Tone control circuit having a frequency controllable filter

Abstract

A tone control circuit is in a receiver adapted to respond to a sequence of control tones alternately selected from a first group of tones in a first band of frequencies and a second group of tones in a second band of frequencies, wherein the two bands are separated by an intermediate band and wherein the time duration of the gap between adjacent tones in the sequence of tones is substantially zero, the tone control circuit including a filter device which is tuned to the frequencies of the control tones as they are received, a plurality of AND circuits corresponding in number to the control tones and respectively having one input coupled to the filter device and another input coupled to the preceding AND circuit so that each AND circuit is operative to produce an output only in the presence of a tone being passed by the filter and an output signal from the preceding AND circuit, the filter device being tuneable either manually or electronically via the outlet signals from the AND circuits.

What I claim is:  .[.1. A communication system comprising a transmitter including a tone-generating circuit for generating tones selected from a first group of tones in a first band of frequencies and for generating tones selected from a second group of tones in a second band of frequencies separate and distinct from said first band of frequencies, said first band of frequencies being separated from second band of frequencies by a substantial intermediate band of frequencies, said tone-generating circuit generating a sequence of tones alternately selected from said first and second groups of tones, the time duration of the gap between adjacent tones being substantially zero, a transmitter output circuit coupled to said tone-generating circuit for transmitting signals corresponding to the sequence of tones a receiver including an input circuit for receiving the signals from said transmitter, a first tone control circuit, a second tone control circuit, said first tone control circuit being coupled to said input circuit and to said second tone control circuit and responsive to the application of tones from said first group of tones to said input circuit to provide first control signals to said second tone control circuit, said second tone control circuit being coupled to said input circuit and to said first tone control circuit and responsive to the application of the tones from said second group of tones to said input circuit to provide second control signals to said first tone control circuit, one of said tone control circuits furnishing an output control signal in response to the application of the last tone in the sequence of tones, and an output signal utilization circuit coupled to said one tone control circuit for utilizing said output control signal therefrom..]. .[.2. The communication system, set forth in claim 1, wherein certain of the tones in said second band of frequencies are harmonics of certain tones in said first band of frequencies..]. .[.3. The communication system set forth in claim 1, wherein said first band of frequencies and said second band of frequencies are in the audio range of frequencies..]. .[.4. The communication system set forth in claim 1, wherein each of said band of frequencies contains at least ten separate tones with each tone being separated from adjacent tones by about 31/3 percent of the frequency thereof..]. .[.5. The communication system set forth in claim 1, wherein the frequency of a tone is slightly greater than 1.035 times the frequency of the next lower tone..]. .[.6. The communication system set forth in claim 1, wherein said transmitter includes a generator for generating a carrier signal that is modulated by the control tones..]. .[.7. The communication system set forth in claim 6, wherein the control tones are applied to the carrier signal by frequency modulation..]. .[.8. The communication system set forth in claim 1, wherein said sequence of tones comprises one tone selected from said first groupo of tones and having a time duration not substantially greater than 40 milliseconds and one tone selected from said second group of tones and having a time duration of at least not substantially greater than 40 milliseconds..]. .[.9. The communication system set forth in claim 1, wherein said sequence of tones comprises two tones selected from said first group of tones having a time duration not substantially greater than 40 milliseconds and two tones selected from said second group of tones each having a time duration of not substantially greater than 40 milliseconds..]. .[.10. The communication system set forth in claim 1, wherein said sequence of tones comprises four tones selected from said first group of tones and each having a time duration not substantially greater than 40 milliseconds and three tones selected from said second group of tones each having a time duration of not substantially greater 
     
        than 40 milliseconds..]. 11. In a communication system for selectively transmitting signals including a sequence of control tones alternately selected from a first group of tones in a first band of frequencies and from a second group of tones in a second band of frequencies separate and distinct from said first band of frequencies, a receiver comprising an input circuit for receiving the signals from an associated transmitter, a first tone control circuit, a second tone control circuit, means coupling said first tone control circuit to said input circuit and to said second tone control circuit, said first tone control circuit having a first tapped filter therein adjustable to a selected one of a plurality of positions respectively corresponding to a selected tone in said first group of tones so that upon the application of the selected tone to said input circuit there is produced first control signals that are applied to said second tone control circuit means coupling said second tone control circuit to said input circuit and to said first tone control circuit, said second tone control circuit having a second tapped filter therein adjustable to a selected one of a plurality of positions respectively corresponding to a selected tone in said second group of tones so that upon the application of the selected tone to said input circuit there is produced second control signals that are applied to said first tone control circuit, one of said tone control circuits furnishing an output control signal in response to the application of the last tone in the sequence of tones, an output signal utilization circuit coupled to said one tone control circuit for utilizing said output control signal 
     
     
        therefrom. 12. The receiver set forth in claim 11, wherein said first tapped filter is adjustable to pass a selected tone in a first band of frequencies and said second tapped filter is adjustable to pass a selected tone in a second band of frequencies, said first band of frequencies being separated from said second band of frequencies by a substantial 
     
     
        intermediate band of frequencies. 13. The receiver set forth in claim 11, wherein said first and second tapped filters are adjustable to pass tones 
     
     
        that are in an audio range of frequencies. 14. The receiver set forth in claim 11, wherein each of said tapped filters is adjustable to pass a selected one of at least ten separate tones, each tone being separated from the adjacent tones by about 31/3 percent of the frequency thereof. 
     
