Changes In Modulation Depth Research Articles

Abnormal handling mechanism and improvement measures of optical DC current transformer in smart grid environment.

An optical DC current transformer anomaly handling mechanism is proposed to address the problem that the conventional DC current transformer anomaly handling mechanism cannot compensate for the defect of capacitor anomaly blocking. First, the implementation principle, modulation loop, demodulation method and its anomaly warning mechanism of the sine-wave modulated all-fibre-optic current transformer (FOCT) are investigated, and the effects of light source intensity and modulation voltage on current decoding are explained. The modulation loop is then simulated and modelled and a FOCT anomaly handling mechanism is proposed based on the Bessel function with real-time dynamic current compensation for small changes in modulation depth. Finally, an integrated dynamic test system for DC current transformers and DC protection is designed, and the actual system operation and fault model is established using the RTDS simulation system. The experiments demonstrate that the proposed FOCT anomaly handling and improvement measures can effectively improve the transient performance of FOCT, and at the same time provide a complete set of testing means for the engineering application and later upgrade and replacement of FOCT.

Distortion of Spectral Ripples Through Cochlear Implants Has Major Implications for Interpreting Performance Scores.

The spectral ripple discrimination task is a psychophysical measure that has been found to correlate with speech recognition in listeners with cochlear implants (CIs). However, at ripple densities above a critical value (around 2 RPO, but device-specific), the sparse spectral sampling of CI processors results in stimulus distortions resulting in aliasing and unintended changes in modulation depth. As a result, spectral ripple thresholds above a certain number are not ordered monotonically along the RPO dimension and thus cannot be considered better or worse spectral resolution than each other, thus undermining correlation measurements. These stimulus distortions are not remediated by changing stimulus phase, indicating these issues cannot be solved by spectrotemporally modulated stimuli. Speech generally has very low-density spectral modulations, leading to questions about the mechanism of correlation between high ripple thresholds and speech recognition. Existing data showing correlations between ripple discrimination and speech recognition include many observations above the aliasing limit. These scores should be treated with caution, and experimenters could benefit by prospectively considering the limitations of the spectral ripple test.

Increase of SRG modulation depth in azopolymers-nanoparticles hybrid materials

Thin films of azopolymer-nanoparticles hybrid materials were fabricated with poly[1-[4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]-1,2-ethanediyl] (PAZO) and different concentrations of Ag and AgAu nanoparticles (NPs). By illuminating the films with polarized interference patterns, surface relief gratings (SRGs) were recorded. It was found that for some concentrations of NPs their modulations and diffraction efficiency were higher than the obtained for PAZO films without NPs. The effect was mainly explained by the increase of the free volume available for the photoisomerization for certain concentrations of NPs. The dependence of the diffraction efficiency on concentration was directly related to changes in modulation depth. When doping with NPs, the maximum efficiency increases more than two times the efficiency without NPs.

Absolute and pixel-wise measurements of vibration amplitudes using time-averaged digital holography

This paper presents scanning time-averaged digital holography, a method for measuring the amplitude distributions of harmonically oscillating objects. Each intensity image reconstructed from the time-averaged digital hologram has the form of a set of fringes that follow a zero-order Bessel function of the first kind. Modulating the phase of the light beam in the interferometer results in shifts in the bright zero-order fringes with changes in modulation depth. The developed method is based on the continuous shift in the zero-order fringe over the object surface. The magnitude of the vibration amplitude is independently evaluated for each pixel. Unlike other digital holographic vibrometry methods based on spatial phase unwrapping or fringe counting, the proposed technique can measure vibration amplitudes without the risk of fringe number confusion or 2π errors. Thus, the new method can be used to measure high-slope amplitude distributions and discontinuous/partially shaded objects. The method was experimentally verified by measuring a bending piezo actuator, and the uncertainty of the method was determined to be in the range of nanometers.

