Patent GB-863174-A (GB)
IPC: H04W88/02
Abstract: 863,174. Apparatus responsive to particular call signals. SCANTLIN, J. R. Sept. 17, 1958 [Sept. 30, 1957; Sept. 2, 1958], No. 29748/58. Class 40 (1). Apparatus adapted to respond to a signal of the kind shown in Fig. 2, having a series of groups of frequency changes each group, by the number of changes, representing a digit, comprises means to count the changes, means to count the groups, an array of gating connections to the stages of the counters to give an indication when the correct digits are received in the correct order and means to reset the counters after each digit if it is incorrect. The signal which consists of two audio tones 26, Fig. 1, sent alternately, is applied to a circuit 10 having a pair of series resonant circuits, the outputs of which are rectified to produce a pair of output signals 28, 29. These are similar except that one is negative and the other positive. The input trigger 11, which consists of a pair of transistors, the collector of each being connected to the base of the other, receives signals 28, 29 and produces from the second transistor a voltage change 31 from every tone change and, by connection through a condenser and a rectifier, a signal 30 having a pulse for every other tone change. Signal 31 is applied to a transistor inverter 12 which produces pulses 32 for every other pulse change. These are combined with pulses 30 at 33 to produce pulses 27 at each tone change which pass to a number counter 16. The inverter 12 also produces output 34 having a pulse for each tone change. This is applied to the digit generator 13 to produce signals 37, 38 for the first tone change of each group of changes. Each pulse 37, 38 therefore corresponds to each group of changes. Each pulse 37, 38 therefore corresponds to a separate digit received. The digit pulse 37 passes to a digit counter 15 and the pulse 38 to a gate circuit 14 which, wheno pen, passes it to reset counters 15 and 16. The outputs from stages of each counter are taken to each of the AND gates 17-21, the connections being such that an output from a gate appears only when there are predetermined counts in the two counters. Where the signal received has five digits the first gate 17 responds to the first digit, the second gate 18 responds to the second digit and so on. The outputs of the first four gates pass to OR gate 22 and the output from this gate passes to the reset gate 14 to block the passage of the reset pulse 38. While the first four correct digits are received, therefore, the counters are not reset. The fifth gate 21, if the fifth digit is correct, passes its output to a trigger 23 for the operation of a relay. The initial single change of the signal shown in Fig. 2 resets the counters. Because the gate 14 is open the first change of the first digit " 3 " will also give a resetting pulse but the next two changes will advance the number counter two counts. The first gate 17 is accordingly connected to respond to a zero count in digit counter and two in counter 16. Its output to gate 22 blocks gate 14 to prevent resetting so that the next digit " 3 " advances counter 16 to five and counter 14 to one. The second gate 18 is connected to respond to this combination and its output again blocks gate 14 to prevent resetting. The output of the fifth gate is not connected to the gate 22 so that the next pulse 38 can reset the counters. An incorrect digit in the first four would fail to produce an output from the corresponding gate 17-20 so that pulses 38 reset the counters. An incorrect fifth digit fails to produce an output to set trigger 23. The digit counter 15 consists of a series of three transistor trigger stages. The number counter is similarly designed but with more stages. Connections are made from certain stages of these counters to a matrix of diodes serving as the AND gates. The diode gates may be replaced by voltage-adding resistors.