GB1143169A - Frequency detecting apparatus - Google Patents

Abstract

1,143,169. Pitch frequency detector. INTERNATIONAL BUSINESS MACHINES CORP. 7 Sept., 1967 [13 Sept., 1966], No. 40869/67. Heading H4R. In a circuit for detecting the fundamental frequency of a voice signal when the fundamental frequency is missing, e.g. due to transmission over a telephone line, the voice signal is applied to a set of band pass filter circuits C1 to C7, each consisting of cascaded filters F and limiters E, the pass frequencies of which are such that each filter circuit passes at most, one harmonic of the fundamental frequency. The outputs of the filter circuits are connected via frequency dividers 2 to 12, and gate circuits, Al to All, to counter circuits 13 to 23 and the gate circuits are controlled by a logic circuit CL, also fed with the outputs of the filter circuits, which determines, in accordance with the particular pattern of energized filter circuits, the range in which the fundamental frequency falls and enables the appropriate gate circuit to feed to a counter circuit a frequency corresponding to the fundamental frequency of the input signal. A filter arrangement for dealing with speech signals with fundamental frequencies in the range 100-350 c/s is described, and appropriate logic circuits to enable the fundamental frequency to be determined are described with respect to Figs. 6 to 20 (not shown). A simplified form of frequency detector which handles only fundamental frequencies in the range 150-270 c/s. is shown in Fig 23, in which filter chains C8, C9 and C10, respond to frequency bands 300- 380, 380-460, and 460-540 c/s respectively. Each filter chain comprises band pass filters F8, F<SP>1</SP>8 &c., half wave rectifiers R8, R<SP>1</SP>8 &c., and limiter circuits E8, E<SP>1</SP>8 &c. providing discrimination against noise and emphasis of any harmonic of the input signal present. The logic circuit CL in this embodiment is such that the measurement is effected on the lowest frequency harmonic present, with all higher harmonics in higher frequency channels blocked by the gates 30 or 36.