Influence of sinusoidal modulation on mode competition and signal distortion in multimode InGaAsP lasers

This paper investigates influence of large-signal modulation on mode oscillation in InGaAsP/InP lasers. The mode competition dynamics under modulation are examined in terms of the temporal trajectories of the total output and the strongest modes as well as their Fourier spectra. The mode coupling induced by the strong spectral gain suppression is evaluated by both the correlation coefficients among the strongest modes and their signal distortion. We show that small and moderate modulations at the multimode hopping of the laser modulate the mode hopping along with modulation of each mode. The mode coupling is characterized by anti-correlation among the modes like as the non-modulated laser. Under modulation with the resonance frequency, the increase in the modulation depth changes mode coupling from anti-correlation to positive correlation and then to complete coupling that correspond to emission of periodic pulses. The mode coupling is characterized by mode competition distortion, which measures the amount of power carried by the mode signal at the frequency of multimode-hopping.

Recovery in Stroke Rehabilitation through the Rotation of Preferred Directions Induced by Bimanual Movements: A Computational Study

Stroke patients recover more effectively when they are rehabilitated with bimanual movement rather than with unimanual movement; however, it remains unclear why bimanual movement is more effective for stroke recovery. Using a computational model of stroke recovery, this study suggests that bimanual movement facilitates the reorganization of a damaged motor cortex because this movement induces rotations in the preferred directions (PDs) of motor cortex neurons. Although the tuning curves of these neurons differ during unimanual and bimanual movement, changes in PD, but not changes in modulation depth, facilitate such reorganization. In addition, this reorganization was facilitated only when encoding PDs are rotated, but decoding PDs are not rotated. Bimanual movement facilitates reorganization because this movement changes neural activities through inter-hemispheric inhibition without changing cortical-spinal-muscle connections. Furthermore, stronger inter-hemispheric inhibition between motor cortices results in more effective reorganization. Thus, this study suggests that bimanual movement is effective for stroke rehabilitation because this movement rotates the encoding PDs of motor cortex neurons.

Comodulation masking release determined in the mouse (Mus musculus) using a flanking-band paradigm.

Comodulation masking release (CMR) has been attributed to auditory processing within one auditory channel (within-channel cues) and/or across several auditory channels (across-channel cues). The present flanking-band (FB) experiment-using a 25-Hz-wide on-frequency noise masker (OFM) centered at the signal frequency of 10 kHz and a single 25-Hz-wide noise FB-was designed to separate the amount of CMR due to within- and across-channel cues and to investigate the role of temporal cues on the size of within-channel CMR. The results demonstrated within-channel CMR in the Naval Medical Research Institute mouse, while no unambiguous evidence could be found for CMR occurring due to across-channel processing (i.e., "true CMR"). The amount of within-channel CMR was dependent on the frequency separation between the FB and the OFM. CMR increased from 4 to 6 dB for a frequency separation of 1 kHz to 18 dB for a frequency separation of 100 Hz. The large increase for a frequency separation of 100 Hz is likely to be due to the exploitation of changes in the temporal pattern of the stimulus upon the addition of the signal. Temporal interaction between both masker bands results in modulations with a large depth at a modulation frequency equal to the beating rate. Adding a signal to the maskers reduces the depth of the modulation. The auditory system of mice might be able to use the change in modulation depth at a beating frequency of 100 Hz as a cue for signal detection, while being unable to detect changes in modulation depth at high modulation frequencies. These results are consistent with other experiments and model predictions for CMR in humans which suggested that the main contribution to the CMR effect stems from processing of within-channel cues.

Optimization of the signal-to-noise ratio of frequency-domain instrumentation for near-infrared spectro-imaging of the human brain

Frequency-domain near-infrared spectro-imaging offers significant advantages over the continuous-wave method in human brain applications. However, the drawback of existing instruments is a low signal-to-noise ratio for measured phase and modulation depth changes caused by cerebral activation. In this paper we show that in the case of the geometry specific for the activated area in the human brain, the SNR can be significantly improved by increasing the modulation frequency. We present the results of two studies: one performed experimentally using a subnanosecond pulsed light source and a spherical absorbing inhomogeneity immersed in a highly scattering solution, and the other performed numerically using Monte Carlo simulations of light transport in an MRI based digital phantom of the adult human head. We show that changes caused by the absorbing inhomogeneity in both phase and modulation depth increase with frequency and reach maximum values at frequencies between 400 and 1400 MHz, depending on the particular source-detector distance. We also show that for the human head geometry an increase of the modulation frequency from 100 to 500 MHz can increase the phase SNR 2-3 times, and the modulation depth SNR up to 10 times.

