Frequency changeable Tone detector to receive and give wrong signals to a Plurality of tone in succession

摘要

1412897 Selective signalling MOTOROLA Inc 7 Nov 1973 [7 Nov 1972] 51635/73 Heading G4H [Also in Division H3] The invention relates to a switchable frequency filter which is capable of responding to a plurality of predetermined tones in rapid succession. The filter has a predetermed energy build up and decay rate in response to the tones, and speed of operation is ensured by temporarily increasing the bandwidth of the filter after detection of a tone frequency to increase the decay rate before selecting the next frequency to which the filter is to respond. The filter 19 comprises operational amplifiers 38, 43, 50 with a resistor 48 and a capacitor 49 which respectively determine the pass frequency and the bandwidth of the filter. Connection of a capacitor 60 in parallel with capacitor 49 increases the bandwidth, and connection of one or more resistances 67 ... 90 in parallel with resistor 48 alters the frequency to be passed. A signal containing an audio part and a sequence of tones constituting an address is received at 10, 11 and passed to amplifier 12 which is held inoperative until a circuit 15 containing filter 19 has detected the required tone sequence. Detection of a frequency causes a counter 21 to be incremented and a timing circuit 22 to switch capacitor 60 into filter 19 so as to increase the bandwidth for a predetermined time. The output from 21 causes a diode matrix 23 to output a signal 24 to the filter to switch in a new combination of resistors 67 ... 90 so as to select the next frequency. When all the tones have been detected, counter 21 actuates an "audio alert" device 30 and enables amplifier 12.