Single-channel Devices Research Articles

Theoretical analysis of HEMT breakdown dependence on device design parameters

A two-dimensional numerical analysis is presented to investigate the breakdown characteristics of single- and double-channel AlGaAs/GaAs HEMTs. The influence of the doped layer thickness and the thickness of an undoped i-layer under the gate is analyzed. Impact ionization is considered to be the dominant breakdown mechanism. All simulations reveal the existence of a high electric field region near the gate contact. Breakdown occurs in the gate-drain region and the (breakdown) path which maximizes the ionization integral is entirely in the AlGaAs layer. For increased donor layer thickness, single-channel devices biased near pinchoff have gate-drain breakdown voltages varying from 8 to 14 V with corresponding peak electric field values in the range of 8.2*10/sup 5/ to 2.4*10/sup 6/ V/cm. The breakdown voltage increases with increasing gate bias mod V /sub gs/ mod due to a screening effect of transverse from longitudinal electric field. Double-channel HEMTs have slightly higher breakdown than single-channel, especially near pinchoff and for thin donor layers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Speech Perception with a Single-Channel Cochlear Implant

Suprasegmental and segmental speech perception tasks were administered to 8 patients with single-channel cochlear implants. Suprasegmental tasks included the recognition of syllable number, syllabic stress, and intonation. Segmental tasks included the recognition of vowels and consonants in three modalities: visual only, implant only, and visual + implant. Results were compared to those obtained from artificially deafened adults using a single-channel vibrotactile device. The patterns of responses for both suprasegmental and segmental tasks were highly similar for both groups of subjects, despite differences between the characteristics of the subject samples. These results suggest that single-channel sensory devices, whether they be cochlear implants or vibrotactile aids, produce similar patterns of speech perception errors, even when differences are observed in overall performance level.

Clinical aspects of vibrotactile devices with deaf children

Longitudinal data will be presented on the speech perception and production abilities of 15 deaf children who use a vibrotactile aid (Tactaid II). Their performance is being compared to that of deaf children who use either a single- or multichannel implant. Initial findings indicate that the performance of the subjects with implants, even those with single-channel devices, is superior to that of the subjects who use the tactile aid. The subjects who achieved the highest scores with the tactile aid tended to have more sophisticated cognitive and linguistic skills that permitted them to develop strategies to decode the vibratory patterns. The importance of prior knowledge of the language in decoding information perceived via the skin is also supported by the data collected from postlingually deaf adults using the same tactile aid. The implications of generalizing data collected on adults to prelingually deaf children will be discussed. [Work supported by NIH.]

Evaluations of single-channel and multichannel tactile aids for the hearing impaired

A number of single-channel and multichannel tactile devices for hearing-impaired persons have been evaluated in our laboratory over the last several years. Both commercially available and experimental devices have been tested, including the Tactaid I, II, and V, Minifonator, Minivib, Queen's University tactile vocoder (both laboratory and wearable versions), and Tacticon TC-1600. Normal-hearing and hearing-impaired adults, and hearing-impaired children, have participated in a variety of training tasks, employing both recorded and live-voice stimuli. These tasks include simple detection of sound, environmental sound identification, syllable rhythm and stress categorization, phoneme identification, word identification, phrase and sentence identification, connected discourse tracking, and a question-and-answer “conversation” task. In the present paper, the results from these evaluations are considered as a whole, to permit generalizations about the kinds of information that can be provided by single-channel, dual-channel, and multichannel devices, with and without the addition of lipreading. In addition, results are discussed in terms of the development of “optimal” training procedures for the use of tactile aids. [Work supported by NSF and NIH.]

The bionic ear: principles and current status of cochlear prostheses.

