Patent ID: 7095845

Claim:
A circuit for detecting a duration of an input signal, comprising: a first band pass filter for inputting said input signal and outputting a first signal having a first frequency; a second band pass filter for inputting said input signal and outputting a second signal having a second frequency; a first hysteresis comparator electrically coupled to said first band pass filter for comparing with said first frequency of said first signal to output a first square wave signal according to a first high voltage and a first low voltage; a second hysteresis comparator electrically coupled to said first band pass filter for comparing with said first frequency of said first signal to output a second square wave signal according to a second high voltage and a second low voltage; a third hysteresis comparator electrically coupled to said second band pass filter for comparing with said second frequency of said second signal to output a third square wave signal according to said first high voltage and said first low voltage: a fourth hysteresis comparator electrically coupled to said second band pass filter for comparing with said second frequency of said second sianal to output a fourth square wave signal according to said second high voltage and said second low voltage; a first detection circuit electrically coupled to said first hysteresis comparator and said third hysteresis comparator for detecting a part of said first signal having said first frequency and a part of said second signal having said second frequency that are higher than said first high voltage and lower than said first low voltage to output a first detection signal according to said first square wave signal and said third square wave signal; and a second detection circuit electrically coupled to said second hysteresis comparator and said fourth hysteresis comparator for detecting a part of said signal first having said first frequency and a part of said second signal having said second frecuency that are higher than said second high voltage and lower than said second low voltage to output a second detection signal according to said second square wave signal and said fourth square wave signal; said duration of said input signal being detected according to said first detection signal and said second detection signal.