EP0069673B1 - Analyseur spectral à filtres communs à deux voies, notamment pour la reconnaissance vocale - Google Patents

Analyseur spectral à filtres communs à deux voies, notamment pour la reconnaissance vocale Download PDF

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Publication number
EP0069673B1
EP0069673B1 EP82401272A EP82401272A EP0069673B1 EP 0069673 B1 EP0069673 B1 EP 0069673B1 EP 82401272 A EP82401272 A EP 82401272A EP 82401272 A EP82401272 A EP 82401272A EP 0069673 B1 EP0069673 B1 EP 0069673B1
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EP
European Patent Office
Prior art keywords
filters
output
filter
input
pass output
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EP82401272A
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German (de)
English (en)
French (fr)
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EP0069673A1 (fr
Inventor
Christian Terrier
Christian Caillon
Daniel Barbier
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Pour L'etude Et La Fabrication De Circuits Integres Speciaux - Efcis Ste
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Pour L'etude Et La Fabrication De Circuits Integres Speciaux - Efcis Ste
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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

Definitions

  • the present invention relates to a spectral analyzer, that is to say a filtering circuit capable of receiving an electrical signal having a certain frequency spectrum and of determining the energy contained in each of several narrow frequency bands of this spectrum.
  • the electrical signal can come from a microphone in front of which one is speaking, and the spectral analyzer is then used to analyze or recognize the speech emitted.
  • the energy spectrum of certain emitted phonemes (and in particular the vowels and sound consonants) is indeed characteristic of these phonemes.
  • FIG. 1 shows a conventional diagram of a spectral analyzer used for speech recognition.
  • the spectral analyzer essentially comprises a series of parallel filtering channels V1 to Vn, a multiplexing system 18 and an analog / digital converter 20.
  • a logic control circuit 22 controls the operation of the filters of the channels V1 to Vn, of the multiplexing system, and of the converter.
  • Each filtering channel Vi comprises a bandpass filter FBi with a narrow band having for example two cutoff frequencies, these filters having a strong rejection outside the range of their cutoff frequencies (for example -40 dB / decade). These may be, for example, fourth order filters.
  • the filtering channels have narrow bandwidths which are substantially adjacent, that is to say that the upper cutoff frequency of a filter is the same as the lower cutoff frequency. of the next filter.
  • the filtering channels can be of variable number, for example 16 or 32 with a logarithmic distribution of the passbands of each filter between 100 Hz and 5000 Hz (the lower cutoff frequency f o of the first filter FB1 being approximately 100 Hz and the upper cutoff frequency of the last FBn filter being approximately 5000 Hz).
  • Each filter is followed by a rectifier without threshold (R1 to Rn) itself followed by an averaging integrator (11 to In) which can be a low-pass second order filter having a cutoff frequency of approximately 25 hertz for lower frequency channels, this frequency may be higher for higher frequency channels.
  • the multiplexing system receives the signals from each channel, that is to say it receives signals which each represent the signal energy contained in a respective narrow band of frequencies. Controlled by the control logic 22, this multiplexing system samples cyclically (with a period of approximately 20 milliseconds because it is at a period of this order that it is estimated that the phonemes are renewed in a normal speech transmission) a signal value at the output of each channel and transmits it to the analog-digital converter 20. The latter therefore receives, during each period of 20 milliseconds, a succession of n signal samples each corresponding to the output of a channel filtering. These samples are converted into digital signals and the output of the spectral analyzer therefore emits successions of digital values which are coefficients representing the energy of the signal in each narrow band of the spectrum.
  • the present invention proposes a spectral analyzer structure which differs slightly from the structure of FIG. 1 as regards the arrangement of the filtering channels and which makes it possible to replace n filters of relatively high order (for example of order 4) by n + 1 filters of lower order (for example 2), without losing on the quality of the filtering in each band.
  • each bandpass filter at two main cutoff frequencies into two simpler filters, each having a main cutoff frequency and having two different outputs which are respectively a lowpass output having this cutoff frequency and a high pass output having the same cutoff frequency.
  • it is used first as a low-pass filter in cascade with another simple high-pass filter with lower cut-off frequency, then in the second step as a filter Cascade high pass with a single low pass filter with higher cutoff frequency.
  • one of the filter outputs is used, and in the second step, the other.
  • two complex filters of different passbands are reconstructed.
  • the result is that with this filter switching, two complex filters are produced with three simpler filters, and, more generally, if this is done for all the filtering channels, n complex filters with n + 1 simpler filters. The size of the circuit is thus significantly reduced.