     
            The receiver set forth in claim 11, and further comprising a holding circuit interconnecting said first and second tone control circuits so as to accommodate tone duration of any length longer than a delay period provided the time gap between adjacent tones in the sequence of tones is 
     
     
        substantially zero. 16. The receiver set forth in claim 11, wherein said first tapped filter has a connection to only one of the taps thereon and said second tapped filter has a connection to only a signal tap thereon, whereby said receiver is responsive to a sequence of two selected tones. 
     
     
          . The receiver set forth in claim 11, wherein said first tapped filter has connections to two of the taps thereon corresponding to two tones in said first group of tones, and said second tapped filter has connections to two gaps thereon corresponding to two tones selected from said second group of tones, whereby said receiver is responsive to a sequence of four 
     
     
        control tones. 18. The receiver set forth in claim 11, wherein said first tapped filter has connections to four of the taps thereon corresponding to four tones selected from said first group of tones, and said second tapped filter has connections to three taps thereon corresponding to three tones selected from said second group of tones, whereby said receiver is 
     
     
        responsive to a sequence of seven selected tones. 19. The receiver set forth in claim 11, wherein the selection of connections to the taps on 
     
     
        said tapped filters is made mechanically. 20. The receiver set forth in claim 11, wherein the connection to the selected tap on said tapped filter 
     
     
        is made electronically. 21. The receiver set forth in claim 11, wherein the selection of the tones to which the receiver is responsive is made by the connectors which connect said tapped filters to the tone receiver 
     
     
        components. 22. A communication receiver for responding to a sequence of first and second control tones, said receiver comprising an input circuit for receiving the control tones, a first filter coupled to said input circuit and tuned to the frequency of the first control tone and providing a signal which has an amplitude dependent upon the frequency of the control tone applied thereto, a first electronic switching device coupled to said first filter and responsive to a signal from said first filter exceeding the threshold of said first switching device to provide a DC voltage of fixed value independent of the amount by which the amplitude of the signal from said first filter exceeds said threshold level, a first delay circuit coupled to said first electronic switching device and operative to produce at its output a first control signal only when the first control tone has persisted for a predetermined fixed duration, a second filter coupled to said input circuit and tuned to the frequency of the second control tone and providing a signal which has an amplitude dependent upon the frequency of the control tone applied thereto, a second electronic switching device coupled to said first delay circuit and to said second filter and responsive to both said first control signal and to a signal exceeding the threshold level of said second switching device to provide a DC voltage of fixed value and independent of the amount by which the amplitude of the signal from said second filter exceeds said threshold level, a second delay circuit coupled to said second electronic switching device and operative to produce at its output a second control signal only when the second control tone has persisted for a predetermined fixed duration, and a control signal utilization circuit coupled to said second 
     
     
        delay circuit for utilizing said second control signal. 23. The communication receiver set forth in claim 22, wherein each of said electronic switching devices is a transistor means having its base coupled to the associated filter and having its collector and emitter coupled to 
     
     
        the associated delay circuit. 24. The communication receiver set forth in claim 22, wherein each of said delay circuits includes a capacitor and resistor coupled in series across the output of the associated electronic switching device, and the output of each delay circuit is the junction of 
     
     
        the resistor and capacitor. 25. A tone control circuit for use in a communication receiver including an input circuit which provides a sequence of control tones and including an output signal utilization circuit, said tone control circuit comprising filter means coupled to the input circuit and having a plurality of control inputs for receiving one at a time signals selectively to tune said filter means to predetermined frequencies, a plurality of AND circuits corresponding in number to the number of control tones in the sequence of control tones and each having first and second inputs and an output, control means for providing at the output thereof a control signal during reception of the first control tone in the sequence of control tones, the output of said control means being coupled to one of the control inputs of said filter means and to the first input of the first of said AND circuits, said filter means being responsive to the application of the control signal from said control means to be turned to the frequency of said first control tone, means coupling the output of said filter means to the second input of each of said AND circuits, said first AND circuit being responsive to the application thereto of both the control signal from said control means and said first control tone for producing at the output thereof an output signal, the output of said first AND circuit and the output of each succeeding AND circuit being coupled respectively to the first input of the next succeeding AND circuit, means coupling the outputs of said AND circuits respectively to the control inputs of said filter means, means coupled to said control means for removing the control signal therefrom after termination of said first control tone, said second AND circuit and each succeeding AND circuit being responsive to the application thereto of both the output signal from the next preceding AND circuit and the associated control tone in the sequence of control tones from said filter means for producing at the output thereof an output signal, said filter means sequentially being tuned to the frequencies of the control tones as they are received by the input circuit in accordance with the sequential application of said output signals to said control inputs, and means coupling the output of the last of said AND circuits to the output signal 
     
