Contains a report on a research project.Purchase Order DDL-B15

Washington, L., Jr.

Witcher, C. M.

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XI. SENSORY REPLACEMENT*Dr. C. M. WitcherL. Washington, Jr.A. VOCATACThe transducer mentioned in the Quarterly Progress Report of July 15, 1956, wascompleted; its speed of operation is significantly superior to that of the previous outputdevice. As stated before, the tactile stimuli are produced by small (3/32 inch in diameter)steel balls which are pushed upward against the finger tip by motion of the armatures ofsix small relays. Figure XI-1 is a sketch of the " sandwich" arrangement in which thestimulator balls are suspended. To assemble this arrangement, the relatively thickmiddle plate was machined with small shoulders that could be spun down around the outeredge of each of the outer plates. Accurate alignment was obtained by pinning the threeplates together at one point before carrying out the final spinning operation. The ballsupport assembly fits into a circular hole in the center of the top cover plate of thetransducer and is clamped in place by a ring, after having been rotated to align the ballswith the ends of the relay armatures. The relays are Allied Control Company type AS3D,with coil resistances of 8945 ohms. After removing the contacts, small brass extensionarms were soldered to the armatures in order to actuate the stimulator balls. Whenconnected to the Schmitt circuit, which will be described below, the elements of thistransducer would respond satisfactorily to an impulse of 8-10 msec duration.In accordance with our plan, a set of Schmitt trigger circuits was developed toreplace the former output circuits of the Vocatac. Each of these trigger circuits wascoupled through a diode and RC filter directly to the output side of one of the tunedfilters (described in the Quarterly Progress Report of Jan. 15, 1956, p. 134).Figure XI-2 is a schematic of the typical Schmitt circuit and coupling network. A veryuseful property of the Schmitt circuit is the fact that the plate current of V 2 can beswitched on or off by varying the dc potential of the grid of V 1 . By proper selection ofthe value of R 5 and of the positive and negative supply voltages, the potentials on thegrid of V 1 that are required for switching the plate current of V 2 on and off can bebrought very close to each other. For the circuit constants shown in Fig. XI-2, theplate current in V 2 was turned on when the dc potential of the grid of V 1 reached nega-tive values in the range from -2. 3 volts to -2. 7 volts. V 2 is switched off for potentialson the grid of V 1 between -1. 5 volts and -1. 8 volts. A bias battery connected in thecommon ground lead maintains the V 1 grids of all the trigger circuits at a positivepotential when no signals are passing through the Vocatac, as can be seen from Fig. XI-2.When the signal-voltage amplitude in any tuned circuit reaches a sufficiently high value,This work was supported in part by Purchase Order DDL-B158.1186 HOLES 0.091 DIAMET0.291 IN EACH DISC0192 0099-01.250, ol , .... . - ,6 STEEL BALLS0 009375 DIAMETERA- /_PEEN OVER EDGESO O 2 BRASS DISCSo 0 I" DIAMETER x 0.011 THFig. XI-1. Ball assembly for tactile transducer.119K RELAYIN301 DIODE IMINPUTFROMTUNEDFILTER- 120VFig. XI-2. Schematic of typical Schmitt circuit used in the Vocatac.vowelsoundsRiIlchannel 2 3 4 5 6Fig. XI-3. Recorded response patterns of the Vocatac. Center frequencies ofchannels: (1) 382 cps; (2) 548 cps; (3) 715 cps; (4) 990 cps; (5) 1529 cps;(6) 3000 cps.119(XI. SENSORY REPLACEMENT)the negative potential applied to the corresponding V 1 grid through the diode and filterwill cause the circuit to switch on. When these circuits were completed, the tunedfilters were all readjusted to give a loaded Q of 10, and the system was adjusted togive a gain characteristic that rises 3 db per octave. This slower rate of rise waspreviously found to be preferable when clipping circuits were