Subband Envelope Research Articles

A modulation-transfer-function-based method for restoring sub-band power envelope from noisy reverberant speech

The concept of the modulation transfer function (MTF) can be successfully applied to evaluating the quality of speech transmission in room acoustics (noisy reverberant environments) as functions of reverberation (reverberation time) and additive noise (signal to noise ratio) (Houtgast and Steeneken, J. Acoust. Soc. Am., 77, 1069-1077, 1985). This paper proposes a method of restoring the power envelope from noisy reverberant speech based on the MTF concept. The proposed method does not need the impulse response and noise conditions in room acoustics to be measured to enhance speech. The proposed approach suppresses the effects of reverberation and noise on the power envelopes by restoring the smeared MTF. We carried out massive simulations of noise-suppression and dereverberation on noisy reverberant speech to objectively evaluate the proposed method. The results revealed that the proposed method could simultaneously work well with both the suppression of noise and dereverberation. We further tested the proposed method as a front-end processor for ASR systems in noisy reverberant environments, and compared it with other methods (MFCC, CMN, spectral subtraction, and RASTA filtering on a constant-bandwidth filterbank). The results demonstrated that the improvement in recognition with the proposed method was more effective than that in extremely noisy reverberant environments.

Estimated threshold base current and light power output of a transistor laser with InGaAs quantum well in GaAs base

We have solved the continuity equation for electrons in the base of an InGaP–GaAs–GaAs heterojunction bipolar transistor laser (TL) in which the position of an InGaAs quantum well (QW) in GaAs base is variable. The injected minority carrier is related to the two-dimensional carrier in QW via virtual states (VSs). The values for optical gain in the QW are obtained by considering subband energies and envelope functions in presence of strain, polarization dependent momentum matrix element and Lorentzian lineshape. Relating the gain with threshold current and the latter with base current via VS current, the threshold base current and power output from the TL are estimated. Good agreement between the calculated and the experimental threshold base currents is obtained and the match for light output power is satisfactory within experimental uncertainty. Our calculated charge distribution in the base shows similar behaviour as in the charge control analysis of the experimental data.

Blind single-channel suppression of late reverberation based on perceptual reverberation modeling

A blind method for suppressing late reverberation from speech and audio signals is presented. The proposed technique operates both on the spectral and on the sub-band domains employing a single input channel. At first, a preliminary rough clean signal estimation is required and for this, any standard technique may be applied; however here the estimate is obtained through spectral subtraction. Then, an auditory masking model is employed in sub-bands to extract the reverberation masking index (RMI) which identifies signal regions with perceived alterations due to late reverberation. Utilizing a selective signal processing technique only these regions are suppressed through sub-band temporal envelope filtering based on analytical expressions. Objective and subjective measures indicate that the proposed method achieves significant late reverberation suppression for both speech and music signals over a wide range of reverberation time (RT) scenarios.

Temporal envelope compensation for robust phoneme recognition using modulation spectrum

A robust feature extraction technique for phoneme recognition is proposed which is based on deriving modulation frequency components from the speech signal. The modulation frequency components are computed from syllable-length segments of sub-band temporal envelopes estimated using frequency domain linear prediction. Although the baseline features provide good performance in clean conditions, the performance degrades significantly in noisy conditions. In this paper, a technique for noise compensation is proposed where an estimate of the noise envelope is subtracted from the noisy speech envelope. The noise compensation technique suppresses the effect of additive noise in speech. The robustness of the proposed features is further enhanced by the gain normalization technique. The normalized temporal envelopes are compressed with static (logarithmic) and dynamic (adaptive loops) compression and are converted into modulation frequency features. These features are used in an automatic phoneme recognition task. Experiments are performed in mismatched train/test conditions where the test data are corrupted with various environmental distortions like telephone channel noise, additive noise, and room reverberation. Experiments are also performed on large amounts of real conversational telephone speech. In these experiments, the proposed features show substantial improvements in phoneme recognition rates compared to other speech analysis techniques. Furthermore, the contribution of various processing stages for robust speech signal representation is analyzed.

