Spectral Degradations Research Articles

Effects of spectral degradation on gated word recognition.

Although much is known about how normal-hearing listeners process spoken words under ideal listening conditions, little is known about how a degraded signal, such as speech transmitted via cochlear implants, affects the word recognition process. In this study, gated word recognition performance was measured with the goal of describing the time course of word identification by using a noise-band vocoder simulation. The results of this study demonstrate that spectral degradations can impact the temporal aspects of speech processing. These results also provide insights into the potential advantages of enhancing spectral resolution in the processing of spoken words.

Spectral degradations in the TIMIT, QuickSIN, NU-6, and other popular bandlimited speech materials

Many popular speech corpora were recorded decades ago using transducers, recording techniques, or post-processing techniques that resulted in spectral degradations. The use of such materials very likely has consequences for measures of speech perception, but this is not often considered in studies that use these materials. We analyzed several of these popular corpora to ascertain spectral degradations that may be present. Spectral degradations we frequently observed included limited extended high-frequency (EHF) (>8 kHz) content, and, for a few corpora, substantial low-frequency roll-off <500 Hz. Two corpora analyzed here, the TIMIT/PRESTO and NU-6, exhibited both high- and low-frequency degradations. Such spectral degradations will likely have an effect on behavioral measures because they do not match the high-fidelity speech signals listeners encounter in their everyday lives. This should be considered when selecting stimuli for speech perception experiments. [Work supported by NIH Grant R01-DC019745]

On the use of the TIMIT, QuickSIN, NU-6, and other widely used bandlimited speech materials for speech perception experiments.

The use of spectrally degraded speech signals deprives listeners of acoustic information that is useful for speech perception. Several popular speech corpora, recorded decades ago, have spectral degradations, including limited extended high-frequency (EHF) (>8 kHz) content. Although frequency content above 8 kHz is often assumed to play little or no role in speech perception, recent research suggests that EHF content in speech can have a significant beneficial impact on speech perception under a wide range of natural listening conditions. This paper provides an analysis of the spectral content of popular speech corpora used for speech perception research to highlight the potential shortcomings of using bandlimited speech materials. Two corpora analyzed here, the TIMIT and NU-6, have substantial low-frequency spectral degradation (<500 Hz) in addition to EHF degradation. We provide an overview of the phenomena potentially missed by using bandlimited speech signals, and the factors to consider when selecting stimuli that are sensitive to these effects.

Super-Resolution Hyperspectral Reconstruction With Majorization-Minimization Algorithm and Low-Rank Approximation

Hyperspectral imaging (HSI) has become an invaluable imaging tool for many applications in astrophysics or Earth observation. Unfortunately, direct observation of hyperspectral images is impossible since the actual measurements are 2-D and suffer from strong spatial and spectral degradations, especially in the infrared. We present in this work an original method for high-resolution hyperspectral image reconstruction from heterogeneous 2-D measurements degraded by integral field spectroscopy (IFS) instrument. A fundamental part of this work is developing a forward model that accounts for the limitations of the IFS instrument, such as wavelength-dependent spatial and spectral blur, subsampling, and inhomogeneous sampling steps. The reconstruction method inverts the forward model using a deterministic regularization framework for edge-preserving. It fuses information from different observations and spectral bands for resolution enhancements. We rely on the Majorize-Minimize memory gradient (3MG) optimization algorithm to solve the inverse problem while considering a low-rank approximation for the unknown to handle the high-dimensionality of the problem.

The relationship between talker acoustics, intelligibility, and effort in degraded listening conditions.

Listening to degraded speech is associated with decreased intelligibility and increased effort. However, listeners are generally able to adapt to certain types of degradations. While intelligibility of degraded speech is modulated by talker acoustics, it is unclear whether talker acoustics also affect effort and adaptation. Moreover, it has been demonstrated that talker differences are preserved across spectral degradations, but it is not known whether this effect extends to temporal degradations and which acoustic-phonetic characteristics are responsible. In a listening experiment combined with pupillometry, participants were presented with speech in quiet as well as in masking noise, time-compressed, and noise-vocoded speech by 16 Southern British English speakers. Results showed that intelligibility, but not adaptation, was modulated by talker acoustics. Talkers who were more intelligible under noise-vocoding were also more intelligible under masking and time-compression. This effect was linked to acoustic-phonetic profiles with greater vowel space dispersion (VSD) and energy in mid-range frequencies, as well as slower speaking rate. While pupil dilation indicated increasing effort with decreasing intelligibility, this study also linked reduced effort in quiet to talkers with greater VSD. The results emphasize the relevance of talker acoustics for intelligibility and effort in degraded listening conditions.