  • a new spectral analyzer structure which includes several elementary filters each comprising a low-pass output and a high-pass output both having the same cut-off frequency for the same filter and different for different filters, and switching means for periodically connecting for a first time interval the elementary filters in groups of two in cascade between a signal input to be analyzed and a transmission channel specific to each group, one elementary filters having its high-pass output (or respectively low-pass) connected to the input of the second elementary filter whose output used is the low-pass output (or respectively high-pass), and for periodically connecting for a second time interval the elementary filters in cascade by groups of two different from the first groups, the output used for an elementary filter penda nt the second time interval being different from the output used during the first time interval.
  • a good method for cutting the frequency band to be analyzed into several narrow bands, with high rejection outside the useful band consists for example in using, to make each band, a bandpass filter having two cutoff frequencies with a slope by +12 dB per octave below the lowest cut-off frequency f i and a slope of -12 dB per octave above the highest cut-off frequency f 1 + 1 , and with a flat part between the two (this response curve has the form illustrated in Figure 4).
  • the corresponding filter can be established by the method of state variables, consisting of starting from the highest degree term AA'p 4 S (p), which is a fourth derivative of the output signal, integrating it four times to obtain the third, second, first derivatives, and the output signal itself, and to constitute from the outputs of each integrator and an input of signal E (p), a circuit which checks the equation (1) .
  • the high-pass filter will have the transfer function:
  • the low-pass filter will have the transfer function:
  • Equation (4) is immediately translated as a circuit ( Figure 2) by noting that from a signal A'p 2 S (p) assumed to exist, we can divide this signal by A '(attenuator 30) , integrate it to obtain a signal pS (p) (integrator 32), and integrate it further to obtain a signal S (p) (integrator 34) which will represent the output of the filter; moreover, we multiply the signal S (p) by a coefficient C '(amplifier 36), we multiply pS (p) by a coefficient B' (amplifier 38) and we thus obtain signals C'S (p) and B'pS (p); in an arithmetic summator 40, a signal E (p) is introduced which will be the input signal of the filter, and the signals B'pS (p) and C'S (p) are subtracted. The output of the adder therefore provides a signal E (p) - B'pS (p) - C'S (p).
  • the output of the second integrator 34 can be used as the output of the filter, but the output of the attenuator 30.
  • this output provides a signal which is p 2 S (p), and which is therefore: which is precisely a transfer function of a second order high pass filter.
  • the cutoff frequency is the same in both cases, it is defined by the polynomial A'p 2 + B'p + C '.
  • the same second order filter first as a low-pass filter associated in cascade with a high-pass filter of lower cut-off frequency, then as a high-pass filter. associated in cascade with a low-pass filter with a higher cut-off frequency. If the cutoff frequency of the filter considered is the same in both cases, two fourth-order bandpass filters, having adjacent frequency bands, will have been produced successively, with only three second-order filters. Similarly, if we have a whole series of n fourth order filters, they can be replaced by n + 1 second order filters.
  • FIG. 5 shows the arrangement of a spectral analyzer which makes it possible to achieve this economy, but it can already be said that the example which has just been given of a filter of the fourth order broken down into two filters of the second order can be generalized, the method remaining the same: a sixth order filter can be decomposed into two third order filters, and even a fifth order filter can be decomposed into a second order filter and a third order filter, with however in the latter case a modification in the sense that two fifth order filters with adjacent frequency bands which will be created using the same filter will not have identical response curve shapes since in one case there will be a slope of 18 dB / octave at low frequency and 12 dB / octave at high frequency and in the opposite case.
  • each channel comprises, as in FIG. 1, a rectifier without threshold and an averaging integrator not shown, and that after the averaging integrators, the various channels are connected to a multiplexing circuit controlled so as to take cyclically, with an overall period of approximately 20 milliseconds, a sample on each channel.
  • the multiplexing circuit At first in the 20 millisecond period, only half of the channels transmit a useful signal, for example the odd numbered channels, and the multiplexing circuit is arranged so as to take samples only on these channels. Secondly, the other half (even numbered channels) transmits useful signals and the multiplexing circuit takes samples on these other channels.
  • Switching means are provided in each channel, with appropriate control means, so that the various filters used can serve alternately in an odd numbered channel and in an even numbered channel depending on whether one is at first or in the second time of the multiplexing cycle.
  • the filters F o to F n are n + 1 for n channels and each filter has a main cutoff frequency, f o to f n , with attenuation for example at 12 dB per octave (second order), and with a low pass output (PB) and a high pass output (PH).
  • PB low pass output
  • PH high pass output
  • the input signal to be analyzed is applied to the inputs of the filters through switches Ko to Kn (for example MOS transistors); the switches of even rank are closed during the first time of the multiplexing cycle and open during the second time.