On-Line Quality Detection of Ultrasonic Wire Bonding via Refining Analysis of Electrical Signal From Ultrasonic Generator

A technique for on-line quality detection of ultrasonic wire bonding is developed. The electrical signals from the ultrasonic generator supply, namely, voltage and current, are picked up by a measuring circuit and transformed into digital signals by a data acquisition system. A new feature extraction method is presented to characterize the transient property of the electrical signals and further evaluate the bond quality. The method includes three steps. First, the captured voltage and current are filtered by digital bandpass filter banks to obtain the corresponding subband signals such as fundamental signal, second harmonic, and third harmonic. Second, each subband envelope is obtained using the Hilbert transform for further feature extraction. Third, the subband envelopes are, respectively, separated into three phases, namely, envelope rising, stable, and damping phases, to extract the tiny waveform changes. The different waveform features are extracted from each phase of these subband envelopes. The principal components analysis method is used for the feature selection in order to remove the relevant information and reduce the dimension of original feature variables. Using the selected features as inputs, an artificial neural network is constructed to identify the complex bond fault pattern. By analyzing experimental data with the proposed feature extraction method and neural network, the results demonstrate the advantages of the proposed feature extraction method and the constructed artificial neural network in detecting and identifying bond quality.

Sub-band temporal modulation envelopes and their normalization for automatic speech recognition in reverberant environments

Abstract Automatic speech recognition (ASR) in reverberant environments is still a challenging task. In this study, we propose a robust feature-extraction method on the basis of the normalization of the sub-band temporal modulation envelopes (TMEs). The sub-band TMEs were extracted using a series of constant bandwidth band-pass filters with Hilbert transforms followed by low-pass filtering. Based on these TMEs, the modulation spectrums in both clean and reverberation spaces are transformed to a reference space by using modulation transfer functions (MTFs), wherein the MTFs are estimated as the measure of the modulation transfer effect on the sub-band TMEs between the clean, reverberation, and reference spaces. By using the MTFs on the modulation spectrum, it is supposed that the difference on the modulation spectrum caused by the difference of the recording environments is removed. Based on the normalized modulation spectrum, inverse Fourier transform was conducted to restore the sub-band TMEs by retaining their original phase information. We tested the proposed method on speech recognition experiments in a reverberant room with differing speaker to microphone distance (SMD). For comparison, the recognition performance of using the traditional Mel frequency cepstral coefficients with mean and variance normalization was used as the baseline. The experimental results showed that by averaging the results for SMDs from 50 cm to 400 cm, we obtained a 44.96% relative improvement by only using sub-band TME processing, and obtained a further 15.68% relative improvement by performing the normalization on the modulation spectrum of the sub-band TMEs. In all, we obtained a 53.59% relative improvement, which was better than using other temporal filtering and normalization methods.

Autoregressive Models of Amplitude Modulations in Audio Compression

We present a scalable medium bit-rate wide-band audio coding technique based on frequency-domain linear prediction (FDLP). FDLP is an efficient method for representing the long-term amplitude modulations of speech/audio signals using autoregressive models. For the proposed audio codec, relatively long temporal segments (1000 ms) of the input audio signal are decomposed into a set of critically sampled sub-bands using a quadrature mirror filter (QMF) bank. The technique of FDLP is applied on each sub-band to model the sub-band temporal envelopes. The residual of the linear prediction, which represents the frequency modulations in the sub-band signal, are encoded and transmitted along with the envelope parameters. These steps are reversed at the decoder to reconstruct the signal. The proposed codec utilizes a simple signal independent nonadaptive compression mechanism for a wide class of speech and audio signals. The subjective and objective quality evaluations show that the reconstruction signal quality for the proposed FDLP codec compares well with the state-of-the-art audio codecs in the 32-64 kbps range.

Time-Frequency Coherent Modulation Filtering of Nonstationary Signals

Modulation filtering is a class of techniques for filtering slowly-varying modulation envelopes of frequency subbands of a signal, ideally without affecting the subband signal's temporal fine-structure. Coherent modulation filtering is a potentially more effective type of such techniques where, via an explicit product model, subband envelopes are determined from demodulation of the subband signal with a coherently detected subband carrier. In this paper we propose a coherent modulation filtering technique based on detecting the instantaneous frequency of a subband from its time-frequency representation. We devise theory to show that coherent modulation filtering imposes a new bandlimiting constraint on the product of the modulator and carrier as well as the ability to recover arbitrarily chosen envelopes and carriers from their modulation product. We then formally show that a carrier estimate based on the time-varying spectral center-of-gravity satisfies the bandlimiting condition. This bandwidth constraint leads to effective and artifact-free modulation filters, offering new approaches for potential signal modification. However, the spectral center-of-gravity does not, in general, satisfy the condition of arbitrary carrier recovery. Finally, the results from modulation-filtering a speech signal are then used to validate the theory.