Grouping and adaptation in modulation detection interference

Modulation detection interference (MDI) can be reduced by gating the masker and signal carriers asynchronously, possibly due to grouping effects. However, as both carriers are generally modulated throughout their presentation, two other factors may also play a role: First, the response to modulation may adapt, thereby reducing the effective masker modulation depth prior to signal presentation. Second, analogous to intensity perception, a dynamic change in modulation depth (increment detection) may be easier to detect than an overall difference across two presentations (level discrimination). A MDI paradigm with 1- and 4.3-kHz carriers was used. The 16-Hz sinusoidal signal modulation was masked by noise modulation with a passband from 8 to 24 Hz. In one experiment, the gating synchrony of carrier and modulator were independently varied. In another experiment, both masker and signal modulated a single 4.3-kHz carrier, with gating synchrony between masker and signal varied. Results suggest that the asynchronous gating of either the carriers or modulators alone is not sufficient to produce a release from MDI. Thresholds were higher for the single carrier than for the two carriers, and gating the masker modulation on prior to the signal did not improve performance. [Supported by NIDCD and Max Kade Foundation.]

Quantification in tissue near–infrared spectroscopy

In near–infrared spectroscopy (NIRS) of tissue, light attenuation is due to: (i) absorption from chromophores of fixed concentration, (ii) absorption from chromophores of variable concentration, and (iii) light scatter. NIRS is usually concerned with trying to quantify the concentrations of chromophores in category (ii), in particular oxy– and deoxyhaemoglobin (HbO2and Hb) and cytochrome oxidase.In the absence of scatter the total light absorption in the medium is a linear sum of that due to each chromophore. In a scattering medium like tissue, this linear summation is distorted because the optical path length at each wavelength may differ. This distorted spectrum is then superimposed upon a further wavelength–dependent attenuation arising from light loss due to scatter, which is a complex function of the tissue absorption and scattering coefficients (μaandμs), scattering phase function, and tissue and measurement geometry. Consequently, quantification of NIRS data is difficult.Over the past 20 years many differing approaches to quantification have been tried. The development of methods for measuring optical path length in tissue initially enabled changes in concentration to be quantified, and subsequently methods for absolute quantification of HbO2and Hb were developed by correlating NIRS changes with an independent measurement of arterial haemoglobin saturation. Absolute determination of tissue optical properties, however, requires additional information over and above the detected intensity at the tissue surface, which must then be combined with a model of light transport to deriveμaandμs. The additional data can take many forms, e.g. the change in intensity with distance, the temporal dispersion of light from an ultrashort input light pulse, or phase, and modulation depth changes of intensity–modulated light. All these approaches are now being actively pursued with considerable success. However, all the approaches are limited by the accuracy of the light transport models, especially in inhomogeneous media.

Influence of relative level on modulation discrimination interference

This experiment examines the effects on modulation discrimination interference (MDI) of differences in level between the signal and masker carriers. The task was to detect a change in depth of amplitude modulation of a 1-kHz signal carrier, in the presence of modulated or unmodulated masker carriers that were either reasonably close to the signal carrier frequency (540 and 1380 Hz) or distant from it (230 and 3300 Hz). The relative level of the signal and masker carriers was systematically varied. The results show that modulated maskers can impair the detection of changes in modulation depth even when the maskers are distant in frequency from the signal carrier and 30–40 dB lower in level than the signal carrier. The magnitude of the interference effect changes only slowly with relative level of the signal and masker carriers, suggesting that the interference depends primarily on across-channel processes.