  • switches Ko to Kn for example MOS transistors
  • switches K'I to K'n are connected downstream of the low-pass outputs of the various filters (except the first filter) to connect these outputs to the other elements of the channels VI to Vn.
  • the switches K'I to K'n are closed and open in phase opposition with the switches KI to Kn.
  • K "I to K” n are connected between the high-pass output of a filter (Fo to Fn-1) and the input of the following filter (FI to Fn). These switches are closed and open in phase with switches K'I to K'n.
  • a switching control circuit 41 acts on the switches, in synchronism with the control of the multiplexing circuit.
  • This switching control circuit is part of a control logic which also has the functions mentioned in connection with FIG. 1, namely the control of the multiplexing, of the analog-digital converter which can be put at the output of the multiplexing circuit, and switching of integration capacities if the filters are switched capacity filters.
  • the filter Fo receives on its input the signal to be analyzed, and has its high-pass output connected to the input of the IF filter whose low-pass output transmits on channel VI by the closed switch K'I a signal filtered by the cascaded FO and FI filters.
  • the frequencies in the narrow band f o , f 1 are therefore transmitted on channel 1.
  • all filters of even rank have their low-pass output isolated from the channel of the same rank, which therefore does not transmit any signal, but receive on their input the signal to be analyzed and have their high-pass output connected to the filter input of odd rank immediately higher; this is isolated from the signal to be analyzed and is connected by its low-pass output to the odd-ranking channel which corresponds to it.
  • the frequencies included in the bands f o , f 1 / f 2 , f3 /.../ f n ⁇ 1 , f n are thus transmitted, and in the second time, they are on the contrary the frequencies of adjacent interlayer strips f 1 , f 2 / f 3 , f4 /.../ f n ⁇ 2 , f n ⁇ 1 .
  • the multiplexing circuit first takes samples on the higher frequency channels and then on those of the lower frequency channels so that the outputs of the filters and of the integrators which follow them in each channel has better time to settle at their new value (the lower frequencies being established more slowly).
  • the filters were connected in series in the following order: high-pass output of a filter connected to the input of a filter with a higher cut-off frequency.
  • the low-pass output of a filter is connected to the input of a filter with a lower cut-off frequency.
  • the low-pass and high-pass outputs of the filter Fi ⁇ 1 preceding the filter in question are respectively connected to two different transmission channels Vi -1 1 and Vi each comprising a rectifier without threshold and an integrator not shown, as in the case of FIG.
  • the low-pass and high-pass outputs of the filter (Fi + 1) according to the filter considered are connected, respectively to the following two channels Vi + 1 and Vi + 2.
  • Switches K'i operating in phase opposition can be provided between the outputs of a filter and the corresponding channels.
  • the switch K'i will be closed when the switch K "i is closed.
  • FIG. 6 it is the even rank filters which have their input permanently connected to the input of the signal to be analyzed, and the odd rank filters which have their low pass and high pass outputs connected by switches K 'i and K'i + 1 to the respective channels Vi and Vi + 1.
  • This arrangement has the advantage of eliminating the switches which were necessary in FIG. 5 between the signal input to be analyzed and the filter inputs.
  • Multiplexing is therefore carried out by taking on each channel two different samples, respectively one during the first time of the cycle and another during the second time.
  • the filters are preferably made in the form of filters with switched capacities, that is to say filters in which each integrator is constituted by an operational amplifier A looped by a capacity Cs of feedback, but which, instead of having an input resistor Re in series (which would define with the capacitance Cs an integration time constant ReCs), has as input circuit an input capacitance in parallel which can be isolated either from the signal input of the integrator, either of the input of amplifier A by two switches, preferably two MOS transistors, T1 and T2, working under the control of complementary signals Q and Q * , or at least signals such as switches are never closed both at the same time.
  • switches preferably two MOS transistors, T1 and T2
  • the integration time constant is inversely proportional to the switching frequency f e .
  • the cutoff frequencies of the filters of the spectral analyzer can therefore be modified by action on the frequency f e ; for example, it may be desired that the spectrum of frequencies analyzed is cut into narrow bands which are not entirely adjacent, that is to say that two bandpass filters corresponding to successive bands do not have a common cutoff frequency which is the high cutoff frequency of one and the low cutoff frequency of the other; in this case, the invention will still remain applicable if the cutoff frequencies are modified, by action on the switching frequency of the capacitors Ce, between the first time and the second time of the multiplexing cycle: a cutoff frequency which is f i initially would become f ' ; , in a second step and, instead of cutting a spectrum into narrow bands of strictly adjacent cut-off frequencies f i ⁇ 1 , f i and f i , f i + 1 we would cut it into two narrow bands f i ⁇