Modulation frequency features for phoneme recognition in noisy speech

In this letter, a new feature extraction technique based on modulation spectrum derived from syllable-length segments of subband temporal envelopes is proposed. These subband envelopes are derived from autoregressive modeling of Hilbert envelopes of the signal in critical bands, processed by both a static (logarithmic) and a dynamic (adaptive loops) compression. These features are then used for machine recognition of phonemes in telephone speech. Without degrading the performance in clean conditions, the proposed features show significant improvements compared to other state-of-the-art speech analysis techniques. In addition to the overall phoneme recognition rates, the performance with broad phonetic classes is reported.

Comparative evaluation of modulation-transfer-function-based blind restoration of sub-band power envelopes of speech as a front-end processor for automatic speech recognition systems

To reduce speech degradation in reverberant environments, we previously proposed a modulation-transfer-function (MTF)-based method of speech dereverberation. By considering the temporal modulation properties of speech, and the exponential decay properties of the power envelope of the impulse response of room acoustics, we obtained the following MTF relation: the sub-band power envelope of reverberant speech that can be represented as a convolution between the sub-band power envelope of clean speech and the power envelope of the impulse response of room acoustics. On the basis of the MTF relation, inverse MTF filtering can be applied to restoring the power envelopes of reverberant speech. Therefore, the impulse response of the room acoustics in this restoration dose not need to be measured at any time since we model the power envelope of the impulse response as an exponential decay function. We have tested how effective this method is as a front-end for automatic speech recognition (ASR) systems in artificial and real reverberant environments. Reverberant speech signals were created by simply convoluting clean speech (AURORA-2J database) with the artificially produced or real impulse responses of room acoustics. A method based on the auditory power spectrum was used as a baseline for comparison. Compared with the baseline, the proposed method for artificial reverberant environments produced a 35.67% relative improvement in the error reduction rate (on average, for reverberation times from 0.2 to 2.0 s), and for real reverberant environments (43 reverberant impulse responses), it produced a 25.78% relative improvement in the error reduction rate. The results demonstrate that our new approach can improve the robustness of speech-recognition systems in reverberant environments, and it performs better than conventional methods.

Electric-field-induced strong mixing betweene1−hh1ande1−hh2excitons in asymmetric double quantum wells

We analyze the electric-field-dependent strong mixing of the two near band-edge ground-state excitons in asymmetric double quantum wells (ADQWs). This excitonic mixing is mainly attributed to the Coulomb interactions between subbands and the valence-subband nonparabolicity. The effect of mixing on the energy levels and oscillator strengths is obtained by a comparison of results including and excluding the Coulomb interaction between different subband pairs which appears in the off-diagonal matrix elements of the Hamiltonian in the calculation of exciton states. We find that a substantial portion of the oscillator strength of the $e1\text{\ensuremath{-}}hh1$ ground-state exciton is due to the $e1\text{\ensuremath{-}}hh2$ subband pair in a bias range of the anticrossing region between pairs of valence subbands. Results also show that excluding the excitonic mixing effect results in significant error in both the energies and the oscillator strengths of the excitons in an ADQW with a thick barrier $(3\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$. Even in an ADQW with a fairly thin barrier $(1.2\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$, the error in the oscillator strengths can be substantial, although the errors in the computed energies may be tolerable. Detailed analysis of ${k}_{\ensuremath{\Vert}}$-dependent Coulomb matrix elements and exciton expansion coefficients reveals that neglecting the off-diagonal elements in the exciton Hamiltonian diminishes the contribution of the ${k}_{\ensuremath{\Vert}}$-dependent subband envelopes around the anticrossing in ${k}_{\ensuremath{\Vert}}$ space to the corresponding excitons. Further, it is demonstrated that the application of almost-degenerate perturbation theory to the uncoupled excitons with the intention of accounting for this mixing effect is to be approached with caution.

Electron transport and optical properties of shallow GaAs/InGaAs/GaAs quantum wells with a thin central AlAs barrier

Shallow GaAs/InGaAs/GaAs quantum well structures with and without a three-monolayer thick AlAs central barrier have been investigated for different well widths and Si doping levels. The transport parameters are determined by resistivity measurements in the temperature range 4–300 K and magnetotransport in magnetic fields up to 12 T. The (subband) carrier concentrations and mobilities are extracted from the Hall data and Shubnikov–de Haas oscillations. We find that the transport parameters are strongly affected by the insertion of the AlAs central barrier. Photoluminescence spectra, measured at 77 K, show an increase of the transition energies upon insertion of the barrier. The transport and optical data are analysed with the help of self-consistent calculations of the subband structure and envelope wavefunctions. Insertion of the AlAs central barrier changes the spatial distribution of the electron wavefunctions and leads to the formation of hybrid states, i.e., states which extend over the InGaAs and the delta-doped layer quantum wells.