     
        utilization circuit of the receiver. 26. The tone control circuit set forth in claim 25, and further comprising a plurality of delay circuits corresponding in number to the number of control tones in the sequence of control tones and coupled respectively between said AND circuits, the last of said delay circuits being coupled between the last of said AND circuits and said output signal utilization circuit, each of said delay circuits being operative to produce an output signal only when the associated 
     
     
        control tone has persisted for a predetermined duration. 27. A tone control circuit for use in a communication receiver including an input circuit which provides a sequence of control tones and including an output signal utilization circuit, said tone control circuit comprising first filter means coupled to the input circuit and having a plurality of control inputs for receiving one at a time signals selectively to tune said first filter means to predetermined frequencies, second filter means coupled to the input circuit and having a plurality of control inputs for receiving one at a time signals selectively to tune said second filter means to predetermined frequencies, a plurality of AND circuits corresponding in number to the number of control tones in the sequence of control tones and each having first and second inputs and an output, control means for providing at the output thereof a control signal during reception of the first control tone in the sequence of control tones, the output of said control means being coupled to one of the control inputs of said first filter means and to the first input of the first of said AND circuits, said first filter means being responsive to the application of the control signal from said control means to be tuned to the frequency of said first control tone, means coupling the output of said first filter means to the second input of said first AND circuit and to the second input of each succeeding odd-numbered AND circuit, means coupling the output of said second filter means to the second input of said second AND circuit and to the second input of each succeeding even-numbered AND circuit, said first AND circuit being responsive to the application thereto of both the control signal from said control means and said first control tone for producing at the output thereof an output signal, the output of said first AND circuit and the output of each succeeding AND circuit being coupled respectively to the first input of the next succeeding AND circuit, means coupling the outputs respectively of said first AND circuit and of each succeeding odd-numbered AND circuit respectively to the control inputs of said second filter means, means coupling the outputs respectively of said second AND circuit and of each succeeding even-numbered AND circuit respectively to the control inputs of said first filter means, means coupled to said control means for removing the control signal therefrom after termination of said first control tone, said second AND circuit and each succeeding AND circuit being responsive to the application thereto of both the output signal from the next preceding AND circuit and the associated control tone in the sequence of control tones from the associated filter means for producing at the output thereof an output signal, said first and second filter means alternately being tuned to the frequencies of the control tones as they are received by said input circuit by the sequential application to said control inputs of said output signals, and means coupling the output of the last of said AND circuits to the output signal utilization circuit of the receiver. 
     
     
            The tone control circuit set forth in 27 and further comprising a first rectifier circuit coupled between said first filter means and the second input of each odd-numbered AND circuit, and a second rectifier circuit coupled between said second filter means and the second input of 
     
     
        each even-numbered AND circuit. 29. The tone control circuit set forth in claim 27, wherein the output of said third AND circuit and the output of each succeeding odd-numbered AND circuit is coupled to said control means for removing the control signal therefrom at least during the presence of said third control tone and during the presence of each succeeding 
     
     
        odd-numbered control tone. 30. A tone control circuit for use in a communication receiver including an input circuit which provides a sequence of control tones and including a DC voltage utilization circuit, said tone control circuit comprising filter means coupled to the input circuit and having a plurality of control inputs for receiving one at a time signals selectively to tune said filter means to predetermined frequencies, a plurality of AND circuits corresponding in number to the number of control tones in the sequence of control tones and each having first and second inputs and an output, control means for providing at the output thereof a control signal during reception of the first control tone in the sequence of control tones, the output of said control means being coupled to one of the control inputs of said filter means and to the first input of the first of said AND circuits, said filter means being responsive to the application of the control signal from said control means to be tuned to the frequency of said first control tone, means coupling the output of said filter means to the second input of each of said AND circuit, said first AND circuit being responsive to the application thereto of both the control signal from said control means and said first control tone for producing at the output thereof an output signal, a plurality of electronic switching circuits corresponding in number to the number of control tones in the sequence of control tones and each having an input and output, the output of said first AND circuit being coupled to the input of the first of said electronic switching circuits and the output of each succeeding one of said AND circuits being respectively coupled to the input of the next succeeding one of said electronic switching circuits, the output of said first electronic switching circuit being coupled to the first input of .[.said first AND circuit.]. .Iadd.the second one of said AND circuits .Iaddend.and the output of each succeeding one of said electronic switching circuits being coupled respectively to the first input of the next succeeding one of said AND circuits means coupling the outputs of said electronic switching circuits respectively to the control inputs of said filter means, said first electronic switching circuit being responsive to the application thereto of the output signal from said first AND circuit for producing at the output thereof a DC voltage, means coupled to said control means for removing the control signal therefrom after termination of said first control tone, said second AND circuit and each succeeding AND circuit being responsive to the application thereto of both the DC voltage from the next preceding electronic switching circuit and the associated control tone in the sequence of control tones from said filter means for producing at the output thereof an output signal, said second electronic switching circuit and each succeeding electronics switching circuit being responsive to the application thereto of the output signal from the next preceding AND circuit for producing at the output thereof a DC voltage, said filter means sequentially being tuned to the frequencies of the control tones as they are received by the input circuit in accordance with the sequential application of said DC voltages to said control inputs, and means coupling the output of the last of said electronic switching circuits to the DC 
     
