Detection circuit of tone signal

• US 6,415,139 B1
• Filed: 11/24/1998
• Issued: 07/02/2002
• Est. Priority Date: 11/26/1997
• Status: Expired due to Term

• Current Assignee: Canon Kabushiki Kaisha (Canon Inc.)
• Original Assignee: OKI Electric Industry Company Limited
• Inventors: Shimbo, Atsushi
• Primary Examiner(s): Trost, William
• Assistant Examiner(s): SOBUTKA, PHILIP
• Application Number:
  US09/198,379
• Time in Patent Office: 1,316 Days
• Field of Search: 455/229, 455/296, 455/212, 455/213, 455/301, 455/307, 455/227, 455/283, 455/225, 379/283, 379/386
• US Class Current: 455/229
• CPC Class Codes: H04Q 1/44 using alternate current H04Q1/50 takes precedence

Abstract

This invention intends to provide a tone signal detection circuit that accurately detects a tone signal regardless of a reception electric field strength. To accomplish this object, the tone signal detection circuit according to the invention is comprised of a first filter that extracts a signal of a specific frequency from an input signal, a second filter that excludes the signal of the specific frequency from the input signal and extracts signals of frequencies near the specific frequency, a subtractor that subtracts an output of the second filter from an output of the first filter, and a judgment circuit to judge whether the tone signal is present or not in the input signal on the basis of an output from the subtractor.

Claims

1. A tone signal detection circuit, comprising:

a first filter that extracts, from an input signal, a band of signals including a signal having a specific frequency

a second filter that extracts, from the input signal, a band of signals that overlap the band of signals extracted by said first filter, excluding the signal having the specific frequency;

a subtractor that subtracts the band of signals extracted by said second filter from the band of signals extracted by said first filter and outputs a subtraction signal; and

a judgment circuit that judges whether a tone signal is present or not in the input signal on the basis of the subtraction signal, wherein said judgment circuit comprises;

a first comparator that compares a level of the subtraction signal with a first level at every occurrence of a specified time interval, outputs an ON-signal when the level of the subtraction signal is greater than the first level, and outputs an OFF-signal when the level of the subtraction signal is less than the first level;

a first counter that counts a frequency during which the ON-signal that is output from said first comparator continues, and outputs a first judgment result indicating that the tone signal is present to an output terminal only when the continued frequency is greater than a first threshold frequency level;

a second comparator that compares the level of the subtraction signal with a second level at every occurrence of a specified time interval, outputs the ON-signal when the level of the subtraction signal is greater than the second level, and outputs the OFF-signal when the level of the subtraction signal is less than the second level;

a second counter that counts a frequency during which the OFF-signal, that is output from said second comparator continues, and outputs a second judgment result indicating that the tone signal is absent to the output terminal only when the continued frequency is greater than a second threshold frequency level; and

a switch that switches the subtraction signal so as to be input to said second comparator when said first counter outputs the first judgment result indicating that the tone signal is present, and switches the subtraction signal so as to be input to said first comparator when said second counter outputs the second judgment result indicating that the tone signal is absent.

2. A tone signal detection circuit as claimed in claim 1, further comprising an electric field strength detection circuit that controls, on the basis of an input electric field strength, a characteristic of said first filter, a characteristic of said second filter, and the first level, the second level, the first threshold frequency level, and the second threshold frequency level of said judgment circuit.

3. A tone signal detection circuit, comprising:

a first filter that extracts, from an input signal, a band of signals including a signal having a specific frequency;

a second filter that extracts, from the input signal, a band of signals that overlap the band of signals extracted by said first filter, excluding the signal having the specific frequency;

a subtractor that subtracts the band of signals extracted by said second filter from the band of signals extracted by said first filter and outputs a subtraction signal;

a lowpass filter that smoothes an output of said subtractor to produce a smoothed subtractor output signal; and

a judgment circuit which judges whether the tone signal is present or not in the input signal on the basis of the smoothed subtractor output signal produced by said lowpass filter, wherein said judgment circuit comprises;

a first comparator that compares a level of the subtraction signal with a first level at every occurrence of a specified time interval, outputs an ON-signal when the level of the subtraction signal is greater than the first level, and outputs an OFF-signal when the level of the subtraction signal is less than the first level;

a first counter that counts a frequency during which the ON-signal that is output from said first comparator continues, and outputs a first judgment result indicating that the tone signal is present to an output terminal only when the continued frequency is greater than a first threshold frequency level;

a second comparator that compares the level of the subtraction signal with a second level at every occurrence of a specified time interval, outputs the ON-signal when the level of the subtraction signal is greater than the second level, and outputs the OFF-signal when the level of the subtraction signal is less than the second level;

a second counter that counts a frequency during which the OFF-signal that is output from said second comparator continues, and outputs a second judgment result indicating that the tone signal is absent to the output terminal only when the continued frequency is greater than a second threshold frequency level; and

a switch that switches the subtraction signal so as to be input to said second comparator when said first counter outputs the first judgment result indicating that the tone signal is present, and switches the subtraction signal so as to be input to said first comparator when said second counter outputs the second judgment result indicating that the tone signal is absent.