Effects of complex-sound phase in the chinchilla auditory nerve

Responses of populations of single auditory nerve fibers in anesthetized chinchillas to three-component stimuli were studied. The carrier frequency ( fc) was 2 kHz and the modulating frequencies ( fm) were 100 and 1000 Hz. The sidebands were −6 dB relative to the center component. The phase of the center component was varied from 0 to 180° in 45° steps, which produced changes in modulation depth. Discharge synchrony to fm=100 Hz decreased with increasing stimulus level for CFs close to fc, as previously shown by other researchers. At 20 dB re:unit threshold, maximum synchrony to 100 Hz was observed for CFs close to fc. The maxima shifted to higher CFs with increase in stimulus level. Synchrony to fc decreased with phase shifts up to 90° and increased for phase shifts between 90° and 180°. At 20 dB, synchrony to the lower sideband was less than that to the upper sideband when the phase shift was 90° and 135°. At 40 dB, 0° and 180° phase shifts produced higher discharge synchrony to the lower sideband. Also at 40 dB, the phase dependence of the synchrony to the two sidebands differed most at the lower CF regions. The changes in discharge synchrony due to phase changes must reflect cochlear nonlinearities because the amplitude ratios of the stimulus components were held constant. [Work supported by DC00341.]

Effects of modulation rate and rate of envelope change on modulation discrimination interference

Modulation discrimination interference (MDI) refers to a reduced ability to detect a change in modulation depth of a target carrier frequency in the presence of other modulated carriers with frequencies remote from that of the target. These experiments examined whether the tuning of MDI in the modulation domain depends primarily on the similarity in modulation periodicity of the target and masking sounds or on the similarity in rate of change of envelope of the target and masking sounds. This question was addressed by manipulating the ac component of the modulation waveform of the target and masker along part of a continuum that ranged from a sinusoid at one end to a square wave at the other; the transition time (rise/fall time) of the modulator was 6.25, 12.5, 25, or 50 ms. When the target and masker carriers were modulated at the same rate [10 periods per second (pps)], MDI decreased with decreasing rise/fall time of the target and with increasing rise/fall time of the masker. Maximum MDI did not occur when the rise/fall times of the target and masker were equal, as might be expected if MDI were caused by perceptual grouping of the target and masker. This pattern of results is consistent with the idea that, for a given carrier, shorter rise/fall times lead to greater salience. When the target and masker had the same rise/fall times, MDI was strongly affected by a difference in modulation rate of the target and masker and slightly affected by the common rise/fall time of the target and masker. Taken together, these results indicate that both modulation periodicity and rate of change of envelope influence MDI.

Intermediate detection of changes in modulation and power spectra

In an effort to examine the shift from modulation-based discrimination for narrow-band complex stimuli to across-frequency spectral comparisons for wideband complex stimuli, the mutual discriminability of 11 stimuli was determined. The five ‘‘base’’ stimuli were the sum of three tones. By altering the relative phases of the tones, modulation depths ranging from maximum (SAM) to minimum (QFM) were generated using five equal log steps. The central frequency was 1000 Hz, and modulation rates ranged from 20 to 240 Hz. Three additional stimuli had envelopes identical to three of the base stimuli, but had periodic fine structures. Three other stimuli had fine structures identical to three of the base stimuli, but no envelope modulation. For observers tested thus far, where the modulation rate was 20 Hz, discriminability is well described as being based on changes in modulation depth. Where the modulation rate was 240 Hz, the discrimination appears to depend on changes in long term power spectra. For intermediate modulation rates, the data do not appear to reflect a gradual change from modulation-based detection to power spectrum-based detection, but the intervening cues appear not to be well defined by our stimuli.

Detection of changes in modulation depth of a target sound in the presence of other modulated sounds

