Patent GB-1498316-A (GB)
Title: Multiple connection device for a digital tdm switching centre
IPC: H04M3/56
Filed: 1976-05-04
Abstract: 1498316 Automatic exchange systems INTERNATIONAL STANDARD ELECTRIC CORP 4 May 1976 [7 May 1975] 18153/76 Heading H4K An electrical switching device for establishing a variable number of multiple connections, i.e. more than two parties, in a TDM digital switching centre, in which each party can be placed in any one of N states each corresponding to a particular type of connection, e.g. conference call, momentary isolation &c., comprises means for converting at an input, the non-linear codes of the samples into linear codes and conversely at an output, a store for storing recent samples, an adder for summing the most recent samples of all parties in each multiple connection and a subtractor for subtracting, in a particular time slot, the coded sample of the respective party from the sum of coded samples corresponding to the multiple connection involving said party. In a particular embodiment the device (2, Fig. 1, not shown) is connected to the time-space-time digital switch (1) of a TDM switching centre together with a plurality of subscribers (at least 256) each one of which is allocated to a time slot t (1-256) of a frame Tn; each time slot is divided into three phases tA, tB and tC. The device, Fig. 2, is connected to the switch (1) through a receive path R and a send path S through which all the subscribers taking part in multiple connections are connected to the device. The device includes registers 3, 14 for storing the incoming and outgoing speech samples respectively, a non-linear to linear code converter 6 and a linear to non-linear code converter 7, a speech memory 4 for storing the speech samples of each subscriber, an adder 18 for adding all the speech samples in a multiple connection, an accumulator 15 storing all the added samples, a subtractor 8 for subtracting a subscriber's own speech sample and a memory 19 which stores the addresses and states of all subscribers. Store 19 receives its information from the marker of the switching centre. Operation.-State El: temporary isolation; AND gates 5, 11 and 17 are disabled and no speech samples are written or read-out of accumulator 15 in a time slot thq hence the subscriber cannot be heard by and cannot listen to other parties. State E2: Conference call; at each time slot of frame Tn corresponding to a subscriber taking part in a conference call, memory 19 is addressed at tA, and memory 4 and accumulator 15 are read out at tB, and the samples from the previous frame T(n - 1) corresponding to a particular subscriber, (from store 4) and to a particular multiple connection (from accumulator 15) are fed to subtractor 8 which, at tC provides the sample to be sent to the subscriber corresponding to each slot. At tC, also, the speech sample from the same subscriber corresponding to frame Tn is stored in store 4 and also added to the output of subtractor 8 by adder 18 and stored in the accumulator 15. Thus all subscribers in each multiple connection can converse with each other. State E3: A subscriber can listen to other parties in a connection but he cannot be heard by the other parties. The operation is as for state E2 except that AND gates 5 are not enabled when the particular subscriber is addressed and thus his speech samples are not stored in store 4 or added to the contents of accumulator 15. State E4: A subscriber can hear only one party in a multiple connection but cannot be heard by any of the other parties. AND gates 5 and 17 are not enabled during the time slot corresponding to the subscriber and memory 19 does not contain the address of the multiple connection but of the particular party which the subscriber will listen to, so that the speech sample sent to the subscriber is that stored in store 4 in the address defined by store 19 and which corresponds to the particular party.