4. A tone signal detection circuit as claimed in claim 3, further comprising an electric field strength detection circuit that controls, on the basis of an input electric field strength, a characteristic of said first filter, a characteristic of said second filter, and the first level, the second level, the first threshold frequency level, and the second threshold frequency level of said judgment circuit.

5. A tone signal detection circuit, comprising:

a first filter that extracts, from an input signal, a band of signals including a signal having a specific frequency;

a second filter that extracts, from the input signal, a band of signals that overlap the band of signals extracted by said first filter, excluding the signal having the specific frequency;

a first lowpass filter that smoothes an output of the first filter, and outputs the smoothed first filter output signal;

a second lowpass filter that smoothes an output of the second filter, and outputs the smoothed second filter output signal;

a subtractor that subtracts the smoothed second filter output signal from the smoothed first filter output signal, and outputs a subtraction signal; and

a judgment circuit that judges whether the tone signal is present or not in the input signal on the basis of the subtraction signal, wherein said judgment circuit comprises;

a first comparator that compares a level of the subtraction signal with a first level at every occurrence of a specified time interval, outputs an ON-signal when the level of the subtraction signal is greater than the first level, and outputs an OFF-signal when the level of the subtraction signal is less than the first level;

a first counter that counts a frequency during which the ON-signal that is output from said first comparator continues, and outputs a first judgment result indicating that the tone signal is present to an output terminal only when the continued frequency is greater than a first threshold frequency level;

a second comparator that compares the level of the subtraction signal with a second level at every occurrence of a specified time interval, outputs the ON-signal when the level of the subtraction signal is greater than the second level, and outputs the OFF-signal when the level of the subtraction signal is less than the second level;

a second counter that counts a frequency during which the OFF-signal that is output from said second comparator continues, and outputs a second judgment result indicating that the tone signal is absent to the output terminal only when the continued frequency is greater than a second threshold frequency level; and

a switch that switches the subtraction signal so as to be input to said second comparator when said first counter outputs the first judgment result indicating that the tone signal is present, and switches the subtraction signal so as to be input to said first comparator when said second counter outputs the second judgment result indicating that the tone signal is absent.

6. A tone signal detection circuit as claimed in claim 5, further comprising an electric field strength detection circuit that controls, on the basis of an input electric field strength, a characteristic of said first filter, a characteristic of said second filter, and the first level, the second level, the first threshold frequency level, and the second threshold frequency level of said judgment circuit.

7. A tone signal detection circuit, comprising:

a first filter that extracts, from an input signal, a band of signals including a signal having a specific frequency;

a second filter that extracts, from the input signal, a band of signals that overlap the band of signals extracted by said first filter, excluding the signal having the specific frequency;

a first lowpass filter that smoothes an output of the first filter, and outputs the smoothed first filter output signal;

a second lowpass filter that smoothes an output of the second filter, and outputs the smoothed second filter output signal;

a subtractor that subtracts the second filter output signal from the smoothed first filter output signal, and outputs a subtraction signal;

a third lowpass filter that smoothes the subtraction signal output from the subtractor, and outputs the smoothed subtraction signal; and

a judgment circuit that judges whether the tone signal is present or not in the input signal on the basis of the smoothed subtraction signal, wherein said judgment circuit comprises;

a first comparator that compares a level of the subtraction signal with a first level at every occurrence of a specified time interval, outputs an ON-signal when the level of the subtraction signal is greater than the first level, and outputs an OFF-signal when the level of the subtraction signal is less than the first level;

a first counter that counts a frequency during which the ON-signal that is output from said first comparator continues, and outputs a first judgment result indicating that the tone signal is present to an output terminal only when the continued frequency is greater than a first threshold frequency level;

a second comparator that compares the level of the subtraction signal with a second level at every occurrence of a specified time interval, outputs the ON-signal when the level of the subtraction signal is greater than the second level, and outputs the OFF-signal when the level of the subtraction signal is less than the second level;

a second counter that counts a frequency during which the OFF-signal that is output from said second comparator continues, and outputs a second judgment result indicating that the tone signal is absent to the output terminal only when the continued frequency is greater than a second threshold frequency level; and

a switch that switches the subtraction signal so as to be input to said second comparator when said first counter outputs the first judgment result indicating that the tone signal is present, and switches the subtraction signal so as to be input to said first comparator when said second counter outputs the second judgment result indicating that the tone signal is absent.

8. A tone signal detection circuit as claimed in claim 7, further comprising an electric field strength detection circuit that controls, on the basis of an input electric field strength, a characteristic of said first filter, a characteristic of said second filter, and the first level, the second level, the first threshold frequency level, and the second threshold frequency level of said judgment circuit.