Amplitude Modulation Envelope Research Articles

Visualizing interferential stimulation of human brains.

Transcranial electrical stimulation (TES) is limited in focally stimulating deep-brain regions, even with optimized stimulation montages. Recently, interferential stimulation (IFS), also known as transcranial temporal interference stimulation (TI, TIS, or tTIS), has drawn much attention in the TES community as both computational and experimental studies show that IFS can reach deep-brain areas. However, the underlying electrodynamics of IFS is complicated and difficult to visualize. Existing literature only shows static visualization of the interfered electric field induced by IFS. These could result in a simplified understanding that there is always one static focal spot between the two pairs of stimulation electrodes. This static visualization can be frequently found in the IFS literature. Here, we aimed to systematically visualize the entire dynamics of IFS. Following the previous study, the lead field was solved for the MNI-152 head, and optimal montages using either two pairs of electrodes or two arrays of electrodes were found to stimulate a deep-brain region close to the left striatum with the highest possible focality. We then visualized the two stimulating electrical currents injected with similar frequencies. We animated the instant electric field vector at the target and one exemplary off-target location both in 3D space and as a 2D Lissajous curve. We finally visualized the distribution of the interfered electric field and the amplitude modulation envelope at an axial slice going through the target location. These two quantities were visualized in two directions: radial-in and posterior-anterior. We hope that with intuitive visualization, this study can contribute as an educational resource to the community's understanding of IFS as a powerful modality for non-invasive focal deep-brain stimulation.

3D-Printed All-Dielectric Electromagnetic Encoders with Synchronous Reading for Measuring Displacements and Velocities.

In this paper, 3D-printed electromagnetic (or microwave) encoders with synchronous reading based on permittivity contrast, and devoted to the measurement of displacements and velocities, are reported for the first time. The considered encoders are based on two chains of linearly shaped apertures made on a 3D-printed high-permittivity dielectric material. One such aperture chain contains the identification (ID) code, whereas the other chain provides the clock signal. Synchronous reading is necessary in order to determine the absolute position if the velocity between the encoder and the sensitive part of the reader is not constant. Such absolute position can be determined as long as the whole encoder is encoded with the so-called de Bruijn sequence. For encoder reading, a splitter/combiner structure with each branch loaded with a series gap and a slot resonator (each one tuned to a different frequency) is considered. Such a structure is able to detect the presence of the apertures when the encoder is displaced, at short distance, over the slots. Thus, by injecting two harmonic signals, conveniently tuned, at the input port of the splitter/combiner structure, two amplitude modulated (AM) signals are generated by tag motion at the output port of the sensitive part of the reader. One of the AM envelope functions provides the absolute position, whereas the other one provides the clock signal and the velocity of the encoder. These synchronous 3D-printed all-dielectric encoders based on permittivity contrast are a good alternative to microwave encoders based on metallic inclusions in those applications where low cost as well as major robustness against mechanical wearing and aging effects are the main concerns.

Multilevel objective measures of interaural phase difference detection

Sensitivity to interaural phase difference (IPD) cues is critical for localizing sound and hearing in background noise. Recent work from multiple laboratories has explored electrophysiologic methods for assaying IPD sensitivity with the aim of developing an objective measure of binaural hearing. Such a measure would be useful in a variety of clinical and research applications such as understanding the effects of aging on binaural hearing; fine-tuning hearing aids/cochlear implants to maximize binaural benefit; and assessing neural damage after a traumatic brain injury. Here, we present data from an ongoing study evaluating a novel “multi-level” (brainstem, midbrain, and cortex) simultaneous measure of IPD acuity. Amplitude modulated (20, 40, and 80 AM) carrier tones (125, 250, and 500 Hz) with IPDs embedded in them (7 deg–180 deg) were used to generate auditory evoked potentials in young, normal hearing listeners. Neural responses corresponding to the carrier tones (frequency following responses), amplitude modulation envelopes (envelope following responses), and cortical change detection (acoustic change complex) were measured simultaneously to provide a multi-level snapshot of neural IPD processing. Parametric data presented here demonstrate feasibility and limitations of this approach for measuring IPD encoding at multiple levels of the auditory system.

