Patent ID: 6415139
Filing Date: 2002-07-02
Classification: H04Q

Abstract:
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.