The field of cochlear prostheses is reviewed, emphasizing the neurophysiological principles necessary for the development of a successful cochlear prosthesis. Pertinent auditory physiology is reviewed, and four conditions are proposed which, if met, should result in speech recognition by the patient with an implant. These conditions are: The surviving neural population must be adequate over the frequency distribution required to deliver the necessary information for speech discrimination. The central neural processing pattern for decoding and recognizing speech must have been established and still persist. The processing of the acoustic signal by the auditory system up to the level of the prosthesis interface must be understood and predictable from the acoustic stimulus. The relationship between the response patterns of the neural elements and the electrical stimulus must be well documented and controllable. The degree to which these conditions can presently be met is discussed, with some suggestions for future development. The function of the cochlear prosthesis is separated into a signal processing section and an electrical stimulus section. Two signal processing strategies are analog processing of the acoustic signal and speech feature extraction from the acoustic signal. Four possible electrical stimulation strategies are direct analog stimulation from the signal processor, pulse amplitude modulation, pulse width modulation, and stimulation designed to optimize the neural responses to electrical stimulation. Some of the present generation cochlear prostheses, including five approved by the FDA as investigational devices, are discussed according to this classification. The clinical results of testing these devices as aids to lip reading and as stand-alone speech reception aids are compared. All are a potential aid to lip reading. Some prosthesis designs have been implanted in patients who then showed exceptional open set speech discrimination. These exceptional patients have used both single channel and multichannel devices and devices with both analog processing and speech feature extraction strategies. These results are encouraging. More consistent and better speech reception is anticipated for the future as this field develops.

Some aspects of temporal coding for single-channel electrical stimulation of the cochlea

Estimates of the useful frequency range for single-channel electrical stimulation of the cochlea range from 400 to 4000 Hz. Psychophysical studies in single-channel implant patients are relevant not only to the practical problem of designing stimulation strategies, but also to questions of temporal processing of pitch in the normal auditory nervous system. Patients with single-channel extracochlear devices participated in several experiments involving stimuli differing in fine temporal structure. Stochastic pulse trains, in which the probability of pulse delivery (p) for a given cycle was less than 1.0, were readily discriminated from ordinary pulse trains. Frequency discrimination using stochastic pulse trains differing only in fine temporal structure (but identical average pulse rates) was as good as with ordinary pulse trains or sinusoids for P ⩾ 0.5, but deteriorated rapidly for P < 0.5. Discrimination of triangular and trapezoidal waveforms from square waves was surprisingly good: rise-times (for 0 to maximum current) as low as 0.08 ms could be discriminated. Conversely, detection of jitter in pulse trains was almost an order of magnitude worse. The results show that frequency discrimination for single-channel electrical stimulation of the cochlea is based on discrimination of inter-pulse periods, and that pulse rates which would be unnatural for acoustically-evoked VIIIth nerve activity — up to 750 Hz — are more useful for coding mid-range frequencies than low-rate stochastic simulations of normal VIIIth nerve firing patterns. The waveform discriminations reported would be obscured by low-pass filtering even at 2000 Hz, and probably depend on changes in relative synchrony among an array of VIIIth nerve units with different thresholds. In general, these results support the use of analog coding schemes with relatively large bandwidth.

Survey of cochlear implant work

The cochlear implant has recently seemed useful enough to be considered by some as a clinical procedure. This paper presents a review of the progress made on cochlear implants worldwide. Data are presented which contrast the results obtained in different labs on similar tests: threshold detection, pitch and loudness scaling, chronaxie, and difference limen tests. Models to account for some of these results are given. Four centers (San Francisco, Vienna, Salt Lake City, and Melbourne) have reported surprisingly good speech comprehension scores. We discuss the psychoacoustic characteristics which may relate to these scores and demonstrate that many implant patients show results which are expected quite typically in cochlear dysfunction. Thus the limits on speech processing in the hard-of-hearing may apply as well to implant patients, regardless of the method of speech coding. We also stress the lack of evidence confirming the assumed greater usefulness of multi-channel implants over single channel devices. Finally, a careful look is given to the risks involved in these procedures, specifically those of bone and tumor growth, device replacement, and the psychological effects from device failure.

