Patent GB-1234419-A (GB)
IPC: H04Q11/04
Filed: 1968-08-27
Abstract: 1,234,419. Automatic exchange systems. STANDARD TELEPHONE & CABLES Ltd. 28 July, 1969 [27 Aug., 1968], No. 40992/68. Heading H4K. Conference connections are effected in a time division multiplex exchange, in which channels of incoming highways are switched over an intermediate highway to channels of outgoing highways, by adjusting a stepping register to have as many effective stages as there are conferees switched over an associated intermediate highway, the register being stepped each time a conferee's channel is switched to the intermediate highway so that the register in each eachange cycle displays the modulations of each and every conferring party. As shown in Fig. 2 in which the modulation employed is deltasigma modulation, an intermediate highway HW has a controlling store CS with a storage compartment for each exchange time slot, inter-highway gates being operated according to address data in the storage compartments in order to connect the send and receive paths of four-wire connections. A party requiring a conference dials the numbers of all required conferees, connections being set up by a call processor and those storage compartments of store CS serving the arbitrarily selected channels for the conference being indicated by a conference mark in a storage cell bj. The presence of mark bj causes the modulation of a channel to be switched to a stepping register SR the number of effective stages of which is adjusted by a so-called combiner control circuit to be the same as the number of conferees. As each modulation is received by SR, in the channel time of the associated conferee, the register is advanced one step so that as the modulation for each conferee is accepted at one end of the register the corresponding modulation of that conferee from the last multiplex cycle is lost by its displacement from the last stage. At the start of each exchange multiplex cycle a scan of all stages of register SR is made and by comparison with a record of previous such scans activity of conferees is judged. With deltasigma modulation idle condition is coded by alternate ones and zeros and if a series of ones is detected in a channel the conferee is presumed to be talking and is nominated as the " source " conferee. A cyclic counter SC, set to as many stages as there are conferees, has injected into it a bit shifted step-by-step with register SR so that the modulation of the " source " conferee is connected to conference highway CHW over gates such as GS1 for each time slot of the conferees as they appear and step SR. As each mark bj is read out of the conferee's storage compartments in CS the conference highway is gated to the outlet channel of the conferee. In the time slot of the " source " conferee, which slot occurs when the mark in counter SC is at the last stage, the conference highway is connected to an idle code generator ICG so that the " source " conferee is sent, not his own modulation, but the idle code decoding to silence. Where such a tdm exchange has more than one intermediate highway each such highway has its control store CS and has a stepping register SR and " source " marking counter SC. As conferees may have the same time slot on different highways it is necessary to provide more than one conference highway so that a " source " conferee can be connected to idle code without coming into conflict with another conferee using the same time slot and to whom the " source " modulation must be switched. Conferencing of the above type may be provided in a system in which intermediate highways have channel store-and-forward stores termed junctors such that the send channel of a connection may be switched in one time slot and the receive channel is switched in a different time slot. Each storage compartment of the junctor is addressed cyclically for switching in one direction and acyclically for switching in the other direction. The use of stepping registers such as SR for conferencing with a nominated " source " conferee is possible in this system if a junctor storage compartment is alotted each conferee and is employed cyclically and for a single time slot. In exchanges in which the modulation employed is PCM the codes may be switched across the exchange in parallel and stepping registers such as SR be provided for each bit of the code. Thus as the PCM code of a conferee is set up on the first stages of the registers SR the code from that conferee in the last exchange cycle is discarded from the last stages. All the PCM codes of all the conferees are available on the corresponding stages of the registers SR. All but the code in the last stage of the registers, this excepted code being the modulation of the conferee whose time slot is currently present and whose channel is being gated, may be combined by addition, with allowance for companding laws if needs be, and the resulting code be supplied over a conferee highway for connection to the conference whose code is not included. Alternatively all codes may be included in the sum but that code standing in the last stages of the registrs SR, this being a modulation from the conferee currently being switched, may be subtracted from the total before this is transmitted.