Showing papers on "Frequency response published in 1975"

Minimum prediction residual principle applied to speech recognition

Abstract: A computer system is described in which isolated words, spoken by a designated talker, are recognized through calculation of a minimum prediction residual. A reference pattern for each word to be recognized is stored as a time pattern of linear prediction coefficients (LPC). The total log prediction residual of an input signal is minimized by optimally registering the reference LPC onto the input autocorrelation coefficients using the dynamic programming algorithm (DP). The input signal is recognized as the reference word which produces the minimum prediction residual. A sequential decision procedure is used to reduce the amount of computation in DP. A frequency normalization with respect to the long-time spectral distribution is used to reduce effects of variations in the frequency response of telephone connections. The system has been implemented on a DDP-516 computer for the 200-word recognition experiment. The recognition rate for a designated male talker is 97.3 percent for telephone input, and the recognition time is about 22 times real time.

A unified analysis of multirate and periodically time-varying digital filters

Abstract: A unified approach to the analysis of periodically timevarying digital filters is introduced. It is shown that this method may be used to describe both time and frequency domain responses. By considering multirate filters as a special class of periodically time-varying filters we have characterized the frequency response of a general multirate filter. A time domain synthesis procedure for periodically varying difference equations is also presented.

Non-linear design for cost of feedback reduction in systems with large parameter uncertainty †

Abstract: Feedback systems containing linear, minimum-phase plants with large parameter uncertainty may be designed to achieve specified performance tolerances over the entire range of parameter uncertainty. The principal ‘cost of feedback’ is in the feedback loop bandwidth, which is generally much larger than that of the system as a whole. This makes the system very sensitive to sensor noise and high-frequency parasitics. It is shown how a non-linear ‘first-order reset element’ (FORE) may be used to drastically decrease the feedback loop transmission bandwidth. One is logically led to FORE by simple, linear feedback frequency response concepts. The paper assumes that the primary design problem is to satisfy quantitative response tolerances to command inputs. However, disturbances at the plant are not neglected, but the specification on such disturbances is in the damping of the step response. An important feature of the non-linear design is that the system response to command inputs is almost exactly that of a lin...

Analysis of linear digital networks

Abstract: A framework is presented for the analysis, representation, and evaluation of digital filter structures. Based on the notation of linear signal-flow graphs and their equivalent matrix representation, a set of general linear digital network properties are reviewed, including precedence relations computability, Tellegen's theorem, interreciprocity, and network transposition. These properties are then utilized in developing time and frequency domain analysis techniques and sensitivity analysis techniques. These techniques, in turn, are applied to the comparison of several basic digital filter structures.

Measurement of loop gain in feedback systems

Abstract: In this design of a feedback system it is desirable to make experimental measurements of the loop gain as a function of frequency to ensure that the physical system operates as analytically predicted or, if not, to supply information upon which a design correction can be based In high loop-gain systems it is desirable that the loop-gain measurement be made without opening the loop This paper discusses practical methods of measuring and interpreting the results for loop gain of the closed-loop system by a voltage injection or a current-injection technique ; extension to the case in which the measurement can be made even though the system is unstable ; and extension to the case in which neither the voltage nor current-injection technique alone is adequate, but in which a combination of both permits the true loop gain to be derived These techniques have been found useful not only in linear feedback systems but also in describing-function analysis of switching-mode converters and regulators

On the theory of self-resonant grids

Abstract: An approximate theory is developed to predict the frequency response of a self-resonant grid. The grid is comprised of capacitive and inductive elements and exhibits a band-stop resonance. The analysis is based upon the derivation, from physical considerations, of an equivalent circuit representation of the grid structure. Predicted results compare well with measured data.

Optical-waveguide filters: Synthesis

Abstract: Analytical formulas for the synthesis of optical-waveguide filters having arbitrary spectral-response characteristics are derived from coupled-mode formalism. Use of these general formulas is illustrated by design of several filters, one of which is a linear power discriminator. The synthesis yields the functional dependence of spatial-perturbation period on the distance along the direction of wave propagation in the waveguide filter. The coupled-mode equations for the functional perturbation forms as determined by the synthesis process were solved numerically to find the actual response characteristics of the filter designs. Excellent agreement was found between the desired characteristics and those of the synthesized filters.

