GB1044991A - A vocoder system - Google Patents

Abstract

1,044,991. Vocoder systems. G. FANT. May 7, 1964 [May 8, 1963], No. 19139/64 Heading H4R. In a vocoder system in which speech signals are analysed to obtain signals representative of the amplitude and formant frequencies of the speech which latter signals are transmitted to a receiver where a synthesizer reconstitutes speech signals from the representative signals, the transmitter includes a synthesizer the output from which is applied to a series of band-pass filters which filter from the synthesized signal a series of successive portions of the frequency spectrum of the signal and the amplitudes of these successive portions of the spectrum are compared with the amplitudes of similar portions selected, by a second series of band-pass filters, from the spectrum of the original speech, the differences in these amplitudes being transmitted to modify the synthesized speech at the receiver and thus produce a more accurate replica of the original speech. Fig. 9 shows a preferred embodiment in which speech from a microphone M is fed via an amplifier F to an analyzer A.N. which extracts voiced-unvoiced signals G1-G2, pitch frequency F0, vowel formant frequencies F1, F2, F3, and consonant formant frequencies K1, and K2. These analogue signals are fed to an analogue to digital converter ADA and also to the transmitter synthesizer OVE. Both the original speech from amplifier F and the synthesized speech from OVE are applied to respective filter banks BPA1 ... BPA6 and BPB1 ... BPB6, the filters dividing the spectrum into frequency ranges of 0-350, 350-900, 900-1600, 1600- 2600, 2,600-1,500 and 4,500-9000. The outputs from corresponding filters in the original speech bank and the synthesized speech bank are rectified in rectifiers LRA1 ... LRA6 and LRB1 . . . LRB6, respectively, and compared in comparator circuits KO1 to KO6 and the difference signal is applied to the analogue to digital converter ADB for transmission to the receiver. At the receiver the digital signals are converted to analogue signals in the units DAA and DAB and the excitation function signals and the spectrum function signals are applied to the synthesizer OVE. The synthesized speech from OVE is applied to a bank of filters BPB1 ... BPB6 similar to those in the transmitter and the resulting contiguous sub-bands are fed to respective modulators MO1 . . . MO6 where their amplitudes are modified in accordance with the difference signals derived by the comparator units KO1 to KO6 in the transmitter to produce a speech signal to be fed to the telephone receivers T. At the transmitter the difference signal from the comparators KO1 . . . KO6 is also fed to a group of modulators MO1 . . . MO6 to operate on the synthesized signal at the transmitter, this modified synthesized signal being fed as side tone to the telephone receivers T. Switch SM is a voiceoperated switch which converts the equipment from the transmit mode to the receiving mode. The vocoder analyser may be of a serial form, Fig. 7 (not shown), or parallel form, Fig. 8 (not shown), formant tracking type and it is also suggested that the system may be used with channel vocoder systems.