These experiments are concerned with the comodulation masking release (CMR) and modulation discrimination interference (MDI) that can occur when modulated carriers are added to a target modulated sound at frequencies remote from the target frequency. Thresholds were measured for detecting increments or decrements in the modulation index (m) of a 1000-Hz carrier (the target) sinusoidally amplitude modulated (SAM) at a 10-Hz rate, with that carrier presented either alone, or with a variety of added sounds. In experiments 1–3, the target had a standard value of m of 0.5. The added sounds had carrier frequencies of either 550 and 1550 Hz or 230 and 3300 Hz, and they were either unmodulated (m=0) or SAM at a 10-Hz rate (in phase with the target), with m=0.25, 0.5, or 0.75. In experiment 1, the target and added sounds were gated on synchronously for a duration of 1000 ms. The added sounds produced MDI, and the amount of MDI increased progressively with increases in the modulation index of the added sounds. In experiment 2, the added sounds were gated on 500 ms before and gated off 50 ms after the 1000-ms target sound. This reduced the MDI, but substantial amounts of MDI remained, especially for the 550- and 1550-Hz added sounds. In experiment 3, the target and added sounds were gated on synchronously for 1550 ms, and the increment or decrement in m was gated on for 1000 ms after a delay of 500 ms. This generally gave rise to more MDI than was found in experiment 2. In experiment 4, the values of m for the target and added sounds were 0.75/0.75, 0.75/1.0, and 1.0/1.0, respectively. The added sound consisted of carriers at 230, 550, 1550, and 3300 Hz. All of the temporal conditions used in experiments 1–3 were examined. The added sounds produced MDI in some conditions and CMR in others. The largest CMR occurred in conditions where the modulation index for the added sounds was 1.0, and the whole target sound was delayed relative to the onset of the added sound. The results are discussed in terms of the factors influencing MDI and CMR, and in terms of the possible role of perceptual grouping mechanisms.

The processing of envelope information in comodulation masking release (CMR) and envelope discrimination

In comodulation masking release (CMR), thresholds for a signal masked by a narrow-band noise are reduced when additional noise is present. To demonstrate CMR, the additional noise must have similar amplitude envelope fluctuations over time as the primary noise band masking the signal. The specific source of information that provides the reduction in masked threshold remains unclear, although it is the focus of recent interest. The envelope of the on-frequency masking band (OFB) changes with the addition of the signal at threshold: in the present work, the importance of these changes in providing the CMR has been investigated. First, CMR thresholds were collected in sinusoidally amplitude-modulated noises which varied either in the phase relationship of their envelopes or in their modulation depth. Then envelope discrimination (ED) thresholds were collected for changes in envelope phase disparity (i.e., envelope correlation) and for changes in modulation depth per se. The patterns of CMR thresholds with envelope phase disparity and with modulation depth are not similar to those of the ED thresholds. A computer simulation was conducted in which the stimulus waveforms were processed through an auditory model which comprised bandpass auditory filters, a square-law nonlinearity, and a sliding temporal window. The envelopes were then extracted from the processed waveforms to determine whether the envelope changes that occur in the signal intervals in the CMR and ED tasks may be similar. The results of this analysis indicate that discriminability of envelope correlation due to the addition of the signal at threshold in the CMR task was insufficient to explain CMR. However, the discriminability of changes in modulation depth due to the addition of the signal is in agreement with thresholds obtained from the CMR task.

Across-channel Masking of Changes in Modulation Depth for Amplitude- and Frequency-modulated Signals

This study examines a form of masking that can take place when the signal and masker are widely separated in frequency and cannot be explained in terms of the traditional concept of the auditory filter or critical band. We refer to this as across-channel masking. The task of the subject was to detect an increment in modulation depth of a 1000-Hz sinusoidal carrier. The carrier could either be sinusoidally amplitude modulated or sinusoidally frequency modulated at a 10-Hz rate. Modulation increment thresholds of this "target" signal were measured for the target alone, and in the presence of two interfering sounds with carrier frequencies of 230 and 3300 Hz. When the interfering sounds were unmodulated, they had no effect on modulation increment thresholds. When the interfering sounds were either amplitude or frequency modulated, thresholds increased. Amplitude modulation (AM) increment thresholds were affected by both amplitude-modulated and frequency-modulated interference. Similarly, frequency modulation (FM) increment thresholds were affected by both amplitude-modulated and frequency-modulated interference. For both types of signal, the interference was tuned for modulation rate; across-channel masking was greatest when the interfering sounds were modulated at rates close to 10 Hz, and declined for higher or lower rates. However, the tuning was rather broad. When the target and interfering sounds were modulated at the same rate, there was no effect of the relative phase of the modulators. Two possible explanations for the results are discussed. One is based on the idea that carriers that are modulated in a similar way tend to be perceptually "grouped". The other is based on the idea that there are "channels" in the auditory system tuned for AM and FM rate. Neither explanation appears completely satisfactory.

