Contains reports on two research projects.National Institutes of Health (Grant MH-04737-03)National Science Foundation (Grant G-16526)National Aeronautics and Space Administration (Grant NsG-496)United States Air Force, Electronic Systems Division (Contract AF19(604)-6102

Dennis, J. B.

Stevens, Kenneth N.

Whitman, E. C.

DSpace@MIT

XIV. SPEECH COMMUNICATION*Prof. K. N. Stevens Prof. J. M. Heinz A. P. PaulProf. M. Halle Dr. A. S. House S. S. ReismanProf. J. B. Dennis Jane B. Arnold E. C. WhitmanW. L. HenkeA. STUDIES OF THE DYNAMICS OF SPEECH PRODUCTIONDuring the past year several cineradiographic films showing lateral views of thespeech mechanism, together with sound recordings, were made in collaboration withthe Speech Transmission Laboratory, Royal Institute of Technology, and the Wenner-Gren Research Laboratory, Norrtull's Hospital, in Stockholm, Sweden. The generalobjectives of this work have been outlined elsewhere. Analysis of the films is now inprogress, and some of this analysis is being carried out by the Speech CommunicationGroup of the Research Laboratory of Electronics. This report presents examples ofsome preliminary results that are emerging from the analysis of the films.The data presented here are taken from tracings made from each frame of the filmthroughout each of several utterances. The tracings represent midsagittal views of theoutline of the vocal tract and of the surrounding structures. The time interval betweenframes on the film is 23 msec. Several measurements showing the motions of variouscomponents of the articulatory mechanism were made from the tracings. Attempts weremade to improve the accuracy of the measurements by averaging groups of tracingsrepresenting contours that remain invariant in shape, such as the mandible, hard palate,and vertebrae, and by using plaster casts to define the contours of the teeth and hardpalate.The illustrative results represent data that provide measures of the activity of thetongue tip and the pharynx. Specifically, these measurements are: (a) the distance inthe midsagittal plane from a point on the alveolar ridge to the tongue surface, measuredalong a fixed line drawn parallel to the posterior mandibular surface when the molarsare approximated; and (b) the average of two measurements of the distance between theanterior surface of the vertebrae and the posterior tongue surface at the levels of thelower surfaces of the second and third cervical vertebrae, respectively. Results forfour utterances by one talker are shown in Fig. XIV-1. The utterances are /h a'te/,/ha'tst/, /ha'to/,and /he'tat/; they were selected because they illustrate the produc-tion of several allophones of one consonant phoneme, /t/, in several vowel environmentsand of two vowels, /E/ and /0,/, in different consonantal environments.Several features of the results shown in Fig. XIV-1, together with related data onThis research was supported in part by the U. S. Air Force (Electronic SystemsDivision) under Contract AF 19(604)-6102; in part by the National Science Foundation(Grant G-16526); and in part by the National Institutes of Health (Grant MH-04737-03and Grant NB-04332-01); additional support was received under NASA Grant NsG-496.QPR No. 71 203.o- -' .. .o ...o ,o ,ha t-0 ---ce-200 -100 0 100 200 300 400 <TIME FROM RELEASE OF INITIAL CONSONANT (MSEC)-200 -100 0 100 200 300TIME FROM RELEASE OF INITIAL CONSONANT (MSEC)Graphs showing measures of displacement of tongue tip relative to alveolar ridge (upper curves) andpharynx width (lower curves) for four utterances as shown. The short vertical lines 40-60 msec tothe right of the time of release of the consonants in the upper graphs represent the times at whichaspiration terminates, as determined from spectrograms of the utterances.0-U-ZLU"Z'0xZ>lULUIoPLLFig. XIV-1.(XIV. SPEECH COMMUNICATION)other measurements for the same utterances, can be used to demonstrate some of thecontextual effects that influence the articulatory activity associated with a given phoneme.The findings must, of course, be interpreted with caution, since they represent data foronly a few utterances by one talker.1. The rate of tongue-tip motion when the /t/ is exploded in intervocalicposition is not the same for all versions of /t/, but depends on the followingvowel and on the presence or absence of a final consonant in the stressed syl-lable. The rate of motion of the tongue tip following the /t/ explosion is greatestin /ha'tat/ and least in /he'te/. These results suggest that the rate of tongue-tip motion depends on how far the tongue tip has to go (greater for /to/ thanfor /ts/), and on whether the tongue tip must return to the consonant positionafter it reaches the vowel "target" position.2. The rate of motion of the tongue tip is considerably faster during theclosing phase of /t/ than during the release. As a