     
        voltage utilization circuit of the receiver. 31. A tone control circuit for use in a communication receiver including an input circuit which provides a sequence of control tones and including a DC voltage utilization circuit, said tone control circuit comprising filter means coupled to the input circuit and having a plurality of control inputs for receiving one at a time signals selectively to tune said filter means respectively to predetermined frequencies, a plurality of AND circuits corresponding in number to the number of control tones in the sequence of control tones and each having first and second inputs and an output, control means for providing at the output thereof a control signal during reception of the first control tone in the sequence of control tones, the output of said control means being coupled to one of the control inputs of said filter means and to the first input of the first of said AND circuits, said filter means being responsive to the application of the control signal from said control means to be tuned to the frequency of said first control tone, means coupling the output of said filter means to the second input of each of said .[.first.]. AND circuits, said first AND circuit being responsive to the applications thereto of both the control signal from said control means and said first control tone for producing at the output thereof an output signal, a plurality of electronic switching circuits corresponding in number to the number of control tones in the sequence of control tones and each having an input and output, the output of said first AND circuit being coupled to the input of the first of said electronic switching circuits and the output of each succeeding one of said AND circuits being respectively coupled to the input of the next succeeding one of said electronic switching circuits, the output of said first electronic switching circuit being coupled to the first input of .[.said first AND circuit.]. .Iadd.the second one of said AND circuits .Iaddend.and the output of each succeeding one of said electronic switching circuits being coupled respectively to the first input of the next succeeding one of said AND circuits, means coupling the outputs of said electronic switching circuits respectively to the control inputs of said filter means, said first electronic switching circuit being responsive to the application thereto of the output signal from said first AND circuit for producing at the output thereof a DC voltage, means coupled to said control means for removing the control signal therefrom after termination of said first control tone, said second AND circuit and each succeeding AND circuit being responsive to the application thereto of both the DC voltage from the next preceding electronic switching circuit and the associated control tone in the sequence of control tones from said filter means for producing at the output thereof .[.and.]. .Iadd.an .Iaddend.output signal, said second electronic switching circuit and each succeeding electronic switching circuit being responsive to the application thereto of the output signal from the next preceding AND circuit for producing at the output thereof a DC voltage, the output of said second AND circuit and the output of each succeeding one of said AND circuits being coupled respectively to an input of the next preceding one of said electronic switching circuits to enable each of said electronic switching circuits to continue to produce said DC voltage as long as the associated control tone is being applied to the next succeeding one of said AND circuits, said filter means sequentially being tuned to the frequencies of the control tones as they are received by said input circuit in accordance with the sequential application of said output signals to said control inputs, and means coupling the output of the last of said electronic switching circuits to the DC voltage utilization 
     
     
        circuit of the receiver. 32. A tone control circuit for use in a communication receiver including an input circuit which provides a sequence of control tones and including an output signal utilization circuit, said tone control circuit comprising first filter means coupled to the input circuit and including a first frequency, determining element having a plurality of inputs thereto, a plurality of electronic switching devices coupled respectively to the inputs of said first frequency-determining element, second filter means coupled to the input circuit and including a second frequency-determining element having a plurality of inputs thereto, a plurality of electronic switching devices coupled respectively to the inputs of said second frequency-determining element, each of said electronic switching devices being closed by the application thereto of a signal to couple in circuit a portion of the associated frequency-determining element and thereby tune the associated filter means to a predetermined frequency, a plurality of AND circuits corresponding in number to the number of control tones in the sequence of control tones and each having first and second inputs and an output, control means for providing at the output thereof a control signal during reception of the first control tone in the sequence of control tones, the output of said control means being coupled to one of the control inputs of said first filter means and to the first input of the first of said AND circuits, said first filter means being responsive to the application of the control signal from said control means to be tuned to the frequency of said first control tone, means coupling the output of said first filter means to the second input of said first AND circuit and to the second input of each succeeding odd-numbered AND circuit, means coupling the output of said second filter means to the second input of said second AND circuit and to the second input of each succeeding even-numbered AND circuit, said first AND circuit being responsive to the application thereto of both the control signal from said control means and said first control tone for producing at the output thereof an output signal, the output of said first AND circuit and the output of each succeeding AND circuit being coupled respectively to the first input of the next succeeding AND circuits, means coupling the outputs respectively of said first AND circuit and of each succeeding odd-numbered AND circuit respectively to the control inputs of said second filter means, means coupling the outputs respectively of said second AND circuit and of each succeeding even-numbered AND circuit respectively to the control inputs of said first filter means, means coupled to said control means for removing the control signal therefrom after termination of said first control tone, said second AND circuit and each succeeding AND circuit being responsive to the application thereto of both the output signal from the next preceding AND circuit and the associated control tone in the sequence of control tones from the associated filter means for producing at the output thereof an output signal, said first and second filter means alternately being tuned to the frequencies of the control tones as they are received by said input circuit by the sequential application to said control inputs of said output signals, and means coupling the output of the last of said AND circuits to the output signal utilization circuit of the receiver. 
     
