			Home About us Contact

Amplitude Modulation (amplitude + modulation)

Distribution by Scientific Domains

Engineering	30%
Psychology	23%
Life Sciences	23%

Kinds of Amplitude Modulation

quadrature amplitude modulation

Selected Abstracts

AM,FM techniques in the analysis of optical coherence tomography signals

JOURNAL OF BIOPHOTONICS, Issue 6-7 2009
Costas