US3431355A - Device for excitation controlled smoothing of the spectrum-channel signals of a vocoder - Google Patents

Device for excitation controlled smoothing of the spectrum-channel signals of a vocoder Download PDF

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US3431355A
US3431355A US445563A US3431355DA US3431355A US 3431355 A US3431355 A US 3431355A US 445563 A US445563 A US 445563A US 3431355D A US3431355D A US 3431355DA US 3431355 A US3431355 A US 3431355A
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channel
unvoiced
spectrum
voiced
pass filter
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US445563A
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Ernst H Rothauser
Erwin F Paulus
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International Business Machines Corp
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International Business Machines 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
    • G10L19/02—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 using spectral analysis, e.g. transform vocoders or subband vocoders

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  • FIG. 2 A VOGODER Sheet 3 of 2 DEVICE FOR EXCITATION CONTROLLED SPECTRUM-CHANNEL SIGNALS OF Filed April 5, 1965 FlGQi FIG. 2
  • the filter arrangement may consist of a pair of low pass filters for each channel, or a variable cut-off low pass filter for each channel, or a triggered integrator for each channel operable in a higher triggering rate or lower triggering rate mode, in each case the filter arrangement being switched between its higher cut-off and lower cutoff mode by the voiced-unvoiced discriminator of the vocoder.
  • This invention relates to methods and arrangements for reducing the noise components contained in the channel voltages when analyzing unvoiced speech segments.
  • one individual channel of the analyzer of a channel vocoder consists of a band-pass filter BP to BP (FIG. 1), a rectifier G to G and a smoothing lowpass filter LP to LP respectively.
  • the output voltages of the smoothing low-pass filters LP to LP should be constant D.C. voltages. This ideal case is easily obtainable for voiced longer-lasting speech segments.
  • the output voltages also contain noise components the energy spectrum of which corresponds to the band width of the smoothing low-pass filter. A noise component gains increasing importance with respect to the constant DC.
  • this invention has for its object to reduce these noise components in the spectrum signals when analyzing unvoiced speech segments.
  • the invention consists in the provision that the noise component of the D.C. signal carried by each spectrum channel after the analysis is reduced by selecting an optimally low value for the upper limiting frequency of the smoothing low-pass filter provided for each spectrum channel in the presence of unvoiced speech segments.
  • each spectrum channel contains two lowpass filters with different upper limiting frequencies both of which are connected, on the one hand, to the associated D.C. circuit and, on the other hand, to a respectively difierent terminal of a controllable switch the output of which carries the respective channel voltage and the control of which is elfected by the output signal of a discriminator provided in a known manner in the excitation channel for the voiced/unvoiced discrimination of the speech signal, in such a manner that in the presence of voiced speech signals the low-pass filter with the higher limiting frequency and in the presence of unvoiced speech signals the low-pass filter with the lower limiting frequency is turned on.
  • This arrangement may be modified in a very advantageous manner by providing only one low-pass filter for each spectrum channel the upper limiting frequency of which is adjustable to a lower value by means of a controllable switch.
  • One modification of the method of improving the speech quality in connection with the analysis of unvoiced speech segments according to the channel vocoder principle consists in the provision that the noise component of the DC. voltage signal carried by each spectrum channel after the analysis is reduced by replacing the smoothing low-pass filter in each spectrum channel with an integrator which is controllable with respect to its integration time and which in the presence of unvoiced speech segments integrates the signals of the channel within an integration interval increased with respect to the normal integration interval.
  • An arrangement for carrying through this method gives especially favorable results due to the provision that for each spectrum channel there is provided an integrator following the DO. circuit and controllable with respect to its integration interval, the control input of which is connected through a switch to two control lines in such a manner that, by means of avoiced/ unvoiced discriminator known per se and included in the excitation channel, said switch in the presence of unvoiced speech segments connects said integrator to one signal generator, whereby the signals of the spectrum channel are integrated within a longer interval of time than in the presence of voiced speech segments when the integrator is connected to another signal generator and the signals of the spectrum channel are integrated within a normal interval of time.
  • FIG. 1 shows the block circuit diagram of the analysis section of a channel vocoder
  • FIG. 2 shows the block circuit diagram. of an arrangement for the excitation-controlled smoothing of the channel voltages by switching to two low-pass filters having different upper limiting frequencies
  • FIG. 3 shows the basic circuit diagram of a low-pass filter having an adjustable upper limiting frequency
  • FIGURE 4 shows the block circuit diagram of an arrangement for the excitation-controlled smoothing of the channel voltages by means of controllable integrators
  • FIG. 5 shows the basic circuit diagram of a controllable integrator.
  • the output voltages contain noise components the energy spectrum of which corresponds to the integration interval of the integration member LP, following the DC. circuit G, (FIG. 1) in each spectrum channel.
  • the band width of a smoothing low-pass filter so that for reducing the noise component it is necessary either to select a low value for the upper limiting frequency of the smoohting low-pass filter or to select a large value for the integration interval of the integrator used.
  • a low upper limiting frequency of the smoothing lowpass filter or a large integration interval of an integrator result in a reduced flow of information, which also has an unfavorable effect on the quality of the synthetic speech.