Measurement of high‐frequency turbulent temperature

Abstract: In many areas of turbulence research of current interest, it is of increasing importance to make reliable measurements of high frequency, small scale temperature fluctuations. Platinum wire resistance thermometers provide adequate spatial resolution, but may, after a period of time, suffer a degradation in frequency response when used in air. Relative effects of various sensor parameters on sensor performance under various flow conditions are also discussed as a guide to sensor selection. However, measurements of actual frequency response of wires in turbulent flow systems using circuits especially developed for the purpose show substantial deviations from the analytic results, as well as a dependence of the frequency response on a probe aging effect, which is possibly due to surface contamination. The measured frequency response was used to correct a temperature spectrum measured in a heated jet. The corrected spectrum agreed well with the expected shape.

Influence of Mistuning on Rotor-Blade Vibrations

Abstract: This analysis is aimed at determining the influence of blade mistiming on the vibratory stress levels of turbine and compressor blades A frequency response analysis for a given rotor configuration shows that a large number of resonances may occur over a frequency band, the width of which is nearly 20% of the mean blade frequency The resonant amplitudes are a function of blade frequency and location on the rotor, and the amount of damping present in the system A parametric study is carried out to evaluate the response levels due to engine order excitation, aerodynamic and mechanical damping, and blade frequency deviation The resulting mode shapes and frequencies are in good agreement with the experimental findings reported earlier in the literature

Dynamics of the head-neck system in response to small perturbations: analysis and modeling in the frequency domain.

Abstract: The response of the head-neck system to forces of small amplitude (up to 15 N) is described. Sinusoidal (0.6–20 Hz) and impulsive (duration: 100 msec) forces are applied in the sagittal plane to the head of the subject who is instructed to resist the disturbancy. In the case of sinusoidal forces of frequency less than about 2 Hz the active effort to resist the disturbancy results in a largely distorted sinusoidal displacement. Above this frequency the response becomes almost linear. The variations with frequency of the amplitude and the phase of the linear response relative to the applied force (transfer function) are used to characterize the dynamics of the system. The transfer functions evaluated from the impulse response are very similar in shape to those obtained with sinusoidal forces. In both cases the results suggest that the system behaves as a quasi-linear second order system with two degrees of freedom. The most prominent nonlinearities, beyond those present in the low frequency range, are related to the properties of the neck muscles. In particular, the transfer functions clearly show that the rigidity of the system increases as a function of the continuous value of the applied force. On the basis of previous work, both the passive properties of the muscles and those pertaining to the neuronal control system are pooled together in the form of viscoelastic parameters. A simple model of the system is introduced and applied to the experimental results. Its main features are 1) the presence of two centers of rotation. 2) the dependency of the viscoelastic parameters (stiffness and viscosity) on the frequency. It is suggested that both these features are necessary and sufficient to account for the observed behaviour above 2 Hz.

Wide-Band Network Modeling of Interacting Inductive Irises and Steps

Abstract: Methods of field and network theory are jointly applied to the problem of deriving wide-band models for interacting inductive irises and steps in standard and oversize lossless rectangular guides. The resulting equivalent network is a cascade of lumped multiports, described by means of their reactance matrix, given in the canonical Foster's form, and of several parallel transmission lines, connecting the interacting discontinuities. The required frequency band and the accuracy of the model can be prescribed at will. The features of the approach are: the solution of the field problem yields a reactance matrix with monotonic convergence properties; small matrices only need be manipulated; the frequency dependence is explicit, so that the field analysis need not be repeated at each frequency point; a true network model (and not a "spot frequency" equivalent circuit) is produced, which is prerequisite for exact synthesis.

A one-dimensional viscoelastic model of cat heart muscle studied by small length perturbations during isometric contraction.

Abstract: To develop a model of heart muscle, we studied cat papillary muscle contracting in a quasi-isometric condition under a fixed inotropic state. The properties of resting muscle were determined by using a step stretch of less than 1.2% of Lmax for initial lengths from 85 to 100% Lmax. The passive force response suggested the model of the passive branch (Fig. 1). All five parameters were small at muscle lengths below 95% of Lmax but increased markedly at longer lengths. The properties of contracting muscle were studied with a sinusoidal length change (amplitude less than 0.15% of Lmax, frequency 0.1-35.0 Hz). The frequency response of active (total minus passive) stiffness suggested the model of the active branch (Fig. 1). We determined the dependency of the elastic elements (K, Ks) and the viscous element (C) on length and time by recording the frequency response at various combinations of length and time Ks varied linearly with active force (FA). K and C exhibited time courses that paralleled FA up to 0.6tmax, and they maintained their values until 1.4tmax. K then fell toward zero, whereas C exhibited a secondary rise before it fell toward zero. K was dependent of length up to 95% of Lmax and then began to decline, but C varied in proportion to muscle length.