Using Reverse Saturable Absorption to Boost Broadband Noise-Like Pulses

Utilizing the nonlinear effects in a photonic crystal fiber to broaden the spectrum and reverse saturable absorption to further improve the flatness of the spectrum, ultra-broadband noise-like pulses (NLPs) were produced in a fiber laser. Guided by numerical results, a broadband spectrum with 290 nm 3-dB bandwidth was obtained, which, to the best of our knowledge, is the broadest bandwidth achieved for NLPs. Spectral degradations of the NLPs have also been observed, and are discussed.

Mixed ligand oxidovanadium(IV) complexes: Synthesis, spectral, structural characterization and catalytic degradations of methyl orange

Mixed ligand oxidovanadium(IV) complexes VO(ox)(im)3·H2O (1), VO(ox)(4-mim)3 (2), K2[VO(ox)2(2-mim)]·2H2O (3) and [V2O2(edta)(phen)2]·11H2O (4) (H2ox = oxalic acid, H4edta = ethylenediaminetetraacetic acid, im = imidazole, 2-mim = 2-methylimidazole, 4-mim = 4-methylimidazole, and phen = 1,10-phenanthroline) have been obtained at room temperature and under hydrothermal condition respectively. 1–3 feature bidentate oxalate with two carboxy oxygen atoms, and the dimeric complex 4 contains hexadentate edta as bridging ligand. 1–4 have been fully characterized by elemental, thermogravimetric and single crystal X-ray structural analyses, as well as IR, UV–vis and EPR spectroscopies. Moreover, 1, 2 and 4 were tested for the catalytic degradations of methyl orange (MO) with H2O2.

Vowel and consonant confusions from spectrally manipulated stimuli designed to simulate poor cochlear implant electrode-neuron interfaces.

Suboptimal interfaces between cochlear implant (CI) electrodes and auditory neurons result in a loss or distortion of spectral information in specific frequency regions, which likely decreases CI users' speech identification performance. This study exploited speech acoustics to model regions of distorted CI frequency transmission to determine the perceptual consequences of suboptimal electrode-neuron interfaces. Normal hearing adults identified naturally spoken vowels and consonants after spectral information was manipulated through a noiseband vocoder: either (1) low-, middle-, or high-frequency regions of information were removed by zeroing the corresponding channel outputs, or (2) the same regions were distorted by splitting filter outputs to neighboring filters. These conditions simulated the detrimental effects of suboptimal CI electrode-neuron interfaces on spectral transmission. Vowel and consonant confusion patterns were analyzed with sequential information transmission, perceptual distance, and perceptual vowel space analyses. Results indicated that both types of spectral manipulation were equally destructive. Loss or distortion of frequency information produced similar effects on phoneme identification performance and confusion patterns. Consonant error patterns were consistently based on place of articulation. Vowel confusions showed that perceptions gravitated away from the degraded frequency region in a predictable manner, indicating that vowels can probe frequency-specific regions of spectral degradations.

Fast Multispectral Imaging by Spatial Pixel-Binning and Spectral Unmixing.

Multispectral imaging system is of wide application in relevant fields for its capability in acquiring spectral information of scenes. Its limitation is that, due to the large number of spectral channels, the imaging process can be quite time-consuming when capturing high-resolution (HR) multispectral images. To resolve this limitation, this paper proposes a fast multispectral imaging framework based on the image sensor pixel-binning and spectral unmixing techniques. The framework comprises a fast imaging stage and a computational reconstruction stage. In the imaging stage, only a few spectral images are acquired in HR, while most spectral images are acquired in low resolution (LR). The LR images are captured by applying pixel binning on the image sensor, such that the exposure time can be greatly reduced. In the reconstruction stage, an optimal number of basis spectra are computed and the signal-dependent noise statistics are estimated. Then the unknown HR images are efficiently reconstructed by solving a closed-form cost function that models the spatial and spectral degradations. The effectiveness of the proposed framework is evaluated using real-scene multispectral images. Experimental results validate that, in general, the method outperforms the state of the arts in terms of reconstruction accuracy, with additional 20× or more improvement in computational efficiency.