  • a discriminator D is used in the excitation channel, as shown in FIG. 1, for making a voiced/unvoiced discrimination sh and sl.
  • This discrimination may be used for controlling either the limiting frequency of the smoothing low-pass filters or the integration interval when using integrators, so that for both casesvoiced and unvoiced speech segmentsoptimum values are set up automatically.
  • FIG. 2 shows the block circuit diagram of a channel vocoder using smoothing low-pass filters controllable with respect to their upper limiting frequency.
  • the subscript i denotes any instant example of a part shown in the ⁇ drawing with the subscript 1 through n.
  • two lowpass filters LP, and LP are provided for each spectrum channel which have differential upper limiting frequencies.
  • a controllable switch S is used for connecting, when analyzing unvoiced speech segments, the low-pass filter LP, with the lower upper limiting frequency through the contact A, into the spectrum channel K, depending on the output signal of the voiced/unvoiced discriminator D.
  • the low-pass filter LP In order to avoid an unnecessary reduction of the flow of information in the presence of voiced speech segments, the low-pass filter LP, with the upper limiting frequency optimal for the analysis of voiced speech segments is connected into the channel by means of the controllable switch. It is also particularly advantageous to use a low-pass filter LP with a reversible upper limiting frequenoy, such as it is shown basically in FIG. 3. It consists essentially of two serially connected partial inductances L, and L, and a capacitor C, which by means of the switch S, may be connected either to the end of partial inductance L, or to the central tap between the two partial inductances.
  • L,, L, and C are selected so that in the two switch positions A, and A,, for the analysis of voiced speech segments and for the analysis of unvoiced speech segments respectively, the optimum upper limiting frequency may be set up.
  • this controllable low-pass filter LP in the arrangement of FIG. 2, the pairs of low-pass filters LP, and LP, are replaced with these filters.
  • triggered integrators Another favorable possibility of realizing a low-pass filter with a controllable upper limiting frequency is obtained by using triggered integrators.
  • an integrator may be imagined as a low-pass filter which in response to the Dirac impulse produces a square pulse of the duration of the integration interval and the output voltage of which is scanned at intervals each comprising one integration interval.
  • an increase in the integration interval corresponds to a reduction in the upper limiting frequency of this low-pass filter.
  • This type of realization may be used to advantage wherever the channel volttages are to be represented digitally since the integrator already supplies a pulse-amplitude modulated signal. This signal then merely requires conversion in an analog-todigital converter.
  • FIG. 4 shows the Iblock circuit diagram of an arrangement in which triggered integrators are used for reducing the noise component of the spectrum signals when analyzing unvoiced speech signals.
  • the triggered integrator I depending on the output signal of the voiced/ unvoiced discriminator D, the triggered integrator I, is connected through the controllable switch S, either to a pulse generator supplying a signal I which consists of square pulses of a given pulse repetition rate which is selected optimally e.g. for the analysis.
  • the integrator has its trigger input a connected via the controllable switch to a generator supplying signals 1 which consist of square pulses with a pulse repetition rate lower than the pulse repetition rate of the signals t but selected optimally for the analysis of unvoiced speech signals.
  • FIG. 5 shows the basiccircuit diagram of such a triggered integrator. From the output of the DC. circuit G, of the respective channel K, a DC. voltage of a constant amplitude or an A.C. voltage the envelope of which changes only very slowly is transmitted in the ideal case via line b to the input of the integrator I,.
  • a DC. voltage of a constant amplitude or an A.C. voltage the envelope of which changes only very slowly is transmitted in the ideal case via line b to the input of the integrator I,.
  • the capacitor C be charged negatively due to a preceding switching process at the transistor TR
  • the voltage at the capacitor is assumed to be -V 0.
  • the transistor T R is cut off, the capacitor is discharged by the collector current i of the transistor TR,.
  • the value of the resistor R is chosen so that the voltage droping thereacross is always low as compared to the voltage at the capacitor C.
  • the voltage at the capacitor is continuously decreased. Wheri due to a positive pulse at the trigger input a of the generator t or t the transistor TR is momentarily turned on, the capacitor is simultaneously recharged to the voltage V
  • the pulse-type charging current causes a pulse at resistor R the maximum amplitude of which is proportional to the voltage change at the capacitor.
  • the signal of the shorttime mean value of the channel filter output voltage is derived at the electrode E and transmitted via the respective channel K',.
  • band pass filter means connected to said input means and adapted to divide the input signal therefrom into a plurality of spectrum channels of different frequency bands
  • D.C. circuit means connected to each filter means and adapted to detect the energy level of the signal in each spectrum channel
  • discriminator means connected to said input means and adapted to yield a signal indicative of the voiced/ unvoiced character of the input speech signal
  • signal smoothing means connected to the output of each of said smoothing means comprises an adjustable said D.C. circuit means in each channel, low pass filter, and said smoothing means having a control input and lbeing said control input is operative to alter the cutoff freadjustable thereby between higher and lower upper quency of said filter. frequency limiting modes of operation, 5 5.
  • said control input being connected to said discriminator each of said smoothing means comprises an integrator means for operation of said smoothing means in having a variable integration interval.
  • each of said smoothing means comprises 3,261,916 7/1966 Bakis 179--1 two low pass filters of differing cutoff frequencies, and 3,102,928 9/ 1963 Schroeder 179-1 switch means under the control of said control input 3,012,098 12/1961 Riesz 179--1 to connect one or the other of said filters in circuit 5 in the Corresponding channeL KATHLEEN H. CLAFFY, Przmary Exammer.

