US3573374A - Formant vocoder utilizing resonator damping - Google Patents

Formant vocoder utilizing resonator damping Download PDF

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US3573374A
US3573374A US700542*A US3573374DA US3573374A US 3573374 A US3573374 A US 3573374A US 3573374D A US3573374D A US 3573374DA US 3573374 A US3573374 A US 3573374A
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signal
amplifier
gain
resonator
resonator circuits
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US700542*A
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English (en)
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Louis R Focht
James M Loe
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Maxar Space LLC
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Philco Ford Corp
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    • G—PHYSICS
    • G10—MUSICAL INSTRUMENTS; ACOUSTICS
    • G10L—SPEECH ANALYSIS TECHNIQUES OR SPEECH SYNTHESIS; SPEECH RECOGNITION; SPEECH OR VOICE PROCESSING TECHNIQUES; SPEECH OR AUDIO CODING OR DECODING
    • G10L19/00—Speech or audio signals analysis-synthesis techniques for redundancy reduction, e.g. in vocoders; Coding or decoding of speech or audio signals, using source filter models or psychoacoustic analysis

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  • signals representative of the frequencies and amplitudes of the formants of a speech sound are transmitted and utilized to control a formant synthesizer which contains three or more resonant circuits.
  • the Q of the resonant circuits must have a high value. This, without damping, produces carry-over" of the signals in the resonant circuits between successive excitation points and hence a low quality synthesized speech wave. Attempts to improve the quality of the synthesized speech wave without damping by adjusting the resonant circuits so that they have a Q value somewhere between the damped and undamped values have not been totally successful.
  • a further object of the present invention is to provide a system in which the Q of the resonant circuits of the synthesizer of a formant vocoder is regulated so as to prevent carryover" ringing of the resonant circuits of the synthesizer between successive pitch pulses.
  • a glottal damping generator which is connected to an output of the pitch generator of the synthesizer and an input terminal of at least one of the formant resonators of the synthesizer, reduces the Q of the resonators to which it is connected for a brief period of time before the occurrence of each successive pitch pulse. This damps the resonant circuits at the same time and to substantially the same degree that the glottis damps the resonant cavities of the human speech mechanism and thus makes the synthesized speech wave a better replica of the spoken speech wave.
  • FIG. 1 is a block diagram of a formant vocoder in accordance with the present invention
  • FIG. 2 is a graph illustrating first formant speech signals produced by the human speech mechanism and formant vocoders.
  • FIG. 3 is a schematic diagram of a portion of the circuit of FIG. 1.
  • a speech signal such as that produced by a standard high-quality microphone (not shown), is supplied to a formant frequency and amplitude detector 2, a pitch detector 4 and a voicing detector 6.
  • Detector 2 produces signals representative of the frequencies F F and F of the first three formants of the speech wave and signals representative of the amplitudes A,, A and A of the first three formants of the speech wave.
  • a suitable formant frequency and amplitude detector for the system of the present invention is described in U.S. Pat. No. 2,458,227, issued to R. Vermeulen et al. on Jan. 4, 1949, entitled Device for Artificially Generating Speech Sounds by Electrical Means.
  • Detector 4 produces a signal the amplitude of which is representative of the frequency of vocal cord excitation, i.e. the the pitch frequency.
  • voicing detector 6 measures the regularity of the spacing between adjacent pitch pulses and produces a signal representative of the ratio of harmonic to nonharmonically related energy present in the speech wave. This information distinguishes voiced sounds from unvoiced sounds. Suitable pitch and voicing detectors for use in the system of the present invention are described in U.S. Pat. No. 3,488,442 of Louis R. Foc'ht, issued Jan. 6, 1970, entitled Single Equivalent Formant Speech Analysis System.” I
  • the signals generated by detectors 2, 4 and.6 are trans mitted by a conventional communication channel to a synthesizer network.
  • the detector signals can be transmitted by continuously varying the amplitude of a radio frequency carrier signal in accordance with the amplitude of the detector output signals.
  • the output of detector 4 is supplied to a frequency-controllable pitch oscillator 8 which generates a signal having a frequency which is a function of the amplitude of the control signal supplied thereto from detector 4. Since, as previously stated, the amplitude of the pitch signal is a function of the pitch frequency, the signal generated by oscillator 8 has a frequency equal to the pitch frequency of the input speech signal.
  • voicing detector 6 The output of voicing detector 6 is supplied to a gating circuit l0.
  • Gating circuit 10 is also coupled to pitch oscillator 8 and to a noise generator 12. Depending upon the amplitude of the voicing signal and accordingly upon whether the speech signal is voiced or unvoiced, gate 10 will pass either the signal from oscillator 8 or the signal from generator 12.
  • the output signal of gate 10 is supplied in parallelto modulators-I4, l6, and 18.
  • Modulators l4, l6, and 18 modulate the amplitude of this signal in accordance with the amplitude of the signals representative of the formant amplitudes A,, A and A respectively.
  • the amplitude modulated signals produced by modulators 14, 16, and 18 are supplied to tuna- -ble resonant circuits 20, 22, and 24, respectively.
  • the tuning of circuits 20, 22, and 24 is controlled by the formant frequenc'y signals F,, F and F respectively.
  • the output signals of the tunable resonant circuits 20, 22, and 24 are processed through band-pass filters, 26, 28, and 30, respectively, which are identical to those used in detector 2 and which remove undesirable harmonic components introduced during the processing of the speech wave, and then combined additively in circuit-31 to form the synthesized speech signal.
  • a glottal damping generator 32 has an input terminal coupledto. pitch generator 8 and an output terminal coupled in parallel to tunable resonators 20 and 22.
  • generator 32 generates damping signals which reduce the Q of the first and second formant resonators 20 and 22. The timing of these signals is set to reduce the Q of resonators 20 and 22 just prior to each successive pitch pulse. Probably because of its physical placement in the human vocal tract the third formant resonant cavity of human speech appears not to be clamped by glottal opening and therefore the third formant resonator is not damped in the synthesizer.
