Gain Frequency Response Research Articles

The Value of Routine Real Ear Measurement of the Gain of Digital Hearing Aids

The main aims of this study were: (1) to determine whether routine real ear insertion gain (REIG) measurement is necessary in fitting digital hearing aids; and (2) to assess the extent to which modifying the frequency-gain response of an aid can lead to better matches to the target in cases where the target gain was not initially achieved. The target formula was selected as NAL-NL1 in the programming software of four types of digital hearing aids. REIG measurements on 42 ears showed that 64% of cases failed to come within +/-10 dB of the target at one or more of the following frequencies: 0.25, 0.5, 0.75, 1, 1.5, 2, and 4 kHz. After adjusting the frequency-gain response of the aids, based on the REIG results, 83% of cases came within +/-10 dB of the target. The target was met more often, both before and after adjustment, for aids with seven gain "handles" than for aids with four gain "handles". The results indicate that REIG measurements can and should be used to achieve more accurate fittings but that accurate adjustments are difficult with some aids.

Gain-frequency response of nearby waveguide modes in vane-loaded gyro-TWT

In this paper, the behavior of a gyro-traveling wave tube (gyro-TWT) amplifier, in which an axis-encircling electron beam synchronously interacts with the azimuthal modes of the vane-loaded cylindrical waveguide interaction structure, has been analyzed. The dispersion characteristics and gain-frequency response of the nearby waveguide modes (TE/sub 01/, TE/sub 21/, and TE/sub 41/) for the vane-loaded gyro-TWT amplifier have been presented. The gain-frequency response of the device has been studied for both the small-orbit and large-orbit configurations. It has been observed that the TE/sub 41/ mode of the device is well separated on the frequency scale for both these configurations. In addition, it has higher device gain in the large-orbit configuration when compared with the same obtained for the TE/sub 01/ and TE/sub 21/ modes.

A method for modelling and optimizing an electrical impedance tomography system

Electrical impedance tomography (EIT) image reconstruction is an ill-posed problem requiring maximum measurement precision. Recent EIT systems claim 60 to 80 dB precision. Achieving higher values is hard in practice since measurements must be performed at relatively high frequency, on a living subject, while using components whose tolerance is usually higher than 0.1%. To circumvent this difficulty, a method for modelling the electronic circuits of an EIT system was developed in order to optimize the circuits and incorporate the model in the reconstruction algorithms. The proposed approach is based on a matrix method for solving electrical circuits and has been applied to the scan-head which contains the front-end electronic circuits of our system. The method is used to simulate the system characteristic curves which are then optimized with the Levenberg–Marquardt method to find optimal component values. A scan-head was built with the new component values and its simulated performance curves were compared with network analyser measurements. As a result of the optimization, the impedance at the operating frequency was increased to minimize the effects of variations in skin/electrode contact impedance. The transconductance and gain frequency responses were also reshaped to reduce noise sensitivity and unintended signal modulation. Integrating the model in the reconstruction algorithms should further improve overall performance of an EIT system.

Multiple coherence of cerebral blood flow velocity in humans

The coherence function has been used in transfer function analysis of dynamic cerebral autoregulation to assess the statistical significance of spectral estimates of gain and phase frequency response. Interpretation of the coherence function and choice of confidence limits has not taken into account the intrinsic nonlinearity represented by changes in cerebrovascular resistance due to vasomotor activity. For small spontaneous changes in arterial blood pressure (ABP), the relationship between ABP and cerebral blood flow velocity (CBFV) can be linearized, showing that corresponding changes in cerebrovascular resistance should be included as a second input variable. In this case, the standard univariate coherence function needs to be replaced by the multiple coherence, which takes into account the contribution of both inputs to explain CBFV variability. With the use of two different indicators of cerebrovascular resistance index [CVRI = ABP/CBFV and the resistance-area product (RAP)], multiple coherences were calculated for 42 healthy control subjects, aged 20 to 40 yr (28 +/- 4.6 yr, mean +/- SD), at rest in the supine position. CBFV was measured in both middle cerebral arteries, and ABP was recorded noninvasively by finger photoplethysmography. Results for the ABP + RAP inputs show that the multiple coherence of CBFV for frequencies <0.05 Hz is significantly higher than the corresponding values obtained for univariate coherence (P < 10(-5)). Corresponding results for the ABP + CVRI inputs confirm the principle of multiple coherence but are less useful due to the interdependence between CVRI, ABP, and CBFV. The main conclusion is that values of univariate coherence between ABP and CBFV should not be used to reject spectral estimates of gain and phase, derived from small fluctuations in ABP, because the true explained power of CBFV in healthy subjects is much higher than what has been usually predicted by the univariate coherence functions.

