GB1113225A - Apparatus for distinguishing between voiced and unvoiced sounds in a speech signal - Google Patents

Abstract

1,113,225. Voiced/unvoiced detector. NATIONAL RESEARCH DEVELOPMENT CORPORATION. 31 Dec., 1965 [5 Jan., 1965], No. 418/65. Heading H4R. A voiced-unvoiced detector, e.g. for use in a vocoder, compares the energy in a low-frequency band of speech, i.e. below 900 c.p.s., with the energy in whichever one of a number of higher frequency pass bands is instantaneously carrying the minimum energy, the higher frequency pass bands including one in the frequency range 900 to 2000 c.p.s. and one in the frequency range 2500 to 6000 c.p.s., and produces an output indication according as whether the ratio of the energies is above or below a predetermined set ratio. The energy in the lowfrequency band may be the maximum value selected from the energies in a number of lowfrequency bands, and the " set ratio " may be adjusted in accordance with an output of a larynx excitation period detector so as to favour an indication of voiced signals when a marked low-frequency periodicity is detected in the speech signal being analysed. Fig. 1 shows a preferred embodiment in which speech on line 1 is applied to band-pass filters 2a, 3a, 4a and 4b, signals in a band in the range 3À5 to 4 kc/s. being applied via a logarithmic amplifier A2a to a minimum selector circuit, which may be of the type disclosed in Specification 1,067,366, to the other input of which is applied a signal from a band in the range 1À2-1À8 kc/s. selected by filter 3a. The output of the minimum selector circuit is applied as one input to a differential circuit 6<SP>1</SP> the other input of which is derived from a maximum selector 8 fed with the outputs of band-pass filter 4a and 4b which select bands in the range 300 to 800 c.p.s. and 50 to 550 c.p.s., so that the output of the differential device is a function of the ratio of the least energy of the higher frequency bands to the most energy of the low frequency bands. The output of the differential device 6<SP>1</SP> is applied to a differential amplifier 9 the other input of which is fed from a peak detector circuit 10 fed with the " stack " signal from a larynx excitation period detector according to Specification 1,113,224 so that the voltage on the base of Q8 of the differential amplifier goes negative to reduce the threshold value at which a signal indicating a voiced sound is produced at the output 15 when the stack signal is high, indicating a marked periodicity in the signal. The peak detector 10 is reset once per cycle of the excitation period by a signal on line 16 from the period detector of Specification 1,113,224. Diodes D3 and D4 in the minimum detector 5 ensure that if the speech signals are absent, or fall below a predetermined level, a predetermined minimum level signal is fed to the differential device 6<SP>1</SP> so that at such times the output of differential amplifier 9 indicates an unvoiced sound.