A multichannel implantable telemetry system for flow, pressure, and ECG measurements.

The design of telemetry systems for chronic implantation within the body have progressed from simple single-channel devices to complex multichannel systems over the last 15 yr. Although chronic measurements of temperature, ECG, and pressure have been reported with good results, measurements such as dimension and blood flow have been difficult because of heavy power requirements. The design to be described is a multiplex system that will measure up to eight individual parameters simultaneously, including blood flow. Flow is measured using an electromagnetic transducer, and by special design, the normal high power requirements of the electromagnetic technique have been reduced to a few hundred milliwatts. The system is powered by two NiCad, rechargeable batteries which are periodically recharged through the intact skin by induction at 250 kHz to an implanted pickup coil. The presently constructed units have been configured to measure ECG, two pressures, temperature and ascending aortic flow.

Present status and future development of the cochlear prosthesis.

Presently devised single channel devices generate relatively primitive sensation of hearing. They provide some enhancement of communication skills for the totally deaf. Definite psychological advantages for the totally deaf have been observed. Pitch discrimination is by the mechanism of "periodicity pitch." No "place" pitch encoding is possible. The recognition of complex sounds is not possible. Multiple segments of auditory nerve must be stimulated in a manner which will stimulate the complex patterns of neural activity necessary for speech discrimination. Electrodes can be optimized and the pathophysiological consequences of electrical stimulation can be determined in experimental animals. The perceptual consequences of electrical stimulation, however, can best be determined in man himself. How much we will have to rely on known and future methods of aural rehabilitation will depend upon how well perceptual speech patterns can be generated by electrical stimulation of the auditory nerve.

The use of new computer technology in chromatography

Recent developments in computer technology have led to decreasing prices and increased computing capabilities of computers and micro processors. A result of this has been the parallel evolution of two types of systems which bring the power of the computer within the budgetary capabilities of many laboratories. The first type uses a powerful micro processor which is built in as an integral part of either a reporting integrator or a processor controlled chromatograph. Both are single channel devices which provide processing of the data to the form of a final printed report. The second is a multichannel system which uses a mini computer with as much as 64K internal memory. The systems are described and compared, and applications are presented.

Mössbauer spectrometry with multiwire proportional counters

MSssbauer spectrometry, as it is currently performed, is inherently a single-channel technique; the (~ channel ~ is defined by the ~,-ray collimation, always present in conventional spectrometers. In the constant-velocity spectrometers the transmission spectrum is obtained by varying step-by-step the value of the relative velocity between the source and the absorber. These spectrometers allow a good absolute calibration for the values of the velocities, but are affected by the instabilities of the detector electronics. In an ~V-channel constant-acceleration spectrometer at best each channel is open only 1/~V seconds per second. These latter spectrometers are less sensitive to instabilities of the associated electronics but they remain essentially single channel devices since also in this case only the y-rays emitted in a small solid angle are counted. Indeed, the most important consideration is that these two types of spectrometers have about the same Figure of merit as to running time necessary to obtain good statistics. I t is thus evident the usefulness of having a MSssbauer spectrometer which would be the analogue of the multichannel analyser for pulse height analysis. The possibility of reducing the running times by a factor of hundreds is particularly interesting when the nuclear levels, which undergo the u transitions involved in the M6ssbauer effect, are populated through a nuclear reaction induced by some incident particles beam (Coulomb excitation (d, p) reactions) and not by a radiaetive decay. On the other hand, a source moving with constant velocity v 0 has velocities between v o and 0 relative to observers distributed at angles between 0 and ~/2 around v o. This suggests the possibility of measuring simultaneously the complete velocity spectrum with a large-angle localizing detector. We think that the multiwire proportional counter (1) (MWPC) is the ideal detector to be used in the construction of a true multiehannel M6ssbauer spectrometer. Let us imagine an experimental set-up as sketched in Fig. 1.