Amplitude Modulation And Nonlinear Self-Interactions of the Geodesic Acoustic Mode at the Edge of MAST

We studied the amplitude modulation of the radial electric field constructed from the Langmuir probe plasma potential measurements at the edge of the mega-ampere spherical tokamak (MAST). The Empirical Mode Decomposition (EMD) technique was applied, which allowed us to extract fluctuations on temporal scales of plasma turbulence, the Geodesic Acoustic Mode (GAM), and those associated with the residual poloidal flows. This decomposition preserved the nonlinear character of the signal. Hilbert transform (HT) was then used to obtain the amplitude modulation envelope of fluctuations associated with turbulence and with the GAM. We found significant spectral coherence at frequencies between 1–5 kHz, in the turbulence and the GAM envelopes and for the signal representing the low frequency zonal flows (LFZFs). We present the evidence of local and nonlocal, in frequency space, three wave interactions leading to coupling between the GAM and the low frequency (LF) part of the spectrum.

Sensitivity to AM incoherence is affected by center frequency and modulation rate

Fine-grained sensitivity to frequency modulation (FM) at slow rates and low-frequency carriers is thought to be due to auditory-nerve phase locking (time code). Alternatively, a unitary code for FM at all rates and carrier frequencies could be based on cochlear conversion of FM to amplitude modulation (AM) (place code). One weakness of the place-coding theory is it cannot readily explain rate- and carrier-dependent trends in FM sensitivity. This study asked whether FM trends could potentially be explained by sensitivity to two AM envelopes that are out of phase (incoherent AM) at separate cochlear locations, thereby simulating the effects of FM. AM discrimination was assessed for two-component complexes centered at low (500 and 1500 Hz) and high (7000 Hz) frequencies, spaced 2/3 or 4/3 octaves apart, and modulated at slow (2 Hz) and fast (20 Hz) rates. Coherent and incoherent two-component AM detection was assessed for the same conditions. Preliminary results show that sensitivity to AM incoherence is best at low center frequencies and slow rates, consistent with trends traditionally found in FM detection that have been attributed to time coding. Findings suggest time coding may not be necessary to explain trends in FM sensitivity. [Work supported by NIH Grant R01DC005216.]

Coupled analysis of nonlinear sloshing and ship motions

A coupled numerical model considering nonlinear sloshing flows and the linear ship motions has been developed based on a boundary element method. Hydrodynamic performances of a tank containing internal fluid under regular wave excitations in sway are investigated by the present time-domain simulation model and comparative model tests. The numerical model features well the hydrodynamic performance of a tank and its internal sloshing flows obtained from the experiments. In particular, the numerical simulations of the strong nonlinear sloshing flows at the natural frequency have been validated. The influence of the excitation wave height and wave frequency on ship motions and internal sloshing has been investigated. The magnitude of the internal sloshing increases nonlinearly as the wave excitation increases. It is observed that the asymmetry of the internal sloshing relative to still water surface becomes more pronounced at higher wave excitation. The internal sloshing-induced wave elevation is found to be amplitude-modulated. The frequency of the amplitude modulation envelope is determined by the difference between the incident wave frequency and the natural frequency of the internal sloshing. Furthermore, the coupling mechanism between ship motions and internal sloshing is discussed.

Low Cost Signal Reconstruction Based Testing of RF Components using Incoherent Undersampling

To meet the testing requirements of high speed components used in modern communication systems, in an efficient and cost effective manner, it is necessary to develop new device performance measurement techniques that are easily scalable to high frequencies. Traditional up/down conversion based transmitter testing architectures are sensitive to the linearity of the mixers and carrier phase noise in the receiver. Direct undersampling based test instrumentation can overcome the limitations imposed by mixers in up/down conversion. A major challenge in direct undersampling based test architecture is to achieve precise phase alignment between different components of the test setup. Such phase alignment of high frequency signals requires the use of expensive test instruments adding to the cost and complexity of the overall test system. To resolve this problem, an incoherent undersampling based test method is developed in this research that eliminates the need for precise phase synchronization between the RF test signal carrier, its amplitude-modulated envelope (generally necessary for measuring amplifier nonlinearity) and the reference sampling clock. A side-benefit, due to the use of signal undersampling, is that signal acquisition is achieved without the use of a Nyquist rate data converter. Multiple RF performance metrics are extracted without the use of a reference receiver. The accuracy of the proposed setup is compared against existing coherent sampling based test setups.