Complete identification of a class of nonlinear systems from steady-state frequency response

Abstract: A solution to the identification problem for a class of nonlinear systems, based on steady-state frequency response, is presented. This class is composed of certain interconnections of stable linear dynamic systems and integer power nonlinearities. Two kinds of measurements are considered. The first involves obtaining the amplitude and relative phase of each harmonic of the steady-state frequency response, while the second involves measuring the amplitudes only. In each case we find the number of measurements sufficient to identify a system for which the highest power present and the bounds on the dimensions of the linear subsystems are known.

On the frequency weighted least-square design of finite duration filters

Abstract: The frequency weighted least-square design of finiteduration filters, in one and two dimensions, continuous and discrete is considered. Some new theoretical results and some practical design techniques for conventional and unconventional filters are presented. In some cases optimum discrete filters can be found conveniently by matrix inversion. In many cases a simple, iterative approximation technique using FFT can be used to carry out the design or to adjust the frequency response of filters.

Method and apparatus for acoustic logging of a borehole

Abstract: A method and apparatus are described for preferentially exciting and for extracting late arrivals at low frequencies in an acoustic investigation of a borehole. The late arrival may be the Stoneley wave whose amplitude is derived by measuring the energy in a predetermined frequency band of a waveform produced with a low frequency transmitter-receiver system. The transmitter produces a highly damped acoustic pulse whose frequency spectrum is selected to preferentially excite a late arrival such as the Stoneley wave and/or tube wave with wavelengths which are significantly greater than the diameter of the borehole while preserving measurable earlier arrivals at higher frequencies. A receiver system is used which provides a generally flat frequency response including low frequencies to produce waveforms of both the earlier arrivals at high frequencies and late arrivals at low frequencies with sufficient extractable amplitudes throughout a wide range of borehole and formation conditions. A method and apparatus are described for measuring inelastic behavior of the formation as well as elastic parameters such as the shear modulus of the material comprising the circumference of the borehole wall by investigating frequency bands in the spectrum of a late arrival such as the Stoneley wave. A display and method for generating the display are described for the investigations of the late arrival.

Flow tracing fidelity of scattering aerosol in laser Doppler velocimetry

Abstract: An experimental method for the determination of the flow-tracing fidelity of a scattering aerosol used in laser Doppler velocimeters was developed with particular reference to the subsonic turbulence measurements. The method employs the measurement of the dynamic response of a flow-seeding aerosol excited by acoustic waves. The amplitude and frequency of excitation were controlled in order to simulate the corresponding values of fluid turbulence components. Experimental results are presented on the dynamic response of aerosols over the size range from 0.1 to 2.0 microns in diam and over the frequency range 100 Hz to 100 kHz. It was observed that unit-density spherical scatterers with diameters of 0.2 micron followed subsonic air turbulence frequency components up to 100 kHz with 98% fidelity.

Dynamic Properties of the Compensation System for Doppler Shifts in the Bat, Rhinolophus ferrumequinum

