Apparatus for attenuating the echo signal in telephone fork circuits for telephone conference calls

#This invention relates to a method of attenuating the echo signal in converting circuits, referred to as telephone fork circuits, intended for telephone conferences held with an odd number of participants, said circuits being useful for matching double-wire telephone lines, each having analog phonic messages transmitted bidirectionally thereover to and from each of the participants, and pairs of double-wire lines, with each line in each pair being respectively adapted to receive and transmit said messages as converted each from and into a digital code of the PCM (Pulse Code Modulation) type.

#The invention is also concerned with an apparatus operating in accordance with the above-noted method.

#The field of application of the method and apparatus according to the invention comprehends in particular, though not solely, digital switching telephone exchanges and private telephone branch exchanges, also referred to as digital PABX (Private Automatic Branch Exchange), wherein there exists a demand for a conference switching phase between messages, coded according to the PCM digital code, from corresponding participants in the conference.

#The features and advantages of the method according to the invention will become apparent from the following detailed description of an apparatus for implementing it, to be taken by way of illustration and not of limitation in conjunction with the accompanying drawings.

#In the drawings:

#FIG. 1 is a schematic view of a telephone branch exchange incorporating the apparatus of this invention; and

#With reference to the drawing views, the numeral 1 designates comprehensively and schematically an apparatus for implementing the inventive method. The apparatus 1 is incorporated to a digital telephone branch exchange 2, e.g. of the PABX (Private Automatic Branch Exchange) type, to which a plurality of double-wire telephone lines, collectively indicated at 3, are led whose other ends are each connected to a subscriber telephone set, not shown, for a participant in a telephone conference.

#The branch exchange 2 includes a converting or interface circuit 4, e.g. of the type available commercially as a SLIC (Subscriber Line Interface Circuit), which is adapted to provide a matching interface between each double-wire telephone line 3 and two groups 5 of double-wire lines 6, 7. Whilst the telephone line 3 is intended for bidirectional transmission of a message or analog phonic signal from and to said participant in the conference, each of the double-wire lines 6 intended is intended, to transmit a message or analog phonic signal and each of the double-wire lines 7 is intended to receive a message or analog phonic signal.

#The message transmitted on the line 6 is denoted ST, and the message received on the line 7 is denoted RT.