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  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • Audiology, Speech & Language Pathology (AREA)
  • Computational Linguistics (AREA)
  • Signal Processing (AREA)
  • Health & Medical Sciences (AREA)
  • Spectroscopy & Molecular Physics (AREA)
  • Human Computer Interaction (AREA)
  • Acoustics & Sound (AREA)
  • Multimedia (AREA)
  • Compression, Expansion, Code Conversion, And Decoders (AREA)
  • Filters That Use Time-Delay Elements (AREA)
  • Meter Arrangements (AREA)
  • Machine Translation (AREA)
US445563A 1965-03-25 1965-04-05 Device for excitation controlled smoothing of the spectrum-channel signals of a vocoder Expired - Lifetime US3431355A (en)

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Application Number Priority Date Filing Date Title
AT275665A AT251656B (de) 1965-03-25 1965-03-25 Verfahren und Anordnungen zur Verbesserung der Sprachqualität in Vocodersystemen

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US (1) US3431355A (xx)
AT (1) AT251656B (xx)
BE (1) BE677136A (xx)
CH (1) CH443413A (xx)
DE (1) DE1276740B (xx)
ES (1) ES324621A1 (xx)
FR (1) FR1471628A (xx)
GB (1) GB1073203A (xx)
NL (1) NL146672B (xx)
SE (1) SE323721B (xx)

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5012519A (en) * 1987-12-25 1991-04-30 The Dsp Group, Inc. Noise reduction system
US5323467A (en) * 1992-01-21 1994-06-21 U.S. Philips Corporation Method and apparatus for sound enhancement with envelopes of multiband-passed signals feeding comb filters
US5768473A (en) * 1995-01-30 1998-06-16 Noise Cancellation Technologies, Inc. Adaptive speech filter
US6108610A (en) * 1998-10-13 2000-08-22 Noise Cancellation Technologies, Inc. Method and system for updating noise estimates during pauses in an information signal

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2810787A (en) * 1952-05-22 1957-10-22 Itt Compressed frequency communication system
US3012098A (en) * 1941-11-22 1961-12-05 Bell Telephone Labor Inc Telephone privacy
US3102928A (en) * 1960-12-23 1963-09-03 Bell Telephone Labor Inc Vocoder excitation generator
US3261916A (en) * 1962-11-16 1966-07-19 Ibm Adjustable recognition system

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3012098A (en) * 1941-11-22 1961-12-05 Bell Telephone Labor Inc Telephone privacy
US2810787A (en) * 1952-05-22 1957-10-22 Itt Compressed frequency communication system
US3102928A (en) * 1960-12-23 1963-09-03 Bell Telephone Labor Inc Vocoder excitation generator
US3261916A (en) * 1962-11-16 1966-07-19 Ibm Adjustable recognition system

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5012519A (en) * 1987-12-25 1991-04-30 The Dsp Group, Inc. Noise reduction system
US5323467A (en) * 1992-01-21 1994-06-21 U.S. Philips Corporation Method and apparatus for sound enhancement with envelopes of multiband-passed signals feeding comb filters
US5768473A (en) * 1995-01-30 1998-06-16 Noise Cancellation Technologies, Inc. Adaptive speech filter
US6108610A (en) * 1998-10-13 2000-08-22 Noise Cancellation Technologies, Inc. Method and system for updating noise estimates during pauses in an information signal

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Publication number Publication date
NL6602974A (xx) 1966-09-26
NL146672B (nl) 1975-07-15
FR1471628A (fr) 1967-03-03
DE1276740B (de) 1968-09-05
SE323721B (xx) 1970-05-11
GB1073203A (en) 1967-06-21
ES324621A1 (es) 1967-02-01
AT251656B (de) 1967-01-10
BE677136A (fr) 1966-07-18
CH443413A (de) 1967-09-15

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