consequence, there is a markedasymmetry in the motion of the tongue tip during the syllables /tat/ and /tst/,where the closure following the vowel element occurs at a faster rate than therelease initiating the vowel.3. The aspiration phase during the release of the initial stop consonants in theseutterances terminates only after the tongue tip has formed a relatively large opening.During this aspiration phase the vocal tract is presumably excited by noise caused byturbulence in the vicinity of the glottis rather than at the constriction formed by thetongue. In this interval the glottis is apparently becoming more constricted in prep-aration for the initiation of vocal-fold vibrations.4. The pharynx width during the generation of these stop consonants dependsstrongly on the vowel environment. In the syllable /he 'tEt/, for example, thepharynx width during the closure for the initial /t/ preceding /6/ is considerablygreater than the width for the final /t/, which precedes silence.5. It is possible to select arbitrarily a point within each vowel at which thevowel "target" configuration is assumed to be reached most closely, which cor-responds to the time when the displacement of the tongue tip is maximal. At thisinstant of time there appears to be a slight undershoot in both tongue-tip dis-placement and pharynx width when the vowel is in the interconsonantal environ-ment, compared with the situation in which there is no consonant termination on thesyllable.K. N. StevensReferences1. K. N. Stevens and S. Ohman, Cineradiographic studies of speech, Speech Trans-mission Laboratory Quarterly Progress and Status Report, Royal Institute of Technology,Stockholm, Sweden, July 15, 1963, pp. 9-11.QPR No. 71 205(XIV. SPEECH COMMUNICATION)B. DESIGN CONSIDERATIONS FOR AN IMPROVED VOCAL TRACT ANALOGWork on the development of an improved dynamic transmission-line analog of thevocal tract has been previously reported by this group. 12 A section of acoustic trans-mission line approximated by electrical inductance and capacitance as shown inFig. XIV-2a is modeled by the configuration of components in Fig. XIV-2b. Since it mustbe possible to vary the effective area of a section over a range of at least 100 to 1, con-trol of the multipliers by the logarithm of the coefficients is convenient and desirable.'1I  L '20e l  C e2O OACOUSTIC AREA a/ACOUSTIC LENGTH a /LC(a)0  /c2el 0-O e21/L(b)Fig. XIV-2. Electrical model of a section of acoustic transmission lineand its representation by analog components.Also, digital control of the multiplier coefficient is appropriate, since it is planned tooperate the analog under the control of a digital computer. Therefore the implementationof the multiplying component in the form of a digitally controlled logarithmic attenuatoris being studied. The principle of the attenuator is described by Fig. XIV-3. The atten-uator is constructed of a cascade of switched sections having individual attenuations of1/4 db, 1/2 db, 1 db, ... , 16 db, and 32 db. The attenuation of each section is switchedin or out by one flip-flop of a register which contains the desired attenuation in binaryform.A complete vocal tract analog will contain perhaps 20 sections of the form illustratedin Fig. XIV-2, and hence at least 40 coefficients must be supplied by the computer. It isexpected that resetting of the coefficient attenuators as often as once every millisecondQPR No. 71 206(XIV. SPEECH COMMUNICATION)might be necessary. The computer, however, would deliver the coefficients in a timesequence, and hence each attenuator will be switching at a different time with respectto the l-msec cycle.In this report two problems concerning the performance of this structure areanalyzed: The first problem is the question of how well the analog represents the differ-ential equation system describing the electrical model. This question has an aspect ofstability and an aspect of error in the natural frequencies of the system. The secondproblem is the amount of noise introduced by the attenuators on account of switchingtransients, errors in adjustment, and the sampled and quantized nature of the coefficientsignals.SWITCHED ATTENUATOR SECTIONS IN CASCADE1/4db 1/2 db * 16db 32dbSIGNAL : SIGNALIN _ SWITCHING OUTSIGNALSFLIP - FLOP REGISTER CONTAINING ATTENUATIONVALUEFig. XIV-3. Principle of a digitally controlled attenuator.1. Stability and AccuracyWhen a set of differential equations is solved on a network of computing amplifiers,the roots of the solution will suffer perturbations, and extraneous roots will be gener-ated by the deviation of amplifier behavior from the ideal mathematical operations thatthey are supposed to perform The nature of these errors has been discussed by Dow 3and MacNee. 