introduced into the initialstages of the Vocatac system.The tactile patterns obtained with the new arrangement exhibited considerably moredetail than those previously obtained. They also appeared to be somewhat more consis-tent for any given talker, but their more detailed space and time variations of structureserved to emphasize differences between voices of different individuals. To facilitateaccurate observation of the patterns, the transducer was replaced by a similar set ofrelays which had not had their contacts removed, and these relays were connected toenergize six of the pens on an Esterline-Angus recorder. Figure XI-3 is a photographof three sets of patterns thus obtained. The first set is for the vowel sounds " a" , " e" ,and " i" , pronounced as in the syllables " ray" , " ree" , and " rye" . The second groupexhibits the syllables just written; and the third is for another set of vowel sounds, " i"," a" , and " i" , pronounced as in " it" , " at" , and " ah" . These sounds were actuallyrecorded on an endless loop of tape and then played back repeatedly through the Vocatacsystem. For any given tape recording, the patterns were invariant under at least a 20-dbchange in signal level. When successive tape recordings were made by the same talkerand then played back to produce printed patterns, the consistency was good, although notabsolutely perfect. Note the differences between the patterns for the three initial " r"sounds in the second group of Fig. XI-3. These are apparently caused by the anticipa-tory effects of the succeeding vowels. A few other combinations of vowels and finalconsonants (" r" and " d" ) were studied, with similar observations.Unfortunately, one discouraging result appeared. When attempts were made toobserve the patterns tactually by one of us (Witcher), a reasonably good Braille reader,much of the spatial and temporal detail was imperceptible. This was still true afterconsiderable time spent in attempting to " learn" to perceive this detail. As we now seeit, further study may be in order along any or all of the following lines. (a) Afterrecording a variety of patterns such as those of Fig. XI-3, an attempt can be made toextract the details that remain essentially invariant when produced by different talkersof the same sex and same approximate age. The system might then possibly be rede-signed to present only these invariant details. (b) Further studies could be conducted todetermine the rate at which a subject can comprehend temporal changes, first involvingtwo stimulus points and then progressively passing to the six stimulus points of thetransducer. (c) Using the present system, experiment with other test subjects who arereasonably proficient Braille readers to determine whether or not they might be able tomanifest more success in pattern learning. (d) Finally, the idea of a Braille-like form120(XI. SENSORY REPLACEMENT)of output may have to be abandoned in favor of a set of tactile stimulators arranged morelike those employed by J. B. Wiesner, N. Wiener, et al. (Quarterly Progress Report,Jan. 15, 1949, et seq.). Perhaps one stimulator might again be assigned to each of thefive fingers and one to some area in the palm of the hand. This procedure would bebased on the hope that tactile patterns characterized by rapid spatial and temporal vari-ations may be capable of transmitting information to the brain at a higher rate when theyhave an opportunity to affect a larger number of nerve fibers.C. M. Witcher, L. Washington, Jr.[Note added in proof. We recently undertook the design and development of adirect-translation reading machine for the blind. The tape output of the machine woulddisplay a facsimile of printed material by a dense array of pin holes that could beperceived as a roughness to touch. If this idea were applied to the output of the Vocatac,it would allow a deaf-blind reader to examine, at his own rate of comprehension, thedetails of patterns such as those shown in Fig. XI-3.]121