Abstract: 1. Echolocation sounds of the batRhinolophus ferrumequinum were played back to the bat as artificial echoes shifted in frequency by a few kHz (0–4 kHz) without distortion of the amplitude and frequency pattern of the sound. The simulated Doppler shifts in the echoes were modulated either sinusoidally or in step functions. 2. The bats did not compensate for frequency shifts within one sound, but in the sounds subsequent to the introduction of the shift. 3. Responses to symmetrical sinusoidal modulations of the echo from −2 kHz to +2kHz demonstrate that in the dynamic situation the bat compensates for positive frequency shifts and ignores negative frequency shifts (Fig. 2). Asymmetrical sinusoidal modulations between 0 Hz and different positive frequency shifts (1, 2, 3, and 4 kHz) yielded amplitude and phase response curves for modulation frequencies between 0.01 and 2 Hz (Fig. 5, 6). The response to step functions gave the time course for positive and negative changes of the frequency shift (Fig. 7). 4. The Doppler compensation system shows nonlinearities: The response of the system depends on the sign of the frequency shift change. Positive changes of the frequency shift in sinusoidal modulations are optimally followed by the system, when it is considered as a sampling system, in which the pulse repetition rate is the sampling rate. The response to negative frequency shift changes is much slower (Fig. 6). With an increase of the modulation frequency the response is DC-shifted. The amplitude and phase response curves depend on the amplitude of the frequency shifts presented in the modulation (Table 1, 2). 5. The maximum compensation between two succeeding bat orientation sounds was 1400 Hz, giving a frequency change of about 10 Hz/msec. This value is about the same as the minimum frequency modulation of the constant frequency portion of the sound detected by the bat. 6. The dynamic properties of the Doppler shift compensation system are discussed in the context to neurophysiological findings on the hearing system of the bat.

A method for determination and control of the frequency response of the constant-temperature hot-wire anemometer

Abstract: The theory of the constant-temperature anemometer has been extended in order to obtain quantitative results for the frequency response. A simple electrical test against which to check the theory has been devised, and the validity of the anemometer equations is demonstrated. Important differences in design philo-sophies and modes of operation are indicated, and results are presented for a design in which high d.c. gain is employed in the servo amplifier. The square-wave response is briefly investigated, and it is concluded that commonly used criteria for determining the frequency response from it should be treated with caution.To measure fluctuations with a hot-wire anemometer in flows containing both velocity and temperature perturbations, the hot wire must be operated at more than one temperature. Variation of the mean wire temperature causes, in general, a variation in the frequency response, as does variation of the mean flow conditions. It is shown that, by simultaneous variation of the gain of the servo amplifier in the anemometer, the frequency response may be held nearly constant over a useful range of both overheat and flow conditions.

Dependence of reproducing gap null on head geometry

Abstract: The spatial frequency response of reproduce heads with infinite depth, but finite pole length, is determined to very good accuracy by superposition of conformal map solutions for simpler geometries. This approach yields closed form approximations of the frequency response which are accurate to better than 7% for any head length-to-gap length ratio and any spatial frequency. In particular, the effect of this ratio on the location of the first gap null in the frequency response spectrum is explored for narrow pole length heads. The results lead to design considerations for extending the useful frequency range of thin film reproduce heads.

Acoustic oscillator fluid velocity measuring device

Abstract: A fluid velocity measuring device for the accurate fast measurement of the velocity of a fluid whether liquid or gas using the time of passage of acoustic energy through the fluid as the frequency determining element in a continuous wave oscillator. The frequencies or periods of oscillations of two anti-parallel acoustic paths are used to compute fluid velocity between the electro-acoustic transducers at the ends of the acoustic paths. The device in accordance with this invention may utilize a single reversing acoustic path or two anti-parallel acoustic paths. The velocity resolution can be selected by the operator in accordance with a stated uncertainty principle limiting the ratio of frequency response to velocity resolution. Mode ambiguities due to fluid phase velocity variations are avoided by varying the path lengths. The frequency range of the oscillator is limited by a phase locked loop.

Acoustic Emission in the Frequency Domain

Abstract: A means for quickly and easily determining the broadband frequency content of acoustic bursts as short as 20 μs in duration has been developed using a video tape recorder and a standard spectrum analyzer. It is shown by examples from several tests on laboratory specimens and on large structures that the frequency content of an acoustic burst is related to the mechanism which produced it and is not affected substantially by the specimen size or by mode conversion due to multiple reflections in the structure. The frequency content of the burst can be changed in two ways, however: by the frequency-dependent attenuation of the propagation medium and in the cases where the medium is dispersive. Results of measurements on the effect of these factors in a variety of structures are given. Although acoustic emissions from many materials tend to be "white noise," several examples of acoustic emissions and exwaneous background noise bursts having distinctive frequency spectra are given which suggest possibilities for discriminating true acoustic emission signals from background noise on the basis of frequency content alone.

