FR2058696A5 - - Google Patents

Abstract

1,266,268. Excitation frequency detector. INTERNATIONAL BUSINESS MACHINES CORP. 4 Sept., 1970 [16 Sept., 1969], No. 42419/70. Heading H4R. In a method of recovering the fundamental frequency of an input signal, e.g. for determination of the pitch frequency of a speech signal, a frequency detector produces a signal whose frequency is either the fundamental frequency "f" or the nth harmonic. In response to the detector output signal a frequency f/n is generated, modulated by the input signal and the presence of frequency components below a predetermined frequency in the modulated frequency are detected. If such components are present the output f/n is selected as the fundamental frequency while if such components are not present the output of the frequency detector is selected as the fundamental frequency. It is shown that if the output of the detector f is the nth harmonic of the input signal fundamental frequency then modulation of the input signal by the frequency f/n produces a D.C. component while if the output of the detector is the true fundamental frequency no D.C. component is produced by the modulation. As described the detector D produces a signal of frequency f which may be the fundamental or second harmonic of an input signal. Generators G1 and G2 produce quadrature signals at the frequency f/2 which modulate the input signal E in modulators M1 and M2. The D.C. or very low frequency components are detected by low pass filters and full wave rectifiers LPF1 and LPF2 and R1 and R2, the signals combined and a threshold device C operated to connect the output of a monostable Mcp to the output S if the D.C. component is detected and the output of detector D to the output S if the D.C. component is not detected. Use of two generators in quadrature prevents an erroneous result if the f/n signal generated by a generator is exactly in quadrature with the input signal fundamental.