Antitheft system

In a burglar alarm system comprising an alarm unit attached to an object to be protected from theft to emit an alarm of a certain frequency in response to illegal conduct and an alarm sensor for emitting an alarm signal upon sensing an alarm from said alarm unit,

• said alarm sensor comprising:
• a detector circuit for generating detected signals for a predetermined time period if a sound pressure value of an input signal containing said alarm and an external noise is not less than a predetermined value;
• a first determination means;
• and a second determination means, wherein when receiving an input signal, said first determination means measures a time more than once required for a predetermined number of waveforms to be counted and determines, based on whether or not each differential between the measured times is within a predetermined time, whether or not the input signal is a random external noise;
• and said second determination means measures a rise time of each of the detected signals generated in each cycle of intermittent waveform of said alarm, measures a rise cycle of each of the detected signals, determines whether or not the detected signals are a reflected sound generated as a result of a reverberation of said alarm and issues an alarm signal.

A burglar alarm method comprising the steps of:

• evaluating a random external noise contained in an input signal containing an alarm, which intermittently generates a certain frequency in response to illegal conduct, and external noises;
• evaluating a reflected sound caused by reverberation of said alarm;
• and
• emitting an alarm signal when the results of the both evaluations indicate that said input signal is an alarm.

A burglar alarm method comprising the steps of:

• generating a detected signal for a predetermined time period when a sound pressure value of an input signal containing an alarm that intermittently generates a certain frequency in response to illegal conduct and an external noise is not less than a predetermined sound pressure value;
• determining whether or not a received input signal is a random external noise, by measuring a time more than once required for a predetermined number of waveforms of the input signal to be counted and determining whether or not each differential between the measured times is within a predetermined time;
• determining, by measuring rise time of one or more detected signals generated in each cycle of the intermittent waveforms of said alarm and measuring a rise cycle of each of the detected signals, whether or not the input signal is a reflect sound caused by reverberation of the alarm;
• and
• generating an alarm signal.