  • Curve A of FIG. 2 illustrates the amplitude spec trum of the first formant of a sound uttered by the human vocal tract. Due to glottal damping, the amplitude of this signal is greatly reduced just prior to each successive pitch pulse, P.
  • Curve B of FIG. 2 illustrates the amplitude spectrum of the first formant signal as synthesized by a conventional formant vocoder. Unlike the speech wave of curve A, the amplitude of this signal is still large just prior to each successive pitch pulse. Thus each ringing signal of the first formant resonator is superimposed on the next ringing signal of that resonator. This superimposition distorts the synthesized speech wave.
  • Curve C of FIG. 2 illustrates the amplitude spectrum of the output signal of the first formant resonator of the present invention. Due to the action of the glottal damping generator 32, the amplitude of the signal from resonator 20 is highly damped before each successive pitch pulse as is the amplitude of sounds uttered by the human vocal tract, curve A. From the foregoing it is apparent that use of the glottal damping generator 32 improves the quality of a formant vocoder because a better electrical analogue of the human vocal tract is obtained.
  • FIG. 3 there is shown a schematic diagram of circuits suitable for use as the pitch generator 8, the resonator 20, and the glottal damping generator 32 of FIG. 1.
  • the signal representative of the pitch rate is supplied to the base of a transistor 33 which forms part of a charging circuit which also includes resistor 34 and capacitor 35.
  • a voltage reference network 36 Connected in shunt with the charging circuit is a voltage reference network 36.
  • the voltage appearing across capacitor 35 is supplied as one input signal to a high-gain differential amplifier 38 and the voltage at the junction of the two resistors of voltage divider network 36 is supplied as the second input signal to differential amplifier 38.
  • Amplifier 38 saturates in the negative direction when the reference voltage is greater than the voltage across capacitor 35 and saturates in the positive direction when the reference voltage is less than the voltage across capacitor 35.
  • the voltage across capacitor 35 is also coupled to th emitter of unijunction transistor 37.
  • Unijunction transistor 37 is biased so that it breaks down, due to the voltage across capacitor 35, at the pitch pulse rate thereby producing pitch pulses across capacitor 39. These pulses are standardized in the single shot multivibrator 41.
  • the output of amplifier 38 is supplied to'the gate of a field effect transistor 40 and to a voltage inverter circuit 42.
  • the source electrode of field effect transistor 40 is coupled to an input of a controllable loss compensating operational amplifier 44.
  • the resistance of transistor 40 and a resistor 43 form the feedback loop for operational amplifier 44 and thus regulate the gain of amplifier 44.
  • The-first formant frequency signal F is supplied to a field effect transistor 45 of a voltage-variable high pass filter circuit 46 and to a field efiect transistor 47 of a voltage variable low pass filter circuit 48by means of potentiometer 50.
  • the filter circuits are isolated by amplifier network 52.
  • the source electrode of field effect transistor 45 is coupled to a second input of operational amplifier 44.
  • the output of amplifier'44 is coupled to switch modulator 14 of FIG. 1 and through an isolating emitter-followernetwork 54 to the source electrode of field effect transistor 47.
  • the damped first formant signal appears at the output of amplifier 44.
  • the operation of the circuit of FIG. 3 is as follows:
  • the amplitude of the pitch signal determines the collector current of transistor 33 and hence the charging rate of capacitor 35.
  • a speech synthesizer comprising:
  • a plurality of resonator circuits at least one of said resonator circuits comprising amplifier means, the bandwidth of said one of said resonator circuits being dependent on the gain of said amplifier means;
  • first means coupled to each of said plurality of resonator circuits for regulating the resonantfrequency of said resonator circuits; 1 second means coupled to each of said plurality of resonator circuits for supplying ringing energy to said resonator circuits upon the occurrence of each of a succession of pulses respectively representative of successive pitch pulses of said speech wave; and
  • damping means coupled to said one of said resonator circuits,'for controlling the bandwidth of said one resonator circuit, the improvement wherein said damping means comprises:
  • third means for controlling the gain of said amplifier means said third means being responsive to a control signal to exhibit between a pair of terminals of said third means a first resistance when said control signal has a first value and a second resistance when said control signal has a second has a second value
  • a differential amplifier for supplying to said third means, in response to an input signal of given magnitude, a control signal having said first value, and for supplying to said third means, in response to an input signal of another magnitude different from said given magnitude, a control signal having said second value, and
  • a speech analyzer comprising first means for producing in response to said speech wave signals representative of the frequencies and amplitudes of a plurality of formants of said speech wave, and second means for producing in response to said speech wave signals representative of the pitch rate of said speech wave; a speech synthesizer comprising oscillator means responsive to said signals produced by said second means to generate a succession of voltage pulses at said pitch rate, a plurality of modulators, third means for coupling said modulators in parallel to said oscillators means, fourth means for supplying each of said signals respectively representative of the amplitudes of said plurality of formants of said speech wave to a difierent one of said modulators, a plurality of resonator circuits each having first and second input terminals, at least one of said resonator circuits having amplifier means, the bandwidth of said one of said resonator circuits being dependent on the gain of said amplifier means, each of said plurality of resonator circuits having said first input terminal thereof coupled to a different one of said modulators, fifth '
  • said analyzer includes means for producing a voicing signal
  • said synthesizer includes a noise generator
  • said third means includes a gating circuit controlled by said voicing signal and coupled to said oscillator means and said noise generator so that, depending upon the amplitude of the voicing signal, either the output signal of said noise generator or the output signal of said oscillator means is coupled to said modulators.