Frequency and spatial correlation functions in a fading communication channel through the ionosphere

Abstract: Equations for the two-frequency two-position mutual coherence functions are derived under the usual parabolic and Markov approximations. These equations are then solved numerically. It is shown that the mutual coherence functions occur naturally in the study of pulse distortion through a random communication channel and in the investigation of signal correlations. Contour plots of correlation functions show the possibility of having equal values at two frequency separations for a given spatial separation. This behavior is explainable in terms of overlapping Fresnel zones. Recently it has been observed that radio signals from communication satellites at a frequency as high as several GHz may experience the scintillation phenomenon when received on the ground [Pope and Fritz, 1971; Skinner et al., 1971; Sessions, 1972; _ Craft and Westerlund, 1972; Taur, 1973]. This came as a surprise since scintillation was not anticipated to occur at such a high frequency. After some experimentation it is now believed that the scintillation is caused by the electron density irregularities in the ionosphere. As is well known, for a given electron density fluctuation, th e rms fluctuation in the refractive index of the ionospheric medium is inversely proportional to the square of the signal frequency. Therefore, under conditions when GHz scintillation occurs, signals with a lower frequency will experience even more severe fading. This implies that the effects of multiple scattering on signal statistics are very important under such conditions. To obtain their statistics correctly, it is desirable to develop a scintillation theory that takes into account the multiple scattering effects. Thanks to the recent advances in the theory of wave propagation in random media, it is now possible to develop such a theory [Liu et al., 1974a; Yeh et al., 1975]. In this paper, we shall apply a similar technique to investigate the frequency and spatial correlations of signals passing through the irregularity slab. In satellite communications the ability to transmit wideband data is limited primarily by the inter-symbol interference which is closely

Laser receiver anti-sun circuit

Abstract: A circuit for preventing laser receiver overload caused by sunlight wherein feedback network provides a d.c. current to cancel the d.c. level of the diode current; and, a receiver design alternative to conventional designs wherein higher sensitivity is provided by maximizing the signal to noise ratio and processing by dual equalization to reshape the frequency response to achieve a flat response over the frequency range of interest.

Transfer-function-parameter estimation from frequency response data: A FORTRAN program

Abstract: A FORTRAN computer program designed to fit a linear transfer function model to given frequency response magnitude and phase data is presented. A conjugate gradient search is used that minimizes the integral of the absolute value of the error squared between the model and the data. The search is constrained to insure model stability. A scaling of the model parameters by their own magnitude aids search convergence. Efficient computer algorithms result in a small and fast program suitable for a minicomputer. A sample problem with different model structures and parameter estimates is reported.

Two-dimensional vibrations analysed by speckle photography

Abstract: Speckle photographs of a laser-illuminated surface vibrating in its own plane give characteristic diffraction spectra. These have been investigated for a surface vibrating sinusoidally along two mutually orthogonal directions with equal amplitudes but differing frequencies. The diffraction patterns obtained are interpreted in terms of the Lissajous figures traced out by each speckle, taking into account the time-of-dwell of a speckle at any point on its trajectory. Results are given for frequency ratios of the two vibrations of 1:1, 2:1, 3:1. 4:1, and for non-integrally related frequencies.

Multi-resonant surface wave resonator

Abstract: A surface wave device having a periodic frequency response characteristic is disclosed. At least two reflective grating structures are located at the surface of the surface wave device such that acoustic surface wave standing wave resonance can occur in the region between the two reflector structures. The spacing between the reflector structures is sufficiently large to insure that standing wave resonance can occur at any of a plurality of discrete resonance frequencies, all falling within the bandwidth of the grating reflectors themselves.

Electronic peak sensing digitizer for optical tachometers

Abstract: A system for accurately digitizing a signal from a high density optical tachometer using a psuedo zero-crossing detector for subsequent use in a speed measuring device. An added enhancement of the system is the ability to double the frequency response of the input signal to thereby provide a more sensitive indication of incremental speed changes.

The frequency response of an amplitude-modulated GaAs luminescence diode

Abstract: The frequency response of an amptitude-modulated GaAs luminescence diode has been studied for optical communication application. The modulated light intensity is found to fan off inversely proportional to the modulation frequency over the frequency range ωτ ≳ 1, where τ is the recombinative lifetime. We show that the falloff characteristic is a result of the complex diffusion process of the minority carrier in the presence of an ac perturbation across the junction. A simple relation which can be employed to determine the minority lifetime accurately to within a nanosecond is derived.