The effect of visual cues on top-down restoration of temporally interrupted speech, with and without further degradations

In complex listening situations, cognitive restoration mechanisms are commonly used to enhance perception of degraded speech with inaudible segments. Profoundly hearing-impaired people with a cochlear implant (CI) show less benefit from such mechanisms. However, both normal hearing (NH) listeners and CI users do benefit from visual speech cues in these listening situations. In this study we investigated if an accompanying video of the speaker can enhance the intelligibility of interrupted sentences and the phonemic restoration benefit, measured by an increase in intelligibility when the silent intervals are filled with noise. Similar to previous studies, restoration benefit was observed with interrupted speech without spectral degradations (Experiment 1), but was absent in acoustic simulations of CIs (Experiment 2) and was present again in simulations of electric-acoustic stimulation (Experiment 3). In all experiments, the additional speech information provided by the complementary visual cues lead to overall higher intelligibility, however, these cues did not influence the occurrence or extent of the phonemic restoration benefit of filler noise. Results imply that visual cues do not show a synergistic effect with the filler noise, as adding them equally increased the intelligibility of interrupted sentences with or without the filler noise.

Temporal B0 field variation effects on MRSI of the human prostate at 7 T and feasibility of correction using an internal field probe.

Spectral degradations as a result of temporal field variations are observed in MRSI of the human prostate. Moving organs generate substantial temporal and spatial field fluctuations as a result of susceptibility mismatch with the surrounding tissue (i.e. periodic breathing, cardiac motion or random bowel motion). Nine patients with prostate cancer were scanned with an endorectal coil (ERC) on a 7-T MR scanner. Temporal B0 field variations were observed with fast dynamic B0 mapping in these patients. Simulations of dynamic B0 corrections were performed using zero- to second-order shim terms. In addition, the temporal B0 variations were applied to simulated MR spectra causing, on average, 15% underestimation of the choline/citrate ratio. Linewidth distortions and frequency shifts (up to 30 and 8 Hz, respectively) were observed. To demonstrate the concept of observing local field fluctuations in real time during MRSI data acquisition, a field probe (FP) tuned and matched for the (19) F frequency was incorporated into the housing of the ERC. The data acquired with the FP were compared with the B0 field map data and used to correct the MRSI datasets retrospectively. The dynamic B0 mapping data showed variations of up to 30 Hz (0.1 ppm) over 72 s at 7 T. The simulated zero-order corrections, calculated as the root mean square, reduced the standard deviation (SD) of the dynamic variations by an average of 41%. When using second-order corrections, the reduction in the SD was, on average, 56%. The FP data showed the same variation range as the dynamic B0 data and the variation patterns corresponded. After retrospective correction, the MRSI data showed artifact reduction and improved spectral resolution. B0 variations can degrade the MRSI substantially. The simple incorporation of an FP into an ERC can improve prostate cancer MRSI without prior knowledge of the origin of the dynamic field distortions.

The effect of listener age on the recognition of dysarthric speech

A growing number of studies have attempted to address the cognitive–linguistic source of intelligibility deficits in dysarthria. These have focused predominantly on the ability of young adults with normal hearing to decipher dysarthric speech. However, dysarthria is commonly associated with aging; older listeners often form primary communication partners. It is also known that older adults exhibit difficulty understanding speech that has been temporally or spectrally degraded. It follows that the spectral and temporal degradations present in dysarthric speech may pose a greater perceptual challenge for older, as opposed to younger, listeners. Twenty younger listeners (mean age = 20 yr) and 15 older listeners with good hearing for their age (mean age = 65 yr) transcribed the speech of individuals with moderate hypokinetic dysarthria. Percent words correct (intelligibility) was calculated and underlying error patterns at the suprasegmental and segmental levels of processing were examined. While the younger and older listener groups achieved similar intelligibility scores, the younger group showed greater reliance on syllabic strength cues to inform word boundary decisions. Similar levels of attention to segmental cues were observed across both groups. These results suggest that the recognition of dysarthric speech may be comparable across younger and older listeners.

Infants’ responses to spectral and temporal degradations of speech signals.

The present study assessed the ability of 6-month-old infants with normal hearing (NH) to discriminate between voiced and unvoiced stop consonants (/aba/versus/apa/) using vocoded disyllables and the head-turn preference procedure. The disyllables were processed by 4- or 32-band noise-excited vocoders in order to (i) degrade temporal fine structure (TFS) cues while preserving spectral- and temporal-envelope cues, (ii) degrade TFS and temporal-envelope cues (temporal envelopes being lowpass filtered at 16 Hz in each frequency band), and (iii) degrade TFS and spectral-envelope cues (temporal-envelope cues being preserved in four broad frequency bands). Overall, the results showed that infants are able to discriminate voicing in each experimental condition. These findings suggest that as adults, 6-month-old infants require minimal spectral information to achieve robust speech discrimination as long as slow (&amp;lt;16 Hz) temporal-envelope cues are available in a few spectral regions. These findings also suggest that the impoverished spectral and temporal cues delivered by current cochlear implants processors can be used by 6-month-old infants to discriminate speech sounds.