Sensory Replacement

Contains research objectives and reports on two research projects

XVIII. SENSORY REPLACEMENTDr. C. M. WitcherL. Washington, Jr.RESEARCH OBJECTIVESAs its title implies, this program has as its ultimate objective the development ofmeans for replacement of the visual and auditory senses in the performance of certainspecific tasks. The areas in which possibilities for visual prosthesis have thus far beenconceived are: (a) development of means to enable the blind to walk about safely andindependently and at normal speed (4-5 ft/sec) when not passing through vehicular traf-fic; (b) development of apparatus to enable the blind to read printed or written material;and (c) invention of devices to permit the blind to recognize familiar objects at a dis-tance, probably through the conversion of visual patterns into auditory patterns. Thusfar, the major effort has been concentrated in the first of these areas, but considerablethought has been given to the second.Up to the present time, one auditory prosthesis project has been initiated. This isthe development of the Vocatac - a device intended to transform voice sounds into tactu-ally perceptible patterns. If this device proves successful, it should be of considerablehelp to the totally deaf who are not familiar with lip reading and to those who are bothblind and totally deaf. A description of this apparatus appears in Section XVIII-B.A. TRAVEL AIDAs mentioned in the Quarterly Progress Report of July 15, 1955, the optical systemnow in use in the travel aid was shown to be quite inadequate. In the receivers, systemsare needed whose circles of confusion do not exceed 0.001 inch in diameter. The imagequality is not important; only the condition that the dimensions of the area imaged by thereceivers should not significantly exceed those of the simple geometric image presentsdifficulties. For some weeks the problem of designing such systems, subject to speci-fied conditions regarding weight, physical dimensions, and so forth, has been underconsideration by the optics design group at Boston University. Quantitative examina-tions have been made of the possibilities of utilizing either an unmodified Maksutovsystem or a Schmidt system. The computations are based on a specified "F" numberof 0.7 or less, an aperture diameter of 2-2.5 inches, and a maximum field of 9.00.Actually, in the receiving systems, the field would not need to exceed 3.60, but it mayprove advisable to use a similar system for the transmitter, which requires a 9.00field (1).The Maksutov system would be more economical to produce, especially in relativelysmall quantities, since the correcting meniscus has only spherical surfaces. However,in this kind of system the focal plane is very nearly tangent to the corrector meniscus.This is not a serious disadvantage in systems that are to be used as receivers becausethe corrector plate could have at its center a hole that is large enough to admit thephotocell without degrading the performance significantly. However, the disadvantagein the case of the light source projector (1) would be serious. At this time, calculations132(XVIII. SENSORY REPLACEMENT)for both systems are nearing completion. It should be mentioned that the Schmidtsystem which has been in use as the projector is of very poor quality, having beendesigned for use in a piece of military infrared equipment in which image quality wasnot important.Calculations recently made by Witcher (2) have indicated that, with acceptableoptics, the present travel aid should be able to provide reliable step-down detectionexcept when signal attenuation caused by a puddle or a wet surface amounts to 33-34 db.This figure is based on the fact that the signal-to-noise ratio reaches a value ofunity when attenuation from these causes amounts to 37 db. Hence, false alarms maybe expected under the worst possible reflection conditions, since values of attenuationas great as 40 db have been observed under such conditions (3).C. M. Witcher, L. Washington, Jr.References1. Quarterly Progress Report, Research Laboratory of Electronics, M.I.T.,Oct. 15, 1954.2. C. M. Witcher, Analysis and design of a travel aid for the blind. Paper presentedat the meeting of the A.A.A.S., Dec. 1955.3. Quarterly Progress Report, Research Laboratory of Electronics, M.I.T.,Jan. 15, 1954.B. THE VOCATACSome years ago J. B. Wiesner, N. Wiener, and others in this Laboratory (1) carriedout theoretical and experimental studies of the possibility of converting speech intoa sequence of tactually perceptible patterns that a totally deaf person might learnto interpret. Unfortunately, the project had to be dropped because of the press of otherwork, and hence the matter remains open. We have often considered the problem ofdesigning a relatively simple apparatus with which a new attack on this problem couldbe initiated. The apparatus has now been constructed as a bachelor's thesis projectby L. Washington, Jr.In the original device (Felix), a number of bandpass filters were used to subdividethe voice range. The condition employed to fix the successive band limits was that eachband should represent approximately an equal amount of energy when averaged overspeech. At first, five channels were used, the outputs being arranged to control theamplitudes of five small vibrators on which the fingers of one hand rested. The vibra-tors all operated at the same frequency, supplied by an auxiliary oscillator. Later, aseven-channel system was used in which the outputs controlled a set of electrical stimu-lators in contact with the forearm.133(XVIII. SENSORY REPLACEMENT)In the present version, use is made of tactile stimuli of the presence-or-absencetype with no amplitude variations. In fact, the outputs of the six channels in the presentapparatus actuate six small round-head pins that are thus brought in contact with thetip of the index finger. The arrangement and spacing of pin positions is the same asthat of dot positions in a standard Braille cell. This scheme has been adopted becauseof the remarkably high rate of speed with which the blind can receive information