Extended delay of broadband signals in stimulated Brillouin scattering slow light using synthesized pump chirp

Judicious chirping of a directly modulated pump laser is used to broaden the intrinsic linewidth of stimulated Brillouin scattering in an optical fiber. The modulation waveform is designed to obtain a spectrum with sharp edges, resulting in phase gradients stronger that those obtained for random pump modulation. The gain and phase frequency response of the slow light process are measured by a vector network analyzer, and the delays obtained for our tailored modulation are compared with the case of random direct modulation. For equal pump powers and gain bandwidths (FWHM), the tailored modulation waveform introduces 30-40% longer delays. Using this technique, pseudo random bit sequences of 5 Gb/s were successfully delayed by up to 120 ps (BER<10(-5)) and 80 ps (BER<10(-9)).

Improved approach for gain-frequency response of vane-loaded gyro-TWT

This paper discusses the simple analytical approach to study the gain-frequency characteristics of the azimuthally periodic vane-loaded cylindrical waveguide interaction structure for the gyro-TWT amplifiers. The azimuthal harmonic effects arise due to the angular periodicity of the vanes have taken into consideration in the derivation of both the cold and hot dispersion relations of the vane-loaded gyro-TWT and, hence, the device gain has been predicted. Thus, results obtained by this approach have been compared with those of others available in the literature.

Optimization of Vane-Parameters for Gain-Frequency Response of Vane-Loaded Gyro-TWT

This paper discusses an analytical method to study the effects of the vane-parameters on the gain-frequency response of an azimuthally periodic vane-loaded cylindrical waveguide interaction structure for the gyro-traveling wave tube (gyro-TWT) amplifier in the small-orbit TE01 waveguide mode configuration. The vane-loaded gyro-TWT enables the higher gain than that of the smooth-wall device. An optimum choice of the vane dimensions causes a change in the frequency corresponding to the peak-gain.

Analysis of a Vane-Loaded Gyro-TWT for the Gain-Frequency Response

A cylindrical waveguide provided with wedge-shaped metal vanes projecting radially inward from the wall of the guide and excited in the transverse-electric mode was analyzed for the cold (beam-absent) dispersion relation. The angular harmonics generated due to the angular periodicity of vanes were taken into account in the cold analysis. The results of the cold analysis of the vane-loaded structure with respect to the axial propagation constant and the cut-off frequency were plugged into the hot (beam-present) dispersion relation of a small-orbit gyro-traveling wave tube (gyro-TWT) in an otherwise smooth-wall waveguide to obtain the hot dispersion relation of a vane-loaded gyro-TWT. The latter was interpreted to obtain the Pierce-type small-signal gain equation of a vane-loaded gyro-TWT. The vane-loaded gyro-TWT enjoys a higher gain than that of the smooth-wall. The beam position, as well as the beam harmonic mode number relative to the waveguide mode number, was optimized for the maximum device gain. The number, angular width, and radial height of vanes were optimized for large gain values without the sacrifice of bandwidth. The competition amongst the close-by waveguide modes was studied. The results obtained by the present analysis were compared with those of the others available in the literature.

Is probe-mic measurement of HA gain-frequency response best practice?

In Brief The authors weigh the main arguments for and against the use of probe-microphone measurements in fitting hearing aids, and then come up with their verdict.

Analysis of a Gyro-TWT in a Cylindrical Waveguide with a Dielectric Lining on its Wall

The cold as well as hot dispersion relation for a gyro-TWT employing a cylindrical waveguide loaded with a dielectric lining on its wall was obtained. An improved approach was given where the effect of dielectric loading was incorporated in the deduction of the hot dispersion relation rather than in an earlier approximate approach where the effect had been considered only in the deduction of the cold dispersion relation. The hot dispersion relation was used to obtain Pierce-type gain expression. The dielectric lining with optimally chosen permittivity and thickness values provided an improvement with respect to the gain as well as bandwidth of the device as compared to a dielectric-free, unloaded device. The gain-frequency responses predicted by the approximate and improved approaches differed more as the permittivity and the thickness of the dielectric lining increased, though the other relevant device parameters like the optimum beam position for maximum gain did not appreciably differ.

Maximizing effective audibility in hearing aid fitting.