Recognition of temporally interrupted and spectrally degraded sentences with additional unprocessed low-frequency speech

Recognition of periodically interrupted sentences (with an interruption rate of 1.5 Hz, 50% duty cycle) was investigated under conditions of spectral degradation, implemented with a noiseband vocoder, with and without additional unprocessed low-pass filtered speech (cutoff frequency 500 Hz). Intelligibility of interrupted speech decreased with increasing spectral degradation. For all spectral degradation conditions, however, adding the unprocessed low-pass filtered speech enhanced the intelligibility. The improvement at 4 and 8 channels was higher than the improvement at 16 and 32 channels: 19% and 8%, on average, respectively. The Articulation Index predicted an improvement of 0.09, in a scale from 0 to 1. Thus, the improvement at poorest spectral degradation conditions was larger than what would be expected from additional speech information. Therefore, the results implied that the fine temporal cues from the unprocessed low-frequency speech, such as the additional voice pitch cues, helped perceptual integration of temporally interrupted and spectrally degraded speech, especially when the spectral degradations were severe. Considering the vocoder processing as a cochlear implant simulation, where implant users’ performance is closest to 4 and 8-channel vocoder performance, the results support additional benefit of low-frequency acoustic input in combined electric-acoustic stimulation for perception of temporally degraded speech.

The modulation transfer function for speech intelligibility.

We systematically determined which spectrotemporal modulations in speech are necessary for comprehension by human listeners. Speech comprehension has been shown to be robust to spectral and temporal degradations, but the specific relevance of particular degradations is arguable due to the complexity of the joint spectral and temporal information in the speech signal. We applied a novel modulation filtering technique to recorded sentences to restrict acoustic information quantitatively and to obtain a joint spectrotemporal modulation transfer function for speech comprehension, the speech MTF. For American English, the speech MTF showed the criticality of low modulation frequencies in both time and frequency. Comprehension was significantly impaired when temporal modulations <12 Hz or spectral modulations <4 cycles/kHz were removed. More specifically, the MTF was bandpass in temporal modulations and low-pass in spectral modulations: temporal modulations from 1 to 7 Hz and spectral modulations <1 cycles/kHz were the most important. We evaluated the importance of spectrotemporal modulations for vocal gender identification and found a different region of interest: removing spectral modulations between 3 and 7 cycles/kHz significantly increases gender misidentifications of female speakers. The determination of the speech MTF furnishes an additional method for producing speech signals with reduced bandwidth but high intelligibility. Such compression could be used for audio applications such as file compression or noise removal and for clinical applications such as signal processing for cochlear implants.

Spectrotemporal Modulations Underlying Speech and Timbre Perception

Event Abstract Back to Event Spectrotemporal Modulations Underlying Speech and Timbre Perception Human speech and music are rich in spectral and temporal modulations, which are fluctuations in either amplitude or frequency. Speech intelligibility, melody perception, and identification of source characteristics (e.g., vocal gender or instrumental timbre) depend on spectrotemporal modulations but can withstand drastic spectral and temporal degradations. We systematically explored which restricted spectrotemporal modulations are essential to the perception of complex sounds. Degraded sentences were obtained by a novel modulation filtering procedure performed on the sound spectrogram. Temporal modulation filtering smeared the amplitude envelope by removing changes above a cutoff in Hz. Spectral modulation filtering smeared the spectral energy across frequency bands by removing changes above a cutoff in cyc/kHz. We further complemented this low-pass filtering with more specific notch-filtering. Speech intelligibility and gender recognition were assessed. We determined that spectral modulations below ~3.75 cyc/kHz, and temporal modulations between 1 and 7 Hz are essential for speech comprehension, whereas gender identification depends on the presence of higher modulations associated with the glottal pulse. We are expanding these psychoacoustic experiments to dissimilarity judgments of orchestral instrument tones that have been normalized in level, tuning and duration. Our goal is to relate the multiple perceptual dimensions of timbre, such as brightness, sharpness of attack, and spectral flux, to underlying acoustic differences in the spectrotemporal modulation spectrum. We will represent the timbral distance in spectrotemporal modulations using multidimensional scaling (MDS) [1]. Then subjects will rate tones along 15 subjective rating scales. We will perform a principal components analysis (PCA) on the timbre ratings to reduce their dimensionality, helping us to assign qualitative labels to the timbre space obtained by the MDS. Our research could be used in the design of optimal signal processing in hearing aids and cochlear implants, as well as in music synthesis and transcription. Acknowledgments We thank D. Wessel and J.-M. Mongeau for helpful discussion.