PECULIARITIES OF ELECTRON TRANSPORT IN THE COUPLED AlGaAs/GaAs QUANTUM WELLS WITH THIN CENTRAL AlAs BARRIER

GaAs / AlGaAs structures with single quantum well (QW) of different width and with coupled quantum wells separated by central barrier of six monolayers of AlAs have been synthesized by MBE. The temperature dependence of resistance, Hall electron concentration and mobility were investigated in the temperature interval 4.2 K <T<300 K . Shubnikov–de Haas (SdH) oscillations and quantum Hall effect were also investigated in high magnetic fields up to 38 T. The influence of thin central barrier on the subband energy structure and electron mobility was analyzed for three values of QW width L=13, 26 and 35 nm. Self-consistent calculations of the subband structure and envelope wave function were carried out.

Theoretical modeling of multiple quantum well lasers with tunneling injection and tunneling transport between quantum wells

Multiple quantum well lasers with tunneling transport of carriers represent a new class of semiconductor lasers. Tunneling can be utilized twofold: as an injection mechanism which drives electrons from a separate confinement heterostructure into active well and also as a mechanism facilitating transport between quantum wells. Since tunneling is normally a very fast process, one can expect that employing the tunneling mechanism for transport of electrons can result in an improvement of modulation bandwidth of multiple quantum well semiconductor lasers. This assertion is justified by an analysis based on the rate equation model (analysis of the tunneling injection) and by determining differential gain (to analyze transport between wells). The analysis, done for 0.98 and 1.55 μm semiconductor lasers, suggests that in tunneling injection lasers it is possible to obtain a substantial increase of intrinsic modulation bandwidth. For the tunneling transport between wells it is shown here within a realistic model including band mixing, that an optimum range of barrier thickness exists for which the differential gain is enhanced and, consequently, the modulation bandwidth improved. A rate equation model, the choice of parameters for the model, as well as the effect of band mixing and well coupling on the optical and differential gain are described. A new formalism for the calculation of optical gain, based on work of Aversa and Iizuka [Aversa and Iizuka, IEEE J. Quantum Electron. 28, 1864 (1992)] is developed. It employs the subband energies and envelope functions determined from the Luttinger–Kohn effective mass equation including band mixing. The study performed for a system of two wells shows that the well coupling substantially shifts the spectral gain peak. The band mixing, in turn, reduces the gain peak as compared to that obtained in the parabolic model. The well coupling enhancement of the differential gain calculated with band mixing is more noticeable than that obtained in the parabolic model. It is true especially for large carrier concentrations and for barrier widths between 20 and 40 Å. Since the relaxation-oscillation frequency depends on the differential gain as fr∝G′, the predicted enhancement of G′ indicates an increase of fr, as well.

On the optical modal gain of coupled quantum wells

Optical modal gain has been investigated for coupled quantum wells. Subband energies and envelope functions were determined by solving the Luttinger–Kohn effective mass equation including band mixing. The analysis was performed for GaAs–Al0.3Ga0.7As system, and shows that well coupling substantially shifts the spectral gain peak and band mixing reduces the gain peak as compared to the parabolic model. The implications for the design of semiconductor lasers based on coupled quantum wells are briefly discussed. ©1999 John Wiley & Sons, Inc. Microwave Opt Technol Lett 22: 301–304, 1999.

Electronic band structure of GaAs sawtooth-doping superlattices.

We report simple calculations (of the Kronig-Penney type) concerning the band structure and charge distribution of GaAs sawtooth-doping superlattices as a function of period and doping concentration. We have obtained, within the effective-mass approximation, analytic expressions for the subband energy levels and envelope wave functions of the system.

Dependence of lifetime on design parameters of an nipi doping superlattice: Results of self-consistent calculations

This paper presents a scheme for calculating recombination lifetimes in a doping superlattice at arbitrary temperatures and forward biases. The scheme involves the self-consistent calculation of sub-band energies, populations and envelope functions, followed by the calculation of lifetime using overlap integrals. Results of these calculations are then presented for a variety of combinations of layer thicknesses and dopings, all at a temperature of 300 K and a forward bias of 1 V. Our results give room temperature lifetimes as high as ∼30 ms for n and p layer thicknesses of 750 Å each, i-layer thickness of 50 Å, and dopings in the n and p layers of 2×10 18 cm −3.