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  • Engineering & Computer Science (AREA)
  • Computational Linguistics (AREA)
  • Signal Processing (AREA)
  • Health & Medical Sciences (AREA)
  • Audiology, Speech & Language Pathology (AREA)
  • Human Computer Interaction (AREA)
  • Physics & Mathematics (AREA)
  • Acoustics & Sound (AREA)
  • Multimedia (AREA)
  • Electrophonic Musical Instruments (AREA)
  • Electrotherapy Devices (AREA)
US700542*A 1968-01-25 1968-01-25 Formant vocoder utilizing resonator damping Expired - Lifetime US3573374A (en)

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US70054268A 1968-01-25 1968-01-25

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US (1) US3573374A (enrdf_load_stackoverflow)
JP (1) JPS4912009B1 (enrdf_load_stackoverflow)
DE (1) DE1903623C3 (enrdf_load_stackoverflow)
GB (1) GB1261552A (enrdf_load_stackoverflow)

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3825685A (en) * 1971-06-10 1974-07-23 Int Standard Corp Helium environment vocoder
US3836717A (en) * 1971-03-01 1974-09-17 Scitronix Corp Speech synthesizer responsive to a digital command input
US6427518B1 (en) * 1998-08-06 2002-08-06 Robert Bosch Gmbh Apparatus for ascertaining a rotation rate and for performing a self-test

Families Citing this family (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
NL8000361A (nl) * 1980-01-21 1981-08-17 Philips Nv Inrichting en werkwijze voor het opwekken van een spraaksignaal.
JPS6113646U (ja) * 1983-10-31 1986-01-27 平谷産業株式会社 組立式板材椅子
JPH0329037U (enrdf_load_stackoverflow) * 1989-08-01 1991-03-22

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2817707A (en) * 1954-05-07 1957-12-24 Bell Telephone Labor Inc Synthesis of complex waves
US2928901A (en) * 1956-04-13 1960-03-15 Bell Telephone Labor Inc Transmission and reconstruction of artificial speech
US3268660A (en) * 1963-02-12 1966-08-23 Bell Telephone Labor Inc Synthesis of artificial speech
US3431362A (en) * 1966-04-22 1969-03-04 Bell Telephone Labor Inc Voice-excited,bandwidth reduction system employing pitch frequency pulses generated by unencoded baseband signal

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2817707A (en) * 1954-05-07 1957-12-24 Bell Telephone Labor Inc Synthesis of complex waves
US2928901A (en) * 1956-04-13 1960-03-15 Bell Telephone Labor Inc Transmission and reconstruction of artificial speech
US3268660A (en) * 1963-02-12 1966-08-23 Bell Telephone Labor Inc Synthesis of artificial speech
US3431362A (en) * 1966-04-22 1969-03-04 Bell Telephone Labor Inc Voice-excited,bandwidth reduction system employing pitch frequency pulses generated by unencoded baseband signal

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3836717A (en) * 1971-03-01 1974-09-17 Scitronix Corp Speech synthesizer responsive to a digital command input
US3825685A (en) * 1971-06-10 1974-07-23 Int Standard Corp Helium environment vocoder
US6427518B1 (en) * 1998-08-06 2002-08-06 Robert Bosch Gmbh Apparatus for ascertaining a rotation rate and for performing a self-test

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DE1903623C3 (de) 1979-11-15
GB1261552A (en) 1972-01-26
DE1903623B2 (de) 1979-03-22
DE1903623A1 (de) 1969-11-20
JPS4912009B1 (enrdf_load_stackoverflow) 1974-03-20

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