DEMODULATION METHOD FOR COMPLEX FM BASE BAND SIGNAL

摘要

PROBLEM TO BE SOLVED: To provide a more efficient method to use a fixed decimal point DSP(digital signal processor) for FM demodulation and decoding. SOLUTION: An integer (2) is added to a value of RL (real number) to calculate (34)1/(RL+2) before using (36) a look-up table to decrease an instruction cycle time, processing in-phase expression of the denominator overcomes a problem of looking up the table. The addition of '2' to the RL causes reduction in the amplitude of a demodulation signal by a constant term K that is substantially uniform. The same constant term K is multiplied by an output in a demodulation process to obtain an excellent approximated demodulation signal. Furthermore, DFT(discrete Fourier Transform) snapshots values of 0 deg., 90 deg., 180 deg., 270 deg. for decoding are selected (46) to decrease the number of multiplication steps so as to use efficient mount of pilot frequency calculation.