inreading Braille (2, 3). In addition to the six tactile stimulator channels, a seventh chan-nel is provided to energize a fairly rugged vibrator that permits perception of the sylla-bic rhythm of speech and furnishes an indication of the occurrence of voiced consonants.Instead of bandpass filters, tuned circuits are used in the present apparatus. Asthis is written, single-tuned circuits with 10 per cent bandwidths are being employed,but provisions have been made to use double-tuned circuits if it is found advisable. Theresonant frequencies of the tuned circuits have been selected on the basis of the so-called importance function given by Fletcher and Galt (4). This function measuresthe relative importance of each frequency in the voice range to reduce errors in theidentification of isolated syllables. By using this function, the part of the voice rangefrom 200 cps to 4500 cps was first divided into six frequency bands, each of which con-tributed equally to the reduction of errors in syllable identification. The resonantfrequency corresponding to each of these bands was chosen at the point where the"importance function" attained its maximum value within that band. The frequenciesthus obtained are: 382, 548, 715, 990, 1529, and 3000 cps. The gain characteristicat the outputs of the tuned filters rises at the rate of 6 db per octave, in accord-ance with the procedure of Potter and Steinberg (5), so that approximately the same200KJONESPLUGFig. XVIII-1. Six-channel Vocatac. Fig. XVIII-2. Schematic of typicaltuned filter of Vocatac.134TUNED FILTER---------------- 1I r------ni I I 20 I-- ICOMMON TO ALL12AT7 CATHODES0- 100IOO1 f V, 12AX70.005,f V, 12AT70.002 tf V 3 6SN7I.Op.f V4 6SN7200pf D, IN93200 f D2 IN9350uf D3 IN9150pf D4 IN91IO f D5 IN340.1f T I  CHICAGO TRANSFORMERCIS - 20.iuf T 2 STANCOR TRANFORMERFig. XVIII-3. Input and typical channel of Vocatac.TI 4LjLURTO R ,2I000a POT100 POT100.. POTlOOK1200,0120K500KIOK470033018K750IOK18K22K5009I MEG POT1000Vi IV-- 2c~JorJ(XVIII. SENSORY REPLACEMENT)amplitude for all frequencies present in each vowel sound is obtained.Figure XVIII- is a photograph of the Vocatac, as the apparatus has been called. Acarbon microphone of good quality is used. The frequency range of carbon microphonesis adequate for this application, and the amplification needed for them is much lessthan that required for others. However, measurements of second-harmonic distortionshould, and will, be made. The small device at the upper right of the chassis is theelectrotactile transducer which converts the electrical outputs of the six channels intoBraille-like tactile patterns. It consists of a set of six small solenoids with plungersconnected to the round-head pins and with provisions for necessary adjustments. Thesix boxes that contain the tuned filters are designed to plug into the chassis interchange-ably, so that any sampling frequency may be assigned to a given position in the tactilepattern.Figure XVIII-2 is a schematic of a typical tuned filter. The input is applied betweenpins 1 and 2 of the Jones plug and the output is taken between pins 2 and 3. The resistorR1 is used to adjust the gain of the channel, and R 2 is used to adjust the circuit Q to avalue of 20. The coils used are type HQA torroids manufactured by United TransformerCorporation. The input impedance at resonance with R1 = 0 is approximately 6500 ohms,which is the value of the combined plate and load resistance of the 12AT7 to which it isconnected. This condition maximizes the gain of the tuned stage for a given circuit Q,but it also effectively doubles the bandwidth. Hence the need for a Q of 20 to obtain10 per cent bandwidth.A schematic of the input circuits and one typical channel is given in Fig. XVIII-3.Three type 12AT7 tubes are used, each section of each tube serving for one channel.All six control grids of these tubes are connected to R 9 . The cathode-follower sectionof the 12AX7 makes it possible to use a low value (500 ohms) for this resistor, so thatthe signal at this point will not be affected materially by grid current drawn by the12AT7 tubes. In the lowest-frequency channel, the gain is such that a signal voltageof 0.18 volt rms at the grid of the 12AT7 will operate the solenoid in the output unit.However, the level is set so that speech frequencies in the neighborhood of 382 cpswill apply a signal voltage of 0.4 volt rms. This is done in order to speed up theoperation of the solenoid. Under this condition, the effective bandwidth for sustainedspeech sounds is increased to approximately 20 per cent. This would appear to beunavoidable with the present scheme for producing the tactile patterns. The perform-ance of a 12AT7 is linear for rms grid voltages less than about 2.0 volts. Calculationsindicate that the average rms voltage of speech should be somewhat less than 1.54 voltswhen the speech level gives 0.4 volt rms within the 10 per cent bandwidth of the 382 cpschannel. The figure 1.54 is the rms voltage obtained from a noise spectrum that(a) is flat from 200 cps to 400 cps, (b) decreases at a rate of 6 db per octave above400 cps, and (c) produces 0.4 volt rms in a band 40 cycles wide centered at 400 cps.136(XVIII. SENSORY REPLACEMENT)Note the control R1 used to set the signal level, and the diodes D1 and D 2 and the gangpotentiometer R 2 and R 3 which are used to clip the speech if the voice level exceedsthat for which R1 was originally set.At present, no tests of the Vocatac have been made, but we plan to start testing inthe next few days.C. M. Witcher, L. Washington, Jr.References1. Quarterly Progress Reports, Research Laboratory of Electronics, M.I.T.,Jan. 15, 1949 to April 15, 1951.2. M. Critchley, Tactile thought, with special reference to the blind, Proc. Roy.Soc. Medicine, Sec. Neurology 46 (1953).3. C. M. Witcher, The teaching of mathematics to the blind, Math. Teacher 48(May 1955).4. H. Fletcher and R. H. Galt, The perception of speech and its relation to telephony,J. Acoust. Soc. Am. 22 (March 1950).5. R. K. Potter and J. C. Steinberg, Toward the specification of speech, J. Acoust.Soc. Am. 22 (Nov. 1950).137