This paper examines why more audibility is not always better than less audibility if hearing-impaired people are to best understand speech. We used speech perception data from 14 normally hearing and 40 hearing-impaired people to quantify the contribution of audibility to speech intelligibility. The quantification revealed that the effectiveness of audibility decreased with hearing loss, and the decrement was greater at high frequencies than at lower frequencies. To apply the Speech Intelligibility Index (SII) model to predict speech intelligibility for hearing-impaired people, we modified the model to take account of effective audibility rather than physical audibility. The modified SII model provided an adequate description of speech performance of people with a wide range of hearing threshold levels. We applied the model to the evaluation of two prescriptions for a sloping audiogram at prescribed levels and at equated loudness levels to demonstrate the necessity of considering loudness and effective audibility in prescribing amplification. Effective audibility is defined as audibility corrected for the effects of level distortion and hearing loss desensitization, and this paper proposes a method of estimating effective audibility from hearing threshold level at different frequencies. The practical implication of considering effective audibility in prescribing hearing aids is that for a given listening level, less gain is provided at frequencies where the hearing is most impaired to allow more gain at frequencies where audibility is most useful. In developing the NAL-NL1 prescription for nonlinear hearing aids, we adopted the modified SII model together with a loudness model to derive optimal gain-frequency response characteristics that maximize predicted speech intelligibility for people with different degrees of hearing losses.

Analysis of a large-orbit gyrotron in a coaxial waveguide under assistant background fields

The dispersion relation of a large-orbit gyrotron in a coaxial waveguide excited in the transverse-electric mode was developed starting from the beam-present wave equation. The relativistic Vlasov's equation was solved under tenuous beam approximations for the perturbed electron distribution function, hence the required current density to be used in the wave equation was found. The analysis was generalized, with respect to background fields, by considering a dc radial electric field and a dc azimuthal magnetic field, over and above conventional dc axial magnetic field. The dispersion relation was solved for the complex frequency for a given propagation constant and the imaginary part interpreted for the growth rate as well as the saturated efficiency of the device. Similarly, for a given frequency the dispersion relation was solved for the complex propagation constant and the imaginary part interpreted for the gain of the device, if configured as an amplifier. Two peaks were obtained in the gain-frequency response corresponding to two points of intersection between the beam-mode dispersion line and the waveguide-mode dispersion curve. The improvement in the growth rate, gain and saturated efficiency was predicted by the application of the additional background fields. A redistribution of beam kinetic energy between the axial and transverse electron velocities led to a remarkable enhanced saturated efficiency at the second peak. A detailed study of the variation of device performance with respect to the gain and saturated efficiency was presented, for a wide range of the dc background field and electron kinetic energy distribution parameters.

Control of the gain-frequency response of a vane-loaded gyro-TWT by beam and magnetic field parameters

The cold dispersion relation of a vane-loaded cylindrical waveguide was combined with the hot dispersion relation of a gyro-TWT in an unloaded waveguide to obtain the hot dispersion relation of a vane-loaded gyro-TWT. Both the device gain and bandwidth were increased by controlling the beam parameters, namely, the voltage, current, and velocity pitch factor of the beam, as well as its location. The device gain and bandwidth characteristics were also found to be highly sensitive to the background magnetic field. Study clearly demonstrated the need to take into account the role of the beam and background magnetic field parameters in efforts to broadband a gyro-TWT by using innovative structures like a vane-loaded cylindrical waveguide. ©2000 John Wiley & Sons, Inc. Microwave Opt Technol Lett 24: 140–145, 2000.

Waveguide Cross Section and Background Magnetic Field Tapers for Broadbanding a Gyro-TWT

The gyro-TWT in a cylindrical waveguide of linearly-tapered cross section was analyzed for the gain-frequency response, using the Pierce-type gain equation. The taper in the waveguide cross section was adjusted for wide device bandwidths, either by changing the taper angle, while keeping the interaction length to be constant, or by changing the interaction length, while keeping the initial and final radii of the waveguide constant. Tapering led to the prediction of wide bandwidths, though at the cost of gain, as compared to a non-tapered device. The range of the DC background magnetic flux density relative to its grazing-point value was identified as a crucial parameter for large gains, with appreciable bandwidths, and minimum mode mixing in a tapered device, the latter in general facing more mode competition than a non-tapered device.