THE MULTIPLICATIVE NOISE COMPONENTS OF ISOMETRIC FORCE FLUCTUATIONS

The experiment tested the relative contribution of additive and multiplicative noise components in isometric force fluctuations. We implemented the Hilbert–Huang transform to analyze variability in the steady state region of isometric force time series generated by participants in three adult age categories: (1) young (20–24 yrs); (2) old (60–69 yrs); and older-old (75–90 yrs). Each participant generated isometric force output in response to constant target force levels (5–40% maximum voluntary contraction). The empirical mode decomposition component of the Hilbert–Huang transform was applied to each isometric force time series resulting in a set of intrinsic mode functions (IMFS). The amplitude modulation envelopes of the IMFS obtained from the modulus of the Hilbert transform were random sequences. These random sequences were the source of the multiplicative noise. Analyses showed significant age group differences in the multiplicative noise structure of the force output. The results provide preliminary evidence that multiplicative noise dominates additive noise in isometric force output and can discriminate among the force fluctuations of healthy adult participants of different ages.

Localizing amplitude‐modulated, high‐frequency tones in free field: Perceptual.

Headphone experiments since the 1950s have shown that high‐frequency tones can be lateralized if they are given low‐frequency amplitude modulation (AM). Lateralization is mediated by the interaural time difference (ITD) of the amplitude envelope, and it resembles lateralization for a comparison sine tone having the frequency of the modulation in the AM tone. However, lateralization is distinct from localization because there are significant physical differences between the idealized AM signals in the headphone experiments and the signals arriving at a listener’s ears in free‐field presentation. In free field, (1) the modulation percentage becomes different for the two ears, (2) a purely amplitude‐modulated source inevitably acquires some frequency modulated component, (3) signals are sometimes overmodulated in one or both ears. Also, the ITD of the AM envelope is generally less than the ITD in the comparison low‐frequency sine tone because the envelope ITD depends on the group delay, not the (larger) phas...

Temporal AM–FM combination for robust speech recognition

A novel method for feature extraction from the frequency modulation (FM) in speech signals is proposed for robust speech recognition. To exploit of the multistream speech recognizers, each stream should compensate for the shortcomings of the other streams. In this light, FM features are promising as complemental features of amplitude modulation (AM). In order to extract effective features from FM patterns, we applied the proposed feature extraction method by the data-driven modulation analysis of instantaneous frequency. By evaluating the frequency responses of the temporal filters obtained by the proposed method, we confirmed that the modulation observed around 4 Hz is important for the discrimination of FM patterns, as in the case of AM features. We evaluated the robustness of our method by performing noisy speech recognition experiments. We confirmed that our FM features can improve the noise robustness of speech recognizers even when the FM features are not combined with conventional AM and/or spectral envelope features. We also performed multistream speech recognition experiments. The experimental results show that combination of the conventional AM system and proposed FM system reduced word error by 43.6% at 10 dB SNR as compared to the baseline MFCC system and by 20.2% as compared to the conventional AM system. We investigated the complementarity of the AM and FM features by performing speech recognition experiments in artificial noisy environments. We found the FM features to be robust to wide-band noise, which certainly degrades the performance of AM features. Further, we evaluated the efficiency of multiconditional training. Although the performance of the proposed combination method was degraded by multiconditional training, we confirmed that the performance of the proposed FM method improved. Through a series of experiments, we confirmed that our FM features can be used as independent features as well as complemental features.

