The main task of the periodical signal frequency evaluation problem is characteristic signal point detection. Nature of a signal often could be changed by some digital operators [1,2]. Evaluation the length in time between two the same characteristic extremum points is frequent choice for frequency determination. There are a lot of applications there the frequency evaluation task must be done in a portable monitoring units. The example is physiological parameters monitoring system. Respiratory rate or heart rate evaluation is one of the tasks in the system. Low power consumption is one of the main requirements for portable systems [3] or devices. Therefore low computational load of the method is big advantage for such applications. It is also desirable the method could be easily implemented in a processors operating in fixed point arithmetic. There is presented a method which could be applied for frequency of the periodical signal evaluation based on relatively long adaptive difference window. The length of the window takes significant part of the signal period.
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