4 In this report, an analysis is given which is particularly adapted to thedesign of a dynamic vocal tract analog and should clarify the principles involved.We suppose that an arbitrary system of linear differential equations is being solvedby a configuration of summers, inverters, and identical integrators, in which only theintegrators fail to have ideal frequency characteristics. A dynamic vocal tract modelconstructed from sections of the type shown in Fig. XIV-2b will meet this assumptionif the controlled attenuators have perfect frequency response.Consider the system of differential equations after transformation to the frequencydomain through substitution of the complex-frequency variable s for time differentiationand division of each equation by the highest power of s that it contains. In using iden-tical nonideal operational amplifiers for the integration operations, one is effectivelyreplacing appearances of -in the transformed equations by the transfer function H(s)sQPR No. 71 207Figure XIV-4.Conventional arrangement for ana-log integration.-6 db/octave1/TFig. XIV-5. Frequency dependence of operational amplifier gain.Figure XIV-6.Perturbation of a natural fre-quency by nonideal integration.- aI I -_--- _-I II ICIIQPR No. 71log Alog w-%W- 0-208(XIV. SPEECH COMMUNICATION)of the nonideal integrator. The result of the replacement is a set of frequency domainequations for the analog model in terms of a new complex-frequency variable s'.1-= H(s'). (1)sSuppose that the original differential equation system has roots or natural frequenciess = sn, s 2 ,  ... , s n .  (2)Then the analog system with nonideal integrators will have roots for every value of s'which yields one of the original natural frequencies (2) when substituted in (1). If the1transfer function H(s) is nearly - in the vicinity of a root s i of the equation system, thenthe analog model will have a root s! very near s.. Since (1) may have a number of solu-1 1tions for any particular value of s, the model will also have extraneous roots. In thefollowing analysis the transformation (1) is constructed for typical amplifier behavior,and the relation between roots of the equation system and roots of the analog model isinvestigated.The standard configuration for a practical analog integrator is shown in Fig. XIV-4.The gain A of a practical operational amplifier is well represented by the curve inFig. XIV-5.A(s) = (3)1 + TSwhere k is the dc gain, T is the time constant of the principal frequency break point,and = k/T is the unity gain crossover frequency. The transfer function of the practicalintegrator is found to beE (s) o WG(s) = = (4)E.(s) 2 k +-- + s+ RC(s-sl)(s-s 2 )where-b ± /b 2 - 4c (5)s2- 2and, in turn,b +k + 1 (6)ST RC RCT1For satisfactory performance as an integrator, it is necessary that we RC- and there-fore b 2 >> 4c. HenceSb +k -1 (7)1 7 RCcQPR No. 71 209(XIV. SPEECH COMMUNICATION)-b- b c22 b cs ---1s2 kRC + T(8)The frequency range utilized for computation is the range for whichand in this range1G(s) RCsRCs'Therefore the appropriate function for use in (1) iswH(s) = RCG(s) c (9)(S-Sl)(S-S 2)and the conversion from the equation system to the analog model is described by thechange of variable(s'-sl)(s'-s 2)s = (10)cAccording to this substitution, each natural frequency s = s of the equation systembecomes a pair of natural frequencies in the analog. To find the relation between rootsof the model and those of the equation system, we solve (10) for s' in terms of s. Settings = a- + jW and using the approximations s 1 >> s and s we , we find-2s' ' + -5 + - iC (11)a a a o kRC + Tcandsb =b + jc - 'c- j. (12)As shown in Fig. XIV-6, s' is the original natural frequency with its real part perturbedaby an amount_ZE 1 (13)- kRC + TcQPR No. 71 210(XIV. SPEECH COMMUNICATION)The root s b is extraneous, but its large negative real part prevents its affecting thesolution seriously. These results agree with those stated by MacNee 4 and with experi-mental findings of Hills 5 and the present authors.The shift of the important root has two components. The positive one, which couldlead to instability if excessive, is proportional to 1/wc The negative component is1 capproximately proportional to - k and leads to exaggerated damping. The minimi-zation of both effects places conflicting requirements on w , and a judicious compromisewill be necessary to ensure optimum results.2. NoiseNoise introduced by the digitally controlled attenuator component can be divided intotwo categories; additive noise - that which is present in the absence of a signal - andmultiplicative noise - that which is proportional in intensity to the signal. The multipli-cative noise arises from the sampling and quantization processes inherent in the digitalorigin of the coefficient signals. The additive noise is introduced primarily by switchingtransients and errors in adjustment.Introduced sampling and quantization noise can be analyzed with the help of the modelin Fig. XIV-7. In