Patent GB-1354854-A (GB)
Title: Integrated circuit decoder responsive to two sequential tones with group call provisions
IPC: H04W88/02
Filed: 1972-05-25
Abstract: 1354854 Radio Signalling MOTOROLA Inc 25 May 1972 [9 June 1971] 24782/72 Heading H4L A decoder in a radio receiver in a paging system responds to two different tones A and B received sequentially which are supplied to terminals 20, 35 respectively. When the tone A applied to the base of transistor 24 reaches a certain peak-to-peak amplitude, transistor 25 is periodically turned on and capacitor 28 is discharged until it reaches a point at which transistor 30 is cut off. When tone A ends, transistor 25 will turn off allowing capacitor 28 to charge via resistor 27, whereupon transistor 30 conducts again. When transistor 30 turns on, a capacitor 31 is charged via resistor 33 and the resulting voltage drop turns off transistor 32 so that its collector voltage rises and then falls as determined by the time constant of 31, 33. Tone B is applied to a threshold detector 38, 39 operating in a similar manner to transistors 24, 25, the time constant of circuit 40, 41 being much shorter than that of 27, 28 and arranged to turn off transistor 42 ahead of the pulse produced at the collector of transistor 32 for tone A. When transistors 32 and 42 are both off, transistors 45, 46 in an AND gate conduct and transistor 50 is turned on to unmute the audio amplifier via terminal 17 and render operative a multivibrator 52, 54, 56 via a transistor 58. The multivibrator controls via transistor 68 an alert tone oscillator 61, 62, 63 including filter 65, to provide a warning tone interrupted at the multivibrator frequency via capacitor 70. To hold the tone oscillator on for the duration of the B tone trigger voltage from transistor 42, transistor 32 is held cut off by a transistor 72 which conducts when transistor 50 conducts. To test the decoder, a capacitor 29 is charged when it is switched on initially and cuts off transistor 32 to turn on transistor 45. Transistor 46 also conducts due to the charge on capacitor 41 holding transistor 42 cut off and transistor 50 is conductive for a short time determined by the time constant of 40, 41. If it is desired to hold the alert tone oscillator operative for a period after the termination of tone B a monostable multivibrator including transistors 74, 75, 76 and diode connected transistor 77 is provided. A capacitor 80 initially is discharged and when transistor 50 conducts transistor 74 is switched on and transistor 75 off thus causing transistor 82 to conduct which co-operates with transistor 45 to hold transistor 50 conducting. Capacitor 80 charges while transistor 74 is conducting until the voltage at the base of transistor 74 falls to the point at which it is cut off to stop the tone oscillator. A group call decoder 19 is described, Fig. 4 (not shown), which is responsive to a signal consisting of tone B alone for a duration longer than for a normal paging cell, this causing the alert tone oscillator to operate continuously. As described, the decoder is constructed as an integrated circuit on a thick film base with certain components connected externally to facilitate changing.