     
            The tone control circuit set forth in claim 32, wherein each of said 
     
     
        frequency-determining elements is a tapped inductor. 34. In a communication receiver for receiving signals including intelligence and a sequence of control tones, a receiver comprising an input circuit which provides the signals including the intelligence and the sequence of control tones, a translating circuit coupled to said input circuit for translating the intelligence into useful form, an output control circuit coupled to said translating circuit and effective in a first condition thereof to render said translating circuit inoperative and effective in a second condition thereof to render said translating circuit operative, filter means coupled to the input circuit and having a plurality of control inputs for receiving one at a time signals selectively to tune said filter means respectively to predetermined frequency.Iadd., .Iaddend.a plurality of AND circuits corresponding in number to the number of control tones in the sequence of control tones and each having first and second inputs and an output, control means for providing at the output thereof a control signal during reception .Iadd.of the .Iaddend.first control tone in the sequence of control tones, the output of said control means being coupled to one of the control inputs of said filter means and to the first input of the first of said AND circuits, said filter means being responsive to the application of the control signal from said control means to be tuned to the frequency of said first control tone, means coupling the output of said filter means to the second input of each of said AND circuits, said first AND circuit being responsive to the application thereto of both the control signal from said control means and said first control tone for producing at the output thereof an output signal, the output of said first AND circuit and the output of each succeeding AND circuit being coupled respectively to the first input of the next succeeding AND circuit, means coupling the outputs of said AND circuits respectively to the control inputs of said filter means, means coupled to said control means for removing the control signal therefrom after termination of said first control tone, said second AND circuit and each succeeding AND circuit being responsive to the application thereto of both the output signal from the next preceding AND circuit and the associated control tone in the sequence of control tones from said filter means for producing at the output thereof an output signal, said filter means sequentially being tuned to the frequencies of the control tones as they are received by said input circuit in accordance with the sequential application of said output signals to said control inputs, and means for applying the output signal from the last of said AND circuits to said output control circuit for actuating said output control circuit from the first condition thereof to the second condition thereof so as to render 
     
     
        said translating circuit operative. 35. A communication system comprising a transmitter including a tone-generating circuit for generating tones selected from a first group of tones in a first band of frequencies and for generating tones selected from a second group of tones in a second band of frequencies separate and distinct from said first band of frequencies, said tone-generating circuit generating a sequence of control tones alternately selected from said first and second groups of tones, and a transmitter output circuit coupled to said tone-generating circuit for transmitting signals corresponding to the sequence of control tones to be transmitted; and a receiver including an input circuit for receiving the signals from said transmitter, filter means coupled to the input circuit and having a plurality of control inputs for receiving one at a time signals selectively to tune said filter means to predetermined frequencies, a plurality of AND circuits corresponding in number to the number of control tones in the sequence of control tones and each having first and second inputs and an output, control means for providing at the output thereof a control signal during reception of the first control tone in the sequence of control tones, the output of said control means being coupled to one of the control inputs of said filter means and to the first input of the first of said AND circuits, said filter means being responsive to the application of the control signal from said control means to be tuned to the frequency of said first control tone, means coupling the output of said filter means to the second input of each of said AND circuits, said first AND circuit being responsive to the application thereto of both the control signal from said control means and said first control tone for producing at the output thereof an input signal, the output of said first AND circuit and the output of each succeeding AND circuit being coupled respectively to the first input of the next succeeding AND circuit, means coupling the outputs of said AND circuits respectively to the control inputs of said filter means, means coupled to said control means for removing the control signal therefrom after termination of said first control tone, said second AND circuit and each succeeding AND circuit being responsive to the application thereto of both the output signal from the next preceding AND circuit and the associated control tone in the sequence of control tones from said filter means for producing at the output thereof an output signal, said filter means sequentially being tuned to the frequencies of the control tones as they are received by said input circuit in accordance with the sequential application of said output signals to said control inputs, and an output signal utilization circuit coupled to the output of the last of said AND 
     
