GB1083386A - Improvements in and relating to electric signal apparatus - Google Patents

Abstract

1,083,386. Vocoder pitch extractor; automatic tuning control. PHILCO-FORD CORPORATION. May 17, 1965 [May 15, 1964], No. 20865/65. Headings H3A and H4R. In a pitch extractor for a vocoder the speech signals are limited to a low-frequency band in band-pass filter 12 and applied to a non- linear circuit 14 which, by intermodulation, introduces the fundamental frequency which may have been absent due to the use of a carbon microphone or a transmission channel deficient at low frequencies. The intermodulated signal is then applied via a low-pass filter, e.g. cut off 300 c/s., to a band-pass filter 18 whose centre frequency and band width are controllable to track the pitch frequency. The output of the band-pass filter is then fed to a frequency discriminator which produces an analogue signal dependent on the pitch frequency. The analogue signal is converted to digital form in unit 24 from which digitals signals are available in either parallel, a shown, or serial form. The digital signals are applied to, e.g. a multiplexer and a transmission system for the vocoder, and to a store 28 which maintains the last known signal value during pauses in speech or during unvoiced sounds. The resulting digital signals are applied, via the digital to analogue converter 30, to control the centre frequency and band-width of the band-pass fiter 18 to ensure that the filter centre frequency stays tuned to the last known pitch frequency. The band-width of filter 18 is arranged to vary with the variation of centre frequency.