An Accurate Ultrasonic Distance Measurement System with Self Temperature Compensation

This study proposes an accurate distance measurement system which has self-temperature-compensation (STC) with the environmental average temperature in space, rather than a single point temperature. This system combines both the amplitude modulation (AM) and the phase modulation (PM) of the pulse-echo technique. The proposed system can reduce error caused by inertial delay and the amplitude attenuation effect when using the AM and PM envelope square waveform (APESW). The proposed system adopts two identical measurement hardware sets using the APESW ultrasonic driving waveform. The first set measures the sound velocity (the environmental average temperature information is also involved) as the result of the temperature compensation data for the second distance measuring set. Without using a temperature sensor, experimental results indicate that the proposed STC distance measurement system can accurately measure the distance. The experimental standard deviation of the linearity with respect to the distance is found to be 0.21 mm at a range of 50 to 500 mm. Moreover, the proposed system's temperature uncertainty effect produced a standard deviation of 0.093 mm, while the temperature sensor system's uncertainty effect produced a standard deviation of 0.68 mm. The STC manner is simple and can be easily adapted for robotic applications for which the temperature sensors can not easily be set up and placed. The main advantages of this STC distance measurement system are: high resolution, low cost, and ease of implementation.

An accurate air temperature measurement system based on an envelope pulsed ultrasonic time-of-flight technique

A new microcomputer based air temperature measurement system is presented. An accurate temperature measurement is derived from the measurement of sound velocity by using an ultrasonic time-of-flight (TOF) technique. The study proposes a novel algorithm that combines both amplitude modulation (AM) and phase modulation (PM) to get the TOF measurement. The proposed system uses the AM and PM envelope square waveform (APESW) to reduce the error caused by inertia delay. The APESW ultrasonic driving waveform causes an envelope zero and phase inversion phenomenon in the relative waveform of the receiver. To accurately achieve a TOF measurement, the phase inversion phenomenon was used to sufficiently identify the measurement pulse in the received waveform. Additionally, a counter clock technique was combined to compute the phase shifts of the last incomplete cycle for TOF. The presented system can obtain 0.1% TOF resolution for the period corresponding to the 40 kHz frequency ultrasonic wave. Consequently, with the integration of a humidity compensation algorithm, a highly accurate and high resolution temperature measurement can be achieved using the accurate TOF measurement. Experimental results indicate that the combined standard uncertainty of the temperature measurement is approximately 0.39 degrees C. The main advantages of this system are high resolution measurements, narrow bandwidth requirements, and ease of implementation.

Envelope pulsed ultrasonic distance measurement system based upon amplitude modulation and phase modulation

A novel microcomputer-based ultrasonic distance measurement system is presented. This study proposes an efficient algorithm which combines both the amplitude modulation (AM) and the phase modulation (PM) of the pulse-echo technique. The proposed system can reduce error caused by inertia delay and amplitude attenuation effect when using the AM and PM envelope square wave form (APESW). The APESW ultrasonic driving wave form causes a phase inversion phenomenon in the relative wave form of the receiver. The phase inversion phenomenon sufficiently identifies the "measurement pulse" in the received wave forms, which can be used for accurate time-of-flight (TOF) measurement. In addition, combining a countertechnique to compute the phase shifts of the last cycle for TOF, the presented system can obtain distance resolution of 0.1% of the wavelength corresponding to the 40 kHz frequency of the ultrasonic wave. The standard uncertainty of the proposed distance measurement system is found to be 0.2 mm at a range of 50-500 mm. The APESW signal generator and phase detector of this measuring system are designed on a complex programmable logic device, which is used to govern the TOF measurement and send the data to a personal computer for distance calibration and examination. The main advantages of this APESW system are high resolution, low cost, narrow bandwidth requirement, and ease of implementation.

Frequency organization of the 40-Hz auditory steady-state response in normal hearing and in tinnitus