Hybrid-mode helix-loading effects on gyro-travelling-wave tubes

A hybrid-mode hot dispersion relation for a gyro-TWT, employing a cylindrical waveguide loaded with a helix supported from the inner wall of the waveguide by a dielectric, has been derived. The dispersion relation is interpreted to obtain the conventional TWT-type gain equation in the Pierce form, including the effect of the circuit loss. This is used to study the gain-frequency response of the device for an initial search of the operating and structure loading parameters, for an enhanced performance in terms of the gain and bandwidth. It is found that a helix-loaded gyro-TWT, while exhibiting a 3dB bandwidth almost similar to an ordinary gyroTWT, gives a large gain both in the cyclotron fundamental and second harmonic operations in the hybrid mode. The relative permittivity of the helix dielectricsupport, and the location of the helix with respect to the envelope are found to have a greater effect than the helix pitch angle on the cold dispersion characteristics of the device. The distributed circuit loss is identified as another handy parameter to shape the gain-frequency response of the device.

Two-stage dielectric-loading for broadbanding a gyro-TWT

A scheme of dielectric-loading for a gyro-traveling wave tube (gyro-TWT) is proposed for a wider bandwidth at a higher gain. The method uses two-section dielectric-loaded cylindrical waveguide as the interaction structure, with the first section overloaded to exhibit a valley between the two peaks in the gain-frequency response, while the other suitably loaded to exhibit a single peak centered at the valley so as to yield an overall wideband response. A bandwidth /spl sim/8.8% at a relatively high small-signal gain /spl sim/30 dB is predicted.

Amplification in Gyro-Travelling-Wave Tubes—Dispersion Relation and Gain-Bandwidth Characteristics

The interaction in a cylindrical waveguide, loaded with a thin hollow beam of electrons gyrating in helical paths to give a gyro-TWT configuration, is investigated. The beam perturbation is estimated by solving the relativistic Vlasov's equation and a dispersion relation describing the beam-wave interaction is obtained for an electron beam of zero guiding centre spread. The dispersion relation is interpreted following the Pierce type approach to a conventional TWT to obtain a gain equation of the well known form G = A+BCN. The influence of the basic parameters such as beam voltage, beam current, velocity pitch factor and DC magnetic flux density on the gain-frequency response is extensively investigated to find a trade-off between the gain and bandwidth. The results show that an increase in beam current improves the gain whereas at higher beam voltages one gets a flat gain-frequency response though at the cost of the gain. A high value of the velocity pitch factor gives a higher gain however at frequencie...

Two‐station in‐orbit testing using swept techniques

AbstractThis paper describes the system, consisting of hardware and software, and two of the two‐station in‐orbit test (IOT) measurements that were developed for INTELSAT as part of their IOTE (in‐orbit test equipment) project. Three systems have been designed and manufactured by SED Systems, and installed in Beijing, Clarksburg and Fucino. The system is capable of two types of synchronized sweeps: sweeps in power and sweeps in frequency; gain transfer and frequency response, respectively, are given as examples. Two‐station measurements are used on complex satellites that contain transmit and receive beams (such as spot beams) that cannot both be seen at the same earth‐station. This paper briefly describes how the measurements are done and discusses some of the more important performance issues.

A Screening Procedure for Modified Simplex in Frequency-Gain Response Selection

The preference for hearing aid frequency-gain responses selected using the modified simplex procedure over that prescribed by NAL-R is not universal. A screening protocol used before individualized fitting may help streamline its use and ensure effective use of clinical time. In this protocol, listeners compare the prescribed frequency-gain response with selected alternate frequency-gain responses. Evaluation with the modified simplex procedure is indicated only when listeners consistently prefer the alternate frequency-gain responses in the screening procedure. Ten hearing aid wearers completed the screening protocol and the full modified simplex procedure. In addition, subjects compared their satisfaction for the prescribed and individually selected frequency-gain responses. The results were examined for efficiency, reliability, sensitivity, and efficacy of the selected frequency-gain response. Findings revealed that the screening protocol allowed reliable identification of listeners who needed fine-tuning of the prescribed frequency-gain response. Furthermore, satisfaction for selected frequency-gain response over prescribed frequency-gain response increased as more differences between the two responses were noted.

Fitting Strategies for Multiple-Memory Programmable Hearing Instruments.

Fitting a multiple-memory programmable hearing instrument presents a new set of challenges to the dispenser, compared to conventional fittings. Rather than having to compromise on a single frequency-gain response (the prescriptive target), it is now possible to create a family of hearing aid responses from which the user can select an appropriate response for a given situation. This philosophy is also applicable in the prescription of compression characteristics. The flexibility designed into the 3M two-channel compression system allows the dispenser to program very different types of signal processing strategies (low frequency compression, high frequency compression, full spectrum compression, or linear processing) into a single instrument to match the requirements of different listening environments and to meet the needs of different users. Utilizing this approach, comfort, speech intelligibility, and sound quality can be optimized in a variety of situations by considering the listener's acoustic environment and the input signals to which the hearing aid must respond.