Spectral and temporal modulations essential to spoken word, gender and timbre identification

Human speech and musical sounds contain complex spectral and temporal modulations. Speech intelligibility, perception of melody, and identification of source characteristics (e.g., speaker gender or musical timbre) depend on spectrotemporal modulations but can be surprisingly robust to drastic spectral and temporal degradations. We systematically explored which restricted spectral and temporal modulations are essential to the perception of complex sounds. Degraded sentences and musical sounds were obtained by a novel modulation filtering procedure performed on the sound spectrogram. Temporal modulation filtering smeared the amplitude envelope by removing changes above particular Hz. Spectral modulation filtering smeared the spectral energy across frequency bands by removing changes above particular cyc/kHz. We further complemented this low-pass filtering with more specific notch-filtering. Speech intelligibility, gender recognition and musical instrument identification were assessed in psychophysical experiments. We determined that spectral modulations below ∼3.75 cyc/kHz, and temporal modulations between 1 and 7 Hz are essential for speech comprehension. Gender identification however required the presence of higher spectral modulations. Similarly the timbre and pitch of instruments was affected differentially by notch filters in these two regions of the modulation spectrum. Our research could be used to guide the design of optimal signal processing in hearing aids and cochlear implants.

Combined Effects of Frequency Compression-Expansion and Shift on Speech Recognition

To explore combined acute effects of frequency shift and compression-expansion on speech recognition, using noiseband vocoder processing. Recognition of vowels and consonants, processed with a noiseband vocoder, was measured with five normal-hearing subjects, between the ages of 27 and 35 yr. The speech signal was filtered into 8 or 16 analysis bands and the envelopes were extracted from each band. The carrier noise bands were modulated by the envelopes and resynthesized to produce the processed speech. In the baseline matched condition, the frequency ranges of the corresponding analysis and carrier bands were the same. In the shift only condition, the frequency ranges of the carrier bands were shifted up or down relative to the analysis bands. In the compression and expansion only conditions, the analysis band range was made larger or smaller, respectively, than the carrier band range. By applying the shift to carrier bands and compression or expansion to analysis bands simultaneously, the combined effects of the two spectral distortions on speech recognition were explored. When the spectral distortions of compression-expansion or shift were applied separately, the performance was reduced from the baseline matched condition. However, when the two spectral degradations were applied simultaneously, a compensatory effect was observed; the reduction in performance was smaller for some combinations compared to the reduction observed for each distortion individually. The results of the present study are consistent with previous vocoder studies with normal-hearing subjects that showed a negative effect of spectral mismatch between analysis and carrier bands on speech recognition. The present results further show that matching the frequency ranges of 1 to 2 kHz, which contain important speech information, can be more beneficial for speech recognition than matching the overall frequency ranges, in certain conditions.

A new bioactive triterpenoid saponin from the seeds of Lactuca scariola Linn.

A new triterpenoid saponin, 3-O-[β-D-galactopyranosyl-(1 → 3)-O-β-D-xylopyranosyl-(1 → 4)-O-α-L-rhamnopyranosyl]-oleanolic acid; has been isolated from the seeds of Lactuca scariola. Its structure was determined by various spectral analysis and chemical degradations. This compound shows antimicrobial activity against various bacteria and fungi.

Accelerated estimation of spectral degradation of white GaN-based LEDs

The intensity and colour (spectral) degradation of light-emitting diodes (LEDs) can significantly affect the reliability of demanding lighting products for which a certain level of intensity and colour homogeneity is required. Intensity degradation is sometimes characterized by manufacturers in a form of a mean half-lifetime, i.e. the time in which the initial intensity of LEDs decreases to 50%. However, additional information on spectral degradation in a batch of LEDs is required for designing and/or maintaining demanding state-of-the-art lighting products. For this purpose, a current-accelerated prediction of spectral degradation of white GaN-based LEDs is presented. The method is based on periodical automated spectral acquisitions of LEDs driven by above nominal currents. The obtained information on current-accelerated spectral degradation is then used for a model-based prediction of spectral degradation of LEDs driven by an arbitrary current. The proposed method was tested on 640 ultra-bright white GaN-based LEDs (from 5 to 20 cd) from six manufacturers. For this purpose, the predicted and measured spectral degradations at the application-specific nominal currents were compared. The method has proved useful for predicting spectral degradation of the eight tested batches by the exponential lifetime prediction model. The achieved up to sixfold acceleration and the mean absolute lifetime prediction error of 15% indicate that the proposed method is feasible for an accelerated estimation of spectral degradation of white GaN-based LEDs. As such, the method may be a valuable tool for designing and maintaining lighting products in numerous existing and emerging applications.