We used the 40-Hz auditory steady-state response (SSR) to compare for the first time tonotopic frequency representations in the region of primary auditory cortex (PAC) between subjects with chronic tinnitus and hearing impairment and normal hearing controls. Frequency representations were measured in normal hearing ( n = 17) and tinnitus ( n = 28) subjects using eight carrier frequencies between 384 and 6561 Hz, each amplitude modulated (AM) at 40-Hz on trials of 3 min duration under passive attention. In normal hearing subjects, frequency gradients were observed in the medial–lateral, anterior–posterior, and inferior–superior axes, which were consistent with the orientation of Heschl’s gyrus and with functional organization revealed by fMRI investigations. The frequency representation in the right hemisphere was ∼ 5 mm anterior and ∼ 7 mm lateral to that in the left hemisphere, corroborating with MEG measurements hemispheric asymmetries reported by cytoarchitectonic studies of the PAC and by MRI morphometry. In the left hemisphere, frequency gradients were inflected near 2 kHz in normal hearing subjects. These SSR frequency gradients were attenuated in both hemispheres in tinnitus subjects. Dipole power was also elevated in tinnitus, suggesting that more neurons were entrained synchronously by the AM envelope. These findings are consistent with animal experiments reporting altered tonotopy and changes in the response properties of auditory cortical neurons after hearing loss induced by noise exposure. Degraded frequency representations in tinnitus may reflect a loss of intracortical inhibition in deafferented frequency regions of the PAC after hearing injury.

Assessment of errors induced by the anomalous dispersion of high frequency shear waves in rheometrical measurements on gelling systems

The principal objectives of the work reported in this paper were two-fold. Firstly, to assess, by means of the numerical simulation of transverse wave propagation within a linear viscoelastic medium, the degree to which wavegroup propagation involving a finite frequency interval (2Δ ω) corresponds to an ‘ideal’ beat (i.e. one for which 2Δ ω → 0). Secondly, to compare the results of shear wave propagation measurements with predictions based on a rheometrical theory, which describes the gelation of materials in terms of their wave dispersion characteristics. The results of the numerical simulations indicate that for a sufficiently small value of the frequency interval Δ ω/ ω, the nodal and anti-nodal regions of the amplitude modulation envelope of wavegroups propagating within a critical-gel may be defined with sufficient accuracy to permit calculation of the corresponding value of the group velocity U. The results of high frequency shear wave dispersion measurements made on a system known to display critical-gel behaviour provide encouraging agreement with theory and the results of the simulations.

Retiming dynamics of modelocked semiconductor laser

The retiming dynamics of ultrashort pulses inside a modelocked semiconductor laser were determined experimentally and compared to theory. The characteristic time constant governing the rate at which mistimed pulses recovered to their equilibrium positions after a temporal displacement of the amplitude modulation envelope was 25 ns. The phase noise power spectrum of the modelocked semiconductor laser was shown to be consistent with the retiming time constant.

Discrimination of amplitude-modulated synthetic echo trains by an echolocating bottlenose dolphin.

Bottlenose dolphins (Tursiops truncatus) have an acute ability to use target echoes to judge attributes such as size, shape, and material composition. Most target recognition studies have focused on features associated with individual echoes as opposed to information conveyed across echo sequences (feature envelope of the multi-echo train). One feature of aspect-dependent targets is an amplitude modulation (AM) across the return echoes in the echo train created by relative movement of the target and dolphin. The current study examined whether dolphins could discriminate targets with different AM envelopes. "Electronic echoes" triggered by a dolphin's outgoing echolocation clicks were manipulated to create sinusoidal envelopes with varying AM rate and depth. Echo trains were equated for energy, requiring the dolphin to extract and retain information from multiple echoes in order to detect and report the presence of AM. The dolphin discriminated amplitude-modulated echo trains from those that were not modulated. AM depth thresholds were approximately 0.8 dB, similar to other published amplitude limens. Decreasing the rate of modulation from approximately 16 to 2 cycles per second did not affect the dolphin's AM depth sensitivity. The results support multiple-echo processing in bottlenose dolphin echolocation. This capability provides additional theoretical justification for exploring synthetic aperture sonar concepts in models of animal echolocation that potentially support theories postulating formation of images as an ultimate means for target identification.

Basic response determinants of single neurons to amplitude modulation in the auditory midbrain.