     
        circuit for utilizing the output signal therefrom. 36. A tone control circuit for use in a communication receiver including an input circuit which provides a control tone and including an output signal utilization circuit, said tone control circuit comprising filter means coupled to the input circuit and including a first frequency-determining element having a plurality of taps thereon and a second frequency-determining element, a connector plug including first and second and third portions, said first portion having a plurality of terminals thereon fixedly coupled respectively to the taps on said first frequency-determining element, said second portion having a terminal fixedly coupled to said second frequency-determining element, said third portion having a plurality of first terminals respectively matable with the terminals in said .[.first.]. .Iadd.second .Iaddend.portion and having a second terminal matable with the terminal in said .[.second.]. .Iadd.first .Iaddend.portion, means associated with said third portion connecting said first terminal to a selected one of said second terminals, whereby mating said third portions with said first and second portions operates to connect a selected section of said first frequency-determining element in circuit with said second frequency-determining element and thereby tune said filter means to the frequency of the control tone, an output circuit coupled to said filter means and responsive to the control tone being coupled therethrough to provide an output signal, and means coupling the output of said output circuit to the output signal utilization circuit of 
     
     
        the receiver. 37. The tone control circuit set forth in claim 36, wherein said first frequency-determining element is an inductor and said second 
     
     
        frequency-determining element is a capacitor. 38. The tone control circuit set forth in claim 36, wherein said first frequency-determining element is an inductor and said second frequency-determining element is a capacitor, one terminal of said capacitor being fixedly coupled to one terminal of said inductor and said connector plug being operative to connect the other terminal of said capacitor to a tap on said inductor so as to define a 
     
     
        parallel resonant circuit. 39. A tone control circuit for use in a communication receiver including an input circuit which provides a sequence of control tones and including an output signal utilization circuit, said tone control circuit comprising filter means coupled to the input circuit and including a first frequency-determining element having a plurality of taps thereon and a second frequency-determining element, a set of electronic switching devices each having a control input and one terminal coupled to said second frequency-determining element, a connector plug including first and second and third portions, said first .[.section.]. .Iadd.portion .Iaddend.having a plurality of terminals thereon fixedly coupled respectively to the taps on said first frequency-determining element.Iadd., .Iaddend.said second portion having a corresponding set of terminals respectively fixedly coupled to the other terminals of said electronic switching devices, said third portion having a plurality of first terminals respectively matable with the terminals in said first portion and a set of second terminals respectively matable with the terminals of said .[.first.].  .Iadd.second .Iaddend.portion, means associated with said third portion for connecting selected ones of said first terminals respectively to selected ones of said second terminals, whereby mating said third portion with said first and second portions enables said electronic switching devices respectively to couple selected sections of said .[.second.]. .Iadd.first .Iaddend.frequency-determining element in circuit with said .[.first.]. .Iadd.second .Iaddend.frequency-determining element, an output circuit having an input coupled to said filter means and having a plurality of outputs respectively coupled to the control inputs of said electronic switching devices, said electronic switching devices sequentially being closed by the application of signals from said output circuit to said control inputs sequentially to tune said filter means to the frequencies of the control tones as they are received by the input circuit, and means coupling the output which is responsive to the last tone in the sequence of control 
     
