Patent GB-1411945-A (GB)
Title: Digital signal generators
IPC: H04M1/27495
Filed: 1971-10-29
Abstract: 1411945 Selective signalling POST OFFICE 30 Oct 1972 [29 Oct 1971] 50373/71 Heading G4H [Also in Division H4] In a push button dialling apparatus in which the whole number to be transmitted can be set up in a storage means prior to its transmission to the exchange means are provided whereby a visual display of the stored number can be set up.  In a first embodiment digits of a required number are fed into a shift register 21A-21D in parallel binary form from a push button operated diode encoding matrix. To obtain a visual display of the stored digits a button 10A is depressed setting bi-stable 10 which causes the stored digits to be read out of the shift register in turn and via a loop including a control circuit 33 back to the shift register input under the control of a 1 Hz oscillator 18, which oscillator also causes gates 45-48 to open thereby causing each binary coded digit in turn to illuminate a numicator 50. A counter 6 counts the number of digits cycled and resets check bistable 10 when the contents of all the shift register stages have been cycled. Each digit of the stored number is hence displayed for ¢ sec. with a ¢ sec. gap between digits. If the number of stored digits is less than the capacity of the shift register 21, a 10 kHz oscillator is energized to rapidly step on the shift register until counter 6 indicates all the stages have been cycled when a digit detector gate 61 determines no digit is present at the output of gates 45-48. To transmit the stored number to the exchange a send switch 11A is actuated to set bi-stable 11 which causes energization of a 10 Hz multivibrator 64. The stored digits are recycled through the shift register as for the visual display operation except that the cycling rate is governed by monostable 71. The output of the 10 Hz multivibrator is used to switch the pulsing relay 68 to supply the dialling pulses. A comparator 60 compares the count value of a binary counter 67 driven by the multivibrator 64 with the binary representation of the digit read out of the shift register and supplied thereto via, the control circuit 33, a code modifier, comprising gates 51-55, which modifies binary 0 (0000) to binary 10 (1010), and gates 56-59. When the counts are equal monostable 71 is switched by the comparator to stop the shift register, and, to inhibit the multivibrator 64 so as to provide the interdigital pause, the monostable 71 resetting counter 67 when it relapses. A relay 66 mutes the receiver during transmission of digits. When the handset is replaced, bi-stable 3 is set the shift register is cleared (all stages set to 1111) and the keyboard inhibited. The 10 kHz multivibrator is set to run during the on hook periods to cycle counter 6, which counter causes bi-stable 3 to reset at the 111 count stage subsequent to the handset next being removed. The clearing of the shift register can be inhibited by the depression of store switch 2 to provide for a recall without dialling feature. Figs. 2 and 3 (not shown) illustrate a similar dialling apparatus in which a separate visual display is provided for each digit so as to simultaneously display the whole number. Bistable storage elements are used in place of the shift register, and a counter and gate arrangement is used to feed each digit in turn to a comparator, the transmission of the digits taking place as in the previous embodiment. Means are included for providing an extra long interdigital pause if the first digit to be transmitted is an access code, e.g. if the digit 9 is transmitted in PBX to gain access to the public system, and several different ways of providing the delay are described.  The dialling apparatus may incorporate an arithmetic unit or may be associated with an arithmetic unit or a comparator over an exchange line. The visual display may utilize decimal numeral indicator tubes or solid state light emitting diodes. Use is made of integrated circuitry. Means may be provided for storing more than one telephone number at a time. Also the transmission of the digits to an exchange may be in a form other than loop disconnect decadic signalling.