the model the sampling operation precedes quantization for conven-ience of analysis. Although the quantization steps are actually proportional to coefficientu(t) v (t)XSIGNAL IN SIGNAL OUTOUT 20 IN) QUANTIZERSAMPLERI IF(t) I ' T -t tc0 XCOEFFICIENTIFig. XIV-7. Model of digital attenuator for analysis ofsampling and quantization noise.QPR No. 71 211(XIV. SPEECH COMMUNICATION)values, the use of uniform steps is adaquate, for the coefficients have slowly varyingvalues.Note that if we fix attention on a particular control waveform f(t), the model is alinear time-varying system with regard to its effect on the signal u(t). Therefore, theeffect of the control waveform on a complex signal can be determined from the effect ona sinusoid through the principle of superposition.Consider, then, u(t) = cos wt. The desired output is w(t) = u(t) [l+f(t)], and the actualoutput is the desired output plus noise.v(t) = w(t) + n(t).Since f(t) is a slowly varying function, its spectrum F(w) is concentrated at low frequen-cies. The spectrum G(w) of the sampler output is F(w) replicated at intervals of 2Tr/Talong the radian frequency axis and multiplied by the squared magnitude system function2 wTsin2  2TwTof the boxcar circuit. This computation is illustrated in Fig. XIV-8. The low-frequencycomponent of G(w) 2 is the desired signal with high frequencies cut down slightly by theF () 20P F (w) 2-4rrT -2TTT 0 2r/T 4/TIx 2IG (w) 12-20T 0 2,T/TFig. XIV-8. Computation of power spectrum of sampled coefficient signal.QPR No. 71 212(XIV. SPEECH COMMUNICATION)sin x/x function. The degradation will be small if the bandwidth of f(t) is small comparedwith 2Tr/T. The other components of G(w) are noise.The quantization operation adds a noise component that has the form of a rectangularq qwave with random amplitude values in the range from -- to as shown in Fig. XIV-9a.62 2'The statistical theory of quantization shows that these amplitude values may be regardedas independent samples from a uniform distribution over this range, if the quantizationbox is small enough so that adjacent samples are likely to differ by more than one step.Experience with quantized systems has shown that this condition on box size does nothave to be well satisfied to achieve apparent independence of the quantization noise.Under these conditions, the power spectrum of the quantization noise will be thesin2 x/x 2 curve shown in Fig. XIV-9b. The sampling and quantization noise componentsreach the attenuator output only through modulation of the signal. For a sinusoidal sig-nal, the noise spectra appear at the output shifted by the signal frequency. For a com-plex signal, the resultant noise will be the superposition of these noise spectra withvarious frequency shifts according to the frequency content of the signal.+q/2 - - - - - - - - - - - -- - - - - - - - - - - - -(a)26-2i/T 2r/T(b)Fig. XIV-9. Form of quantization noise as a random-amplitudesquare wave and its power spectrum.The additive component of noise originating from switching transients takes the formof a train of short pulses having pseudorandom amplitudes and a uniform time spacingT. Such a waveform has a uniform power density in the audio-frequency range if thepulses have independent amplitudes. An error in adjustment of an attenuator sectionwill cause some of the switching signal to appear at the attenuator output. If each hasan unrelated error of this type, the corresponding noise component will be a random-amplitude square wave having the form of spectrum shown in Fig. XIV-9b.QPR No. 71 213(XIV. SPEECH COMMUNICATION)These sources of noise in the attenuator output make a careful choice of parametersnecessary in the design of the attenuator. At present, an attenuator design is being eval-uated with respect to noise contributions and their effect on the performance of an analogvocal tract model.J. B. Dennis, E. C. WhitmanReferences1. E. C. Whitman, An Improved Dynamic Analog of the Vocal Tract, S.M. Thesis,Department of Electrical Engineering, M.I.T., January 1963.2. E. C. Whitman, A transistorized articulatory speech synthesizer, QuarterlyProgress Report No. 68, Research Laboratory of Electronics, M.I.T., January 15, 1963,pp. 164-167.3. P. C. Dow, An analysis of certain errors in electronic differential analyzers,IRE Trans., Vol.EC-6, pp. 255-260, December 1957.4. A. B. MacNee, Some limitations on the accuracy of electronic differentialanalyzers, Proc. IRE 40, 304-308 (1952).5. F. B. Hills, Application Studies of Combined Analog-Digital Computation Tech-niques, Report ESL-FR-165, Electronic Systems Laboratory, M.I.T., February 1963.6. B. Widrow, A study of rough amplitude quantization by means of Nyquist samplingtheory, IRE Trans., Vol. PGCT-3, No. 4, pp. 266-276, December 1956.QPR No. 71 214

Speech Communication

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