network (10) is also connected by way of its own switched input and output lines (102, 101). The bus (3) is connected through a respective switched (12) and one or more impedances (11) to earth, each impedance representing that of a part. Intercommunication between posts, for example between two posts (6, 8) and the public network (10) is effected by time division multiplexing in which the relevant input and output lines are rapidly switched by a control circuit (2). In addition, the bus impedances are switched into the network to ensure that the total impedance seen by the bus remains constant and thus avoids signal degradation. For example, for a conference between (m) phones and a larger number (n) phones, (n-1) bus impedances are provided and (n-m) impedances are switched into the network.