Contains reports on three research projects.Lincoln Laboratory, Purchase Order DDL-B158Department of the ArmyDepartment of the NavyDepartment of the Air Force under Contract AF 19(122)-45

XIV. SENSORY REPLACEMENTDr. C. M. WitcherL. Washington, Jr.A. OPTICAL PROBEThe optical probe, described in the Quarterly Progress Reports of July 15, 1953,and July 15, 1955, has been further improved by the substitution of a transistorizedmultivibrator circuit for the neon lamp relaxation oscillator previously used. Thecircuit was developed by Carl R. Hurtig, of this Laboratory. Its description, togetherwith some additional information concerning the new probe, is given in Section X. Notethat substantial reduction in size was achieved and that the three 30-volt hearing aidB-batteries were replaced by a single 1. 5-volt pen-light cell.The American Foundation for the Blind, New York, N. Y., has appropriated fundsfor the production of 50 models of the probe for distribution to interested blind peopleon a loan basis. These units are being produced by Dunn Engineering Associates,Cambridge, Massachusetts.B. TRAVEL AID OPTICAL SYSTEMThe calculations carried out by the optics design group at Boston University (Quar-terly Progress Report of Jan. 15, 1956) did not lead to a system capable of meeting therequirements previously given. A new attempt to design a suitable system is now underway at the American Optical Company, Southbridge, Massachusetts.C. VOCATACThe device for converting speech sounds into tactile patterns, the Vocatac, wasdescribed in the Quarterly Progress Report of Jan. 15, 1956. On completion of theinstrument, it was found that the tactile patterns produced were unsatisfactory in tworespects: they were not at all consistent - being strongly dependent on voice level atthe microphone; and they often failed to provide any indication of consonant sounds orof the shorter transition elements of speech.In order to obtain complete consistency, we would apparently need an automaticvolume control (avc) circuit whose performance was "ideal," in the sense that it wouldprovide a perfectly constant output level for a wide range of input levels while operatingwith absolutely no time delay. This being impossible, an alternative procedure had tobe found. J. C. R. Licklider and I. Pollack have shown (1) that infinite clipping ofspeech has very little effect on its intelligibility. It therefore appeared that a satis-factory substitute for the "ideal" avc circuit might be the subjection of speech to aconsiderable amount of clipping. Accordingly, the input section of the instrument wascompletely redesigned; its circuit schematic is shown in Fig. XIV-1. Note the two setsof diodes used to provide clipping, one at the output of the microphone transformer and*This work was supported in part by Purchase Order DDL-B158. (See the Introduction.)CarbonMicrophoneFig. XIV-1. Redesigned input section of the Vocatac.(XIV. SENSORY REPLACEMENT)the other between the amplifier section and the cathode follower section of the 12AT7.These diode arrangements are similar to those employed by Licklider and Pollack (1).The total amount of clipping thus obtained is 43-55 db, depending on input voice level.It would seem that not much more clipping than this could be tolerated, since the con-sequent required increase in gain would bring the noise up to the point at which it mightbegin to actuate the output transducer. (Note that for the clipping figures just given,the ganged potentiometers r 2 and r 3 constituting the clipping control are set to give areading of 0. 5 volt rms at the output sides of the diode clippers.) This circuit has pro-vided some improvement in pattern consistency, but much more is to be desired.One cause of the second defect in the tactile patterns is the insufficient speed ofoperation of the transducer. Another may be the charging time of the output filters. Inthe present transducer each solenoid plunger, together with its associated push rod andcoupling rod, represents a relatively large mass. Therefore, a new transducer is beingconstructed in which the solenoids are replaced by small relays with much less massivearmatures. The tactile patterns are produced by steel balls, 3/32 inch in diameter, thatare brought up against the finger tip by pressure of the relay armatures. The outputcircuits are being replaced by Schmidt trigger circuits (2). This type of trigger circuitwould seem to be ideal for the present application, since it can be turned on and off bya very small variation in the dc potential applied to the grid of one of the triodes. Theuse of these circuits should also yield a further improvement in pattern consistency.References1. J. C. R. Licklider and I. Pollack, J. Acoust. Soc. Am. 20, 42-51 (Jan. 1948).2. O. S. Puckle, Time Bases (John Wiley and Sons, Inc., New York, 1951), pp. 81-83.

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Sensory Replacement

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