Dial tone detector

Claim 3

A method as defined in claim 1, wherein said dual tone signal is dial tone, and said conductor is a telephone line.

Claim 4

A method as defined in claim 2, wherein said first predetermined frequency is 350 hertz and said second frequency is 440 hertz.

Claim 5

A method as defined in claim 1, wherein said step of detecting the periods of said successive cycles is further comprised of comparing the amplitude of said signal on the conductor to a further threshold signal for detecting successive zero crossings of said signal on the conductor, and measuring the duration of time between three such successive zero crossings.

Claim 6

A method as defined in claim 2, wherein said step of detecting the periods of said successive cycles is further comprised for comparing the amplitude of said signal on the conductor to a further threshold signal for detecting successive zero crossings of said signal on the conductor, and measuring the duration of time between three such successive zero crossings.

Claim 7

A method as defined in claim 3, wherein said step of detecting the periods of said successive cycles is further comprised of comparing the amplitude of said signal on the conductor to a further threshold signal for detecting successive zero crossings of said signal on the conductor, and measuring the duration of time between three such successive zero crossings.

Claim 8

Apparatus for detecting the presence of dial tone signal on a telephone line comprised of tip and ring leads, said signal being comprised of a first tone signal of first predetermined frequency and a second tone signal of second predetermined frequency, comprising: (a) a transformer having a primary winding connected to said tip and ring leads, for receiving and coupling signals carried by said tip and ring leads to a secondary winding, thereof, (b) comparator means having a first input connected to said transformer, for receiving said signals coupled thereacross, and (c) a microprocessor for: (i) generating and applying a first threshold signal to a second input of said comparator means such that an output of said comparator means generates a first control signal in the event a predetermined one of said received signals has an amplitude greater than said first threshold signal, (ii) receiving said first control signal, and in response generating and applying a second threshold signal to said second input of the comparator means such that said output generates a succession of second control signals corresponding to zero crossings of said received signal, (iii) receiving said second control signals and measuring the duration of time between successive ones thereof, wherein said durations correspond to the periods of a predetermined number of successive cycles of said received signal, and (iv) in the event successive ones of said detected periods are different, detecting an average value of said periods, and in the event said average value corresponds to a frequency which is greater than or equal to said first predetermined frequency and less than or equal to said second predetermined frequency, generating a signal indicative of the presence of said dial tone signal.

Claim 9

Apparatus for detecting the presence of dial tone signal on a telephone line comprised of tip and ring leads, said signal being comprised of a first tone signal of first predetermined frequency and a second tone signal of second predetermined frequency, comprising: (a) a transformer having a primary winding connected to said tip and ring leads, for receiving and coupling signals carried by said tip and ring leads to a secondary winding, thereof, (b) comparator means having a first input connected to said transformer, for receiving said signals coupled thereacross, and (c) a microprocessor for: (i) generating and applying a first threshold signal to a second input of said comparator means such that an output of said comparator means generates a first control signal in the event a predetermined one of said received signals has an amplitude greater than said first threshold signal, (ii) receiving said first control signal, and in response generating and applying a second threshold signal to said second input of the comparator means such that said output generates a succession of second control signals corresponding to zero crossings of said received signal, (iii) receiving said second control signals and measuring the duration of time between successive ones thereof, wherein said durations correspond to the periods of a predetermined number of successive cycles of said received signal, and (iv) in the event successive ones of said detected periods are different, detecting an average value of said periods, and in the event said average value corresponds to a frequency which is greater than or equal to said first predetermined frequency and less than or equal to said second predetermined frequency, generating a signal indicative of the presence of said dial tone signal.

Claim 10

Apparatus as defined in claim 7, wherein said dual tone signal is a dial tone and said conductor is a telephone line comprised of tip and ring leads.

Claim 11

Apparatus as defined in claim 8, wherein said secondary winding is connected via a resistor and capacitor series circuit to said first input, for AC coupling said signals coupled by the transformer.

Claim 12

Apparatus as defined in claim 9, further including means for superimposing said AC coupled signals on a 2.50 volt DC signal.

Claim 13

Apparatus as defined in claim 10, wherein said first threshold signal is approximately 2.49 volts and said second threshold signal is approximately 2.50 volts.

Claim 16

A method as defined in claim 12, wherein the steps of detecting the periods of said successive cycles is further comprised of comparing the amplitude of said superimposed signal to a further threshold signal in the form of a DC signal having an amplitude of approximately 2.50 volts, for detecting successive zero crossings of said signal on the conductor, and measuring the duration of time between three such successive zero crossings.