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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)
  • Filters That Use Time-Delay Elements (AREA)
  • Measurement Of Resistance Or Impedance (AREA)
  • Monitoring And Testing Of Transmission In General (AREA)
EP82401272A 1981-07-10 1982-07-06 Analyseur spectral à filtres communs à deux voies, notamment pour la reconnaissance vocale Expired EP0069673B1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
FR8113674A FR2509500A1 (fr) 1981-07-10 1981-07-10 Analyseur spectral a filtres communs a deux voies, notamment pour la reconnaissance vocale
FR8113674 1981-07-10

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EP0069673A1 EP0069673A1 (fr) 1983-01-12
EP0069673B1 true EP0069673B1 (fr) 1986-05-07

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US (1) US4459542A (ja)
EP (1) EP0069673B1 (ja)
CA (1) CA1182921A (ja)
DE (1) DE3270980D1 (ja)
FR (1) FR2509500A1 (ja)

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Publication number Priority date Publication date Assignee Title
CA1284179C (en) * 1986-04-11 1991-05-14 Cal Corporation Method and apparatus for simultaneous instantaneous signal frequency measurement
GB2237649B (en) * 1989-09-08 1994-02-09 Gale P Michael Weighted channelized receiver
US5291125A (en) * 1992-09-14 1994-03-01 The United States Of America As Represented By The Secretary Of The Air Force Instantaneous frequency measurement (IFM) receiver with two signal capability
JP3505197B2 (ja) * 1993-04-12 2004-03-08 三菱電機株式会社 波形整形回路
JPH08213881A (ja) * 1995-02-02 1996-08-20 Fujitsu Ltd 周波数制御回路
FR2738628B1 (fr) * 1995-09-08 1997-10-24 Sextant Avionique Dispositif optique de determination de l'orientation d'un solide
JP3357807B2 (ja) * 1997-01-13 2002-12-16 株式会社東芝 受信装置および移相器
US6495998B1 (en) 2000-09-28 2002-12-17 Sunrise Telecom Corp. Selectable band-pass filtering apparatus and method
US6915264B2 (en) * 2001-02-22 2005-07-05 Lucent Technologies Inc. Cochlear filter bank structure for determining masked thresholds for use in perceptual audio coding
US11606154B2 (en) * 2021-01-26 2023-03-14 Rohde & Schwarz Gmbh & Co. Kg Wideband spectrum analyzer

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US3519926A (en) * 1966-10-31 1970-07-07 Electro Optical Ind Inc Digital wave analyser having sequentially scanned substantially identical,low pass filters
GB1153267A (en) * 1967-03-10 1969-05-29 Hitachi Ltd Filter Circuit
US3581192A (en) * 1968-11-13 1971-05-25 Hitachi Ltd Frequency spectrum analyzer with displayable colored shiftable frequency spectrogram
US4137510A (en) * 1976-01-22 1979-01-30 Victor Company Of Japan, Ltd. Frequency band dividing filter
JPS6016582B2 (ja) * 1977-03-04 1985-04-26 日本電気株式会社 デイジタル周波数分析装置
JPS584307B2 (ja) * 1979-10-23 1983-01-25 富士通株式会社 スペクトル分析器

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FR2509500A1 (fr) 1983-01-14
CA1182921A (en) 1985-02-19
EP0069673A1 (fr) 1983-01-12
US4459542A (en) 1984-07-10
DE3270980D1 (en) 1986-06-12
FR2509500B1 (ja) 1983-10-21

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