Patent GB-1364609-A (GB)
Title: Speech detector for a communication system
IPC: H04J3/17
Filed: 1972-10-19
Abstract: 1364609 Multiplex pulse code signalling J Y QUEFFEULOU and G VAUTRIN 19 Oct 1972 [19 Oct 1971] 48236/72 Heading H4L Means are disclosed for detecting the presence of speech signals in a time assignment speech interpolation P.C.M. time division multiplex system in which the threshold used for the detection of P.C.M. speech signals for transmission is dependent upon the corresponding received P.C.M. signals. As described, 256 subscriber lines or trunks 1, Fig. 1, are connected to a multiplexer-demultiplexer 2. The samples are converted to P.C.M. at 3, compressed at 23 and applied to the time-sharing switching network 5 under the control of signals from a circuit 9. In the opposite direction the compressed P.C.M. signals from network 5 are expanded at 24, decoded at 4 and converted to separate analogue signals in the demultiplexer 2. A speech detector 10 is connected to the output of compresser 23 and operates on a threshold determined by threshold selector 31 coupled to the received highway 14, the output of detector 10 being supplied to the control circuit 9. In this embodiment there are two alternative thresholds. The output of network 5 is converted to analogue form at 6 and supplied to carrier apparatus 8 for transmission via lines 16 to the remote station. It is assumed that 128 transmission lines 16 are available for the 256 input signals. Incoming signals on lines 16 are similarly converted to P.C.M. at 7 for application to the network 5. Alternatively, the transmission may be by P.C.M. via time-sharing line 17. A signalling transmitter 12 and receiver 15 transmit and receive orders for the numbers of lines to be connected and disconnected. The speech detector, Fig. 2 (not shown) comprises as many logic circuits, as there are time slots in the multiplexer. A number p of coded samples of each trunk are compared with the threshold to decide whether a signal represents a useful speech signal and the resultant signal is then supplied to a processer which has two periods, a complete period which is the time required for processing the threshold comparison of all the trunks = 1 ms. and a unitary period which is the time for processing the threshold comparison of one trunk = 3À9 Ás. The unitary period is divided into p sub-unitary periods which are each equal to the time slot duration. The detector connects or disconnects the trunks only under the following conditions (1) Trunk passing from passive to active state. This is confirmed only if, for N complete periods during which the signal of a given slot has been compared p times with the threshold, at least one signal among the p signals has exceeded the selected threshold this being followed by a silent period of duration T to prevent connection of an already connected circuit. In this case the detector supplies to the control circuit a "first transfer signal" which is a demand for the connection of a circuit whose address is supplied at the same time; (2) Holding a trunk in the active state for a time T<SP>1</SP> exceeding time T and (3). Passage of a trunk from an active to a passive state when a silent period which lasts longer than T is detected. In this case the disconnection is immediate if the trunk engagement had been shorter than T and is delayed until T<SP>1</SP> if the engagement had been longer than T. Conditions (2) and (3) give rise to a "second transfer signal" to perform the disconnection of the circuit. The comparison between the P.C.M. signals and the selected thresholds are made by detecting the binary value 1 or 0 of selected digits or digit combinations. Assuming that the P.C.M. signals consist of a sign bit and six bits ABCXYZ, comparison with the first threshold depends on the value of bit A, the most significant digit (low sensitivity) and comparison with the second threshold depends on the Boolean quantity (A+B+C), (high sensitivity). The selection between the two thresholds depends on the Boolean quantity (A+BC) of the return P.C.M. signals. If this condition is satisfied the low sensitivity threshold is used. Otherwise the high sensitivity threshold is in operation. The system prevents the sole presence of signals induced in the transmission slot by signals or noise in the reception slot from establishing connections.