GB841306A - Improvements in or relating to electrical apparatus for determining the fundamental frequency of a complex wave - Google Patents

Abstract

841,306. Determination of voice pitch frequency. WESTERN ELECTRIC CO. Inc. June 7, 1957 [June 13, 1956], No. 18219/57. Class 40 (4). [Also in Group XL (c)] The pitch frequency of the function g(t) of voice currents of the form g(t) sin (#t+#), where # is the formant frequency, is obtained by forming an auxiliary current g(t) cos (#t+#), squaring and adding these two currents to obtain {g(t)}<SP>2</SP> whose frequency is then obtained by known methods. As shown, the voice currents are passed via a band-pass filter 2, and an equalizer 3 to networks N 1 , N 2 which produce outputs which differ in phase by 90 degrees but are otherwise identical. These are squared at 11 and 12 respectively and added at 13 to produce the required output {g(t)}<SP>2</SP>. This may be passed to a square-rooter 14 to produce an output g(t) whose frequency is then obtained. Since, however, g(t) is always positive its peaks and those of {g(t)}<SP>2</SP> always coincide so that it is sufficient to measure the frequency of occurrence of the peaks of this latter function. The function g(t) or {g(t)}<SP>2</SP> may thus be fed to a circuit 15 comprising a low-pass filter 21 (0-250 c.p.s.) followed by a counting-device 23 which produces an output via low-pass filter 24 (0-25 c.p.s.) to lead 17 as a measure of the pitch frequency. Alternatively or in combination with the above circuit 15 a circuit 16 may be provided for giving a pulse output on lead 18 at the beginning of each fundamental pitch period. This circuit may comprise a succession of circuits each comprising a shaping network 31 (or 34) emphasizing low frequencies and a detector 32 (or 35). The networks N 1 , N 2 may be of any suitable type provided they introduce a 90 degree phase difference between their outputs and provided they have otherwise the same phase - frequency and amplitude - frequency characteristics. Correction for these latter may then be effected by the single equalizer 3 in the input circuit. The circuit may be used in a vocoder system of the type described in Specification 466,327.