the amplitude value of a sinusoidal wave is calculated by operating means which processes absolute values of two data samples which are sampled from the sinusoidal wave at two points spaced 90 ° electrical degrees apart thereby producing an electrical signal proportional to the amplitude value . the operation means comprises a comparator , a combination of an adder and a subtractor , or a combination of a comparator and an adder . the output of the comparator or the combination is multiplied with a predetermined coefficient and then added together to produce said signal .