     
        tones to the output signal utilization circuit of the receiver. 40. The tone control circuit set forth in claim 39, wherein each of said electronic switching devices is a transistor means having a base which is said control input and a pair of output electrodes in the form of a collector and an emitter, one of said output electrodes being coupled to the associated tap and the other of said output electrodes being coupled to said second frequency determining element. .Iadd. 41. A selective receiver for indicating the presence of a predetermined tone grouping in tone encoded paging signals comprising electric filter means sequentially tunable to pass individual tones of said grouping in a predetermined order, means for detecting said tone encoded signals and coupling same to the input of said filter means, sequence detector means coupled to the output of said filter means and arranged to cause said filter means to pass sequentially in said predetermined order the tones of said grouping upon the prior passage of preceding tones of said tone grouping, and indicator means responsive only to the passage through said filter means of the last of the tones of said grouping for indicating said receiver is being paged. .Iaddend..Iadd. 42. A selective receiver as in claim 41 wherein said sequence detector means includes counter means responsive to the output of said filter means for counting the number of tones passed through said filter means and for actuating said indicator means when a number corresponding to the number of tones in said tone grouping is counted. .Iaddend..Iadd. 43. A selective receiver as in claim 41 wherein said filter means is controllable to selectively pass distinct individual tones, and further including sequence control means responsive to said sequence detector means for controlling said filter means to sequentially pass tones in said predetermined order. .Iaddend. .Iadd. 44. A selective receiver as in claim 43 wherein said sequence control means is coupled to said filter means to quiescently tune said filter means to pass the first tone of said predetermined order and wherein said sequence detector means includes counter means responsive to the output of said filter means for counting the number of tones passed and for actuating said indicator means when all tones of said grouping are counted. .Iaddend..Iadd. 45. A selective receiver as in claim 44 wherein said counter means includes monostable multivibrator means and register means, each coupled to the output of said filter means, said monostable multivibrator means being responsive to at least the first tone passed through said filter means for operating said sequence control means and causing said filter means to tune to the second tone in said grouping, and said register means being arranged to be nonresponsive to said first tone pulse and to count the remaining tones in said sequence. .Iaddend..Iadd. 46. A selective receiver as in claim 45 wherein said counter means includes gate means coupled to the outputs of said filter means and of said monostable multivibrator means for rendering said register means nonresponsive to said first tone of said predetermined order and responsive to all succeeding tones of said grouping. .Iaddend..Iadd. 47. A selective receiver according to claim 41 wherein said sequence control means includes means for successively connecting said filter means to different impedance terminals for causing said filter means to pass different tones and means for separately changing the impedance applied to each terminal for changing the coding of said receiver. .Iaddend. .Iadd. 48. A selective receiver according to claim 41 wherein said sequence detector means is responsive to the prior passage of a tone by said electric filter means to connect said electric filter means to a different tuning impedance terminal in said predetermined order and wherein said sequence detector means further includes an impedance device capable of providing at each of several connecting points, greater in number than said tuning impedance terminals, a different impedance and connection means for selectively connecting each of said tuning impedance terminals to different ones of said connection points. .Iaddend..Iadd. 49. A selective receiver for indicating the presence of a predetermined frequency grouping in frequency encoded paging signals comprising: electric filter means controllable to selectively pass individual frequencies of said grouping, means for detecting said frequency encoded signals and coupling same to the input of said filter means, sequence control means coupled to said filter means for tuning same to sequentially pass frequencies in a predetermined order, sequence detector means connected to the output of said filter means for detecting the passage of the frequencies in said grouping, and indicator means responsive only to the last output of said sequence detector means for indicating said receiver is being paged, said sequence control means being coupled to the output of said filter means for operation in response to the outputs thereat. .Iaddend. .Iadd. 50. A selective receiver as in claim 49 wherein said sequence control means is preset to cause said filter means to pass the first frequency in said predetermined order and to pass successively the succeeding frequencies in said predetermined order upon the passage of their preceding frequency. .Iaddend..Iadd. 51. A selective receiver as in claim 50 wherein said sequence control means is constructed to return to its preset condition after at least one subsequent frequency of a detected encoded frequency grouping fails to pass through said filter means. .Iaddend..Iadd. 52. A selective receiver as in claim 50 wherein said sequence control means is operable in response to said sequence detector means to cause said sequential tuning of said filter means. .Iaddend..Iadd. 53. A selective receiver as in claim 52 wherein said sequence detector means includes counter means responsive to the output of said filter means for counting the number of distinct frequencies passed and for actuating said indicator means. .Iaddend..Iadd. 54. A selective receiver as in claim 53 wherein said sequence control means is reset by said counter means. .Iaddend..Iadd. 55. A selective receiver as in claim 54 wherein said sequence control means is operable to cause said sequential tuning of said filter means in response to said counter means. .Iaddend. .Iadd. 56. A selective receiver as in claim 55 wherein said counter means includes monostable multivibrator means and register means, said monostable multivibrator means being responsive to at least the first frequency output of said filter means, for operating said sequence control means and causing said filter means to tune to the second frquency of said predetermined order, and said counter circuit being arranged to be nonresponsive to said first frequency and to count the remaining frequencies of said predetermined order. .Iaddend..Iadd. 57. A selective receiver as in claim 56 wherein said sequence control circuit is controlled and reset by both said monostable multivibrator means and said counter circuit means. .Iaddend..Iadd. 58. A selective receiver for indicating the presence of a predetermined tone grouping in tone encoded paging signals comprising: active electric filter means arranged to have its bandpass range tunable in accordance with external circuit means, a plurality of external circuit means each arranged to be separately coupled to said active filter and to tune its bandpass