Amplitude-modulated (AM) signals represent important components of environmental sounds. While single-cell responses to AM tones in the central auditory system were often studied using repetitive modulation, owing to its presence in vocalization signals, the AM response has not been fully depicted in terms of receptive field in the stimulus domain. This study was aimed to characterize the receptive field of AM response with respect to nonrepetitive AM stimuli and to understand how complex acoustic signals may be coded in the brain. A novel AM stimulus was implemented with a random envelope and a systemic change in intensity across trials. From 393 single units recorded in the inferior colliculus (IC) of urethane-anesthetized rats, responses to the AM stimulus were first characterized in terms of dot-raster pattern. Three types of response were identified: type I showing a monotonic response to mainly the steady states of the AM envelope and type II to rising phases of the AM envelope with a clear intensity preference. Type III showed a mixed response of both type I and type II. A small number of units, called type IV, responded to both rising and falling phases of the modulation. Using perispike averaging, the AM receptive field, or "level temporal receptive field" (LTRF), was displayed in a "stimulus level versus perispike time" plane. The LTRF, particularly of the type II response, clearly revealed triggering features of the cell. The triggering features are consistent with the representation of the cell's response in a receptive space formed by the Cartesian axes of the velocity of amplitude modulation, the intensity of the sound, and the range of modulation. We therefore considered these stimulus parameters as the three basic determinants of the AM response in the auditory midbrain.

An alpha modulation index for electroencephalographic studies using complex demodulation.

An automated technique for measuring the relative amount of amplitude modulation of electroencephalographic (EEG) alpha activity is developed to increase the number of existing tools for differentiating the various types of alpha activity. EEG data collected from 12 normal males is used to characterize alpha modulation frequency characteristics. From these findings, a complex demodulation method is constructed to extract the amplitude modulation envelope of alpha activity from an epoch of EEG data while disregarding both continuous amplitude alpha activity and activity outside the alpha band. A threshold technique is then used to determine the relative amount of modulation contained within the data epoch. This metric is termed the alpha modulation index (AMI). Good correlation (R2 = 0.86) is found when automated scoring results are compared with manual scoring of physiologic EEG alpha modulation. The flexibility of this technique makes it easily adaptable to other EEG frequency bands and applications.

Auditory responsive cortex in the squirrel monkey: neural responses to amplitude-modulated sounds.

The neural response to amplitude-modulated sinus sounds (AM sound) was investigated in the auditory cortex and insula of the awake squirrel monkey. It was found that 78.1% of all acoustically driven neurons encoded the envelope of the AM sound; the remaining 21.9% displayed simple On, On/Off or Off responses at the beginning or the end of the stimulus sound. Those neurons with AM coding were able to encode the AM sound frequency in two different ways: (1) the spikes followed the amplitude modulation envelopes in a phase locked manner; (2) the spike rate changed significantly with changing modulation frequencies. As reported in other species, the modulation transfer functions for rate showed higher modulation frequencies than the phase-locked response. Both AM codings exhibited a filter characteristic for AM sound. Whereas 46.6% of all neurons had the same filter characteristic for both the spike discharge and the phase-locked response, the remaining neurons displayed combinations of different filter types. The discharge pattern of a neuron to simple tone or noise bursts suggests the behaviour of this neuron when AM sound is used as the stimulus. Neurons with strong onset responses to tone/noise bursts tended to have higher phase-locked AM responses than neurons with weak onset responses. The spike rate maxima for AM sound showed no relation to the tone/noise burst discharge patterns. Varying modulation depth was encoded by the neuron's ability to follow the envelope cycles and not by the non-phase-locked spike rate frequency. The organization of the squirrel monkey's auditory cortex has previously been established by an anatomical study. We have added two new fields using physiological parameters. All fields investigated showed a clear functional separation for time-critical information processing. The best temporal resolution was shown by the primary auditory field (AI), the first-temporal field (T1) and the parainsular auditory field (Pi). The neural data in these fields and the amplitude modulation frequency range of squirrel monkey calls suggest a similar correlation between vocalization and perception as in human psychophysical data for speech and hearing sensation. The anterior fields in particular failed to follow the AM envelopes. For the first time in a primate, the insula was tested with different sound parameters ranging from simple tone bursts to AM sound. It is suggested that this cortical region plays a role in time-critical aspects of acoustic information processing. The observed best frequencies covered the same spectrum as AI. As in the auditory fields, most neurons in the insula encoded AM sound with different filter types. The high proportion of neurons unable to encode AM sound (40.6%) and the low mean best modulation frequency (9.9 Hz) do not support a prominent role of the insula in temporal information processing.