The continuum between comodulation masking release (CMR) and profile analysis (PA): Modulation depth

The existence of a continuum between CMR and PA was investigated by varying the modulation depth of five sinusoidal components. In the unmodulated condition (PA), a 2.0‐kHz tone was presented either along (reference), or with four flanking tones presented to the same or opposite ear. In the modulated conditions (CMR), each tone was sinusoidally amplitude modulated (SAM) from 20% to 100%. The signal was an increment to the 2.0‐kHz tone. The conditions were run with or without a random level variation of +10 dB about a median value of 50 dB per SAM tone. Randomizing level impaired performance in the reference condition at low modulation depths, and a masking release occurred with both monaural and dichotic presentation of the flanking SAM tones. At high modulation depths (60%–100%), randomizing level did not impair performance, probably because the signal produced a change in modulation depth that could be used as a cue. In this case, there was no masking release.

The role of envelope information in CMR tasks

In comodulation masking release (CMR), thresholds for a signal masked by a narrow‐band noise are reduced when additional noise is present, provided that the additional noise has the same amplitude envelope fluctuations over time as the original masking band. Recently, some speculation has appeared in the literature as to the specific source of information which provides the reduction in masked threshold. In the present work, the importance of changes in the envelope of the masker band, due to the addition of the signal at threshold, in providing the CMR, has been investigated. First, CMR thresholds were collected in sinusoidally amplitude‐modulated noises which varied either in the phase relationship of their envelopes or in their modulation depth. Then, envelope discrimination (ED) thresholds were collected for changes in envelope phase disparity and for changes in modulation depth per se. The data indicate that the patterns of CMR thresholds with envelope phase disparity and with modulation depth vary differently from that of the ED thresholds. A simulation was then conducted in which the stimulus waveforms were processed through a model that includes an auditory filter [equivalent rectangular bandwidth (ERB) filtering], a square‐law nonlinearity, and a sliding temporal window [an equivalent rectangular duration (ERD) filter]. The envelopes were then extracted from the processed waveforms to determine what actual changes in the envelopes were occurring in the signal intervals in both CMR and ED. The results of this waveform analysis indicate that discriminability of envelope changes due to the addition of the signal at threshold in the CMR task are insufficient to explain CMR.

Factors affecting thresholds for sinusoidal signals in narrow-band maskers with fluctuating envelopes.

When a signal is higher in frequency than a narrow-band masker, thresholds are lower when the masker envelope fluctuates than when it is constant. This article investigates the cues used to achieve the lower thresholds, and the factors that influence the amount of threshold reduction. In experiment I the masker was either a sinusoid (constant envelope) or a pair of equal-amplitude sinusoids (fluctuating envelope) centered at the same frequency as the single sinusoid (250, 1000, 3000, or 5275 Hz). The signal frequency was 1.8 times the masker frequency. At all center frequencies, thresholds were lower for the two-tone masker than for the sinusoidal masker, but the effect was smaller at the highest and lowest frequencies. The reduced effect at high frequencies is attributed to the loss of a cue related to phase locking in the auditory nerve. The reduced effect at low frequencies can be partly explained by reduced slopes of the growth-of-masking functions. In experiment II the masker was a sinusoid amplitude modulated at an 8-Hz rate. Masker and signal frequencies were the same as for the first experiment. Randomizing the modulation depth between the two halves of a forced-choice trial had no effect on thresholds, indicating that changes in modulation depth are not used as a cue for signal detection. Thresholds in the modulated masker were higher than would be predicted if they were determined only by the masker level at minima in the envelope, and the threshold reduction produced by modulating the master envelope was less at 250 Hz than at higher frequencies. Experiments III and IV reveal two factors that contribute to the reduced release from masking at low frequencies: The rate of increase of masked threshold with decreasing duration is greater at 250 Hz than at 1000 Hz; the amount of forward masking, relative to simultaneous masking, is greater at 250 Hz than at 1000 Hz. The results are discussed in terms of the relative importance of across-channel cues and within-channel cues.