range to pass different tones of said grouping, means for detecting said tone encoded signals and for coupling same to the input of said active filter means, sequence control means arranged to sequentially connect each of said plurality external circuit means to said active filter to cause said active filter to sequentially pass tones in a predetermined order, sequence detector means connected to the output of said active filter means for detecting the passage of the tones in said grouping in said predetermined order and indicator means responsive only to the last output of said sequence detector means for indicating that said receiver is being paged. .Iaddend. .Iadd. 59. A selective receiver as in claim 58 wherein said active filter means is constructed to have its bandpass range varied in response to externally connected impedance means, and wherein each of said external circuit means comprises impedance means for tuning said active filter means to pass a distinct tone of said tone grouping. .Iaddend..Iadd. 60. A selective receiver as in claim 59 wherein said paging signals are sequential tone encoded signals and said predetermined order is the order in which said tones occur in said grouping and wherein said sequence control means comprises switch means for individually coupling each of said impedance means to said active filter means in a sequential manner to pass tones in the sequence which the tones occur in said grouping. .Iaddend..Iadd. 61. A selective receiver as in claim 60 wherein said switch means is coupled to the output of said active filter means and are operable in response thereto. .Iaddend..Iadd. 62. A selective receiver as in claim 61 wherein said switch means is preset to cause said active filter means to pass the first tone of said grouping and to pass the succeeding tones of said grouping upon the passage of the preceding tones of said grouping. .Iaddend..Iadd. 63. A selective receiver as in claim 62 wherein said switch means is constructed to return to its preset condition after at least one subsequent tone of a detected tone encoded grouping fails to pass through said filter means. .Iaddend. .Iadd. 64. A selective receiver as in claim 60 wherein said switch means is coupled to said sequence detector means and is operable in response thereto to cause said sequential tuning of said filter means. .Iaddend..Iadd. 65. A selective receiver as in claim 64 wherein said sequence detector means includes counter means responsive to the output of said active filter means for actuating said indicator means. .Iaddend..Iadd. 66. A selective receiver as in claim 65 wherein said switch means is coupled to said counter means to be controlled and reset thereby. .Iaddend..Iadd. 67. A selective receiver as in claim 66 wherein said counter means includes monostable multivibrator means and register means, said monostable multivibrator means being responsive to at least the first tone output of said active filter means, for operating said sequence control means and for causing said active filter means to tune to the second tone in said grouping, and said register means being arranged to be nonresponsive to said first tone pulse and to count the remaining tones in said grouping. .Iaddend..Iadd. 68. A selective receiver as in claim 67 wherein said switch means is coupled to said monostable multivibrator means and is arranged to switch from its preset condition to cause said active filter means to pass the second tone of said grouping and wherein said switch means is coupled to said register means to cause said active filter means to pass the remaining tones in said grouping. .Iaddend. .Iadd. 69. A method of receiving and indicating the presence of a predetermined tone grouping in tone encoded paging signals comprising detecting said tone encoded paging signals, passing said tone encoded signals through an electric filter arranged to pass only the first tone of said grouping, thereafter sequentially retuning said filter to pass the remaining tones in said grouping in the sequence in which they are arranged in said grouping, and causing an output indication only upon the passage of all of said tones in said grouping through said filter. .Iaddend..Iadd. 70. A method of receiving and indicating the presence of a predetermined tone grouping as in claim 69 including the step of counting the number of tones passed through said filter and causing said indication upon the counting of the total number of tones in said grouping. .Iaddend..Iadd. 71. A method of receiving and indicating the presence of a predetermined tone grouping as in claim 69 wherein said filter comprises an active filter arranged to have its bandpass range tuned in accordance with a plurality externally connected circuit means, and wherein the sequential retuning of said active filter is carried out by sequentially connecting each of said plurality of circuit means to said active filter to cause said filter to pass tones in the sequence in which they occur in said grouping. .Iaddend. .Iadd. 72. A decoder for determining the presence of an n tone grouping in tone encoded paging signals, wherein n corresponds to the number of tones in said grouping, comprising: an active electric filter arranged to have its bandpass tuned in accordance with external impedance means, n impedance means each arranged to tune said active filter to pass separately the different tones of said tone grouping, n switch means for coupling each of said plurality of impedance means to said active filter, and gate means arranged to operate said n switch means in response to the output of said active filter to cause said active filter to sequentially pass tones in the sequence in which the tones are arranged in said grouping, counter means coupled to the output of said active filter for counting the tones passed through said active filter and providing an actuating output only upon the passage through said active filter of n tones whereby presence of an actuating output at said counter means indicates presence of said predetermined tone grouping. .Iaddend..Iadd. 73. A decoder as in claim 72 and wherein said n switch means are sequentially operated in response to said counter means. .Iaddend..Iadd. 74. A decoder as in claim 72 wherein said n tone grouping is made up of tones selected from m tones and wherein said n impedance means are selected from a group of m impedance means each arranged to tune said active filter to pass a different one of said m tones and connector means for separately connecting selected ones of said m impedance means to said n switch means, said selected impedance means being arranged to tune said active filter to pass separately the different tones of said tone grouping. .Iaddend. .Iadd. 75. A selective receiver for indicating the presence of a predetermined modulation grouping in modulation encoded paging signals comprising: modulation selection means sequentially adjustable to pass individual tones of said grouping in a predetermined order, means for detecting said modulation encoded signals and coupling same to the input of said modulation selection means, sequence detector means coupled to the output of said modulation selection means and arranged to cause said modulation selection means to pass sequentially in said predetermined order the modulations of said grouping upon the prior passage of preceding modulations of said grouping, and indicator means responsive to the passage through said modulation selection means of all the modulations of said grouping for indicating said receiver is being paged.