Patent GB-1349787-A (GB)
Title: Apparatus for receiving and registering alternative call code  signals of two call codes
IPC: H04Q1/442
Filed: 1971-06-03
Abstract: 1349787 Radio signalling AGA AB 3 June 1971 [12 June 1970] 18746/71 Heading H4L Apparatus for receiving and registering calls in a telecommunication system, e.g. a mobile radio system, includes means for responding to a station calling code and an alternative signal, e.g. a group calling code. As described, the calling code consists of three signals X, Y, Z, X and Y each representing a code position which can be occupied by any of the digits 0 to 9 and Z, representing a position which can be occupied by any of the digits 1 to 9. The digits occupying positions X, Y are common to a group of receiver stations and a group call code for these stations takes the form X, Y, G where G is occupied by the digit 0. As shown in Fig. 1 incoming coded signals are supplied to a bandpass filter F which can be switched between four different selection states, corresponding to X, Y, Z and G, under the control of a counter R which is advanced one step each time the filter F has completed the reception of a code element and thus registers the signals as they are received. Considering the reception of a call XYZ, initially the filter F is switched to pass the first code element X which is rectified at D and integrated at I to operate a trigger T after a predetermined time interval and advance the counter R to position Y. An astable multivibrator A, B is assumed to have a signal on output B at this time, Fig. 2 (not shown) so that the output from trigger T locks it in that position via gate 5 to enable gate 3. During the reception of code element Y this procedure is repeated and the Counter R is switched to output Z but since the multivibrator AB always has an output B when the code element Y ceases which then changes to output A the filter F is first switched to receive the alternative signal G. Since this signal does not appear the multivibrator changes back to state B so that gate 1 is enabled and filter F is switched to receive code element Z. This locks the multivibrator to the B output and when signal Z ceases the counter R provides an output control signal. A timer K resets the counter if the intervals between code elements are unduly long. To receive a call XYG, assuming the multivibrator is in its B state when signal X commences, Fig. 3 (not shown) this signal is passed by filter F which is in its initial state, and when it ceases the multivibrator is locked in what is then its state A via gate 4 but immediately changes to state B. During the pause between the X and Y signals it changes back to state A so that when signal Y appears the filter F cannot immediately be switched to receive this signal but only after the multivibrator has again changed to the B state where it is locked via AND gate 5. When signal Y ceases the multivibrator changes to state A so that filter F is switched to receive code element G but during the pause between the Y and G signals the multivibrator changes to state B so that filter F is switched to receive signal Y when G appears. When the multivibrator again changes state, gate 2 is enabled so that filter F passes the signal G and the multivibrator is locked on the A output and remains locked until signal G and the output of trigger T ceases and the counter delivers the control signal. In a modification, Fig. 4 (not shown), the multivibrator AB is replaced by a monostable device and the apparatus always first switches the filter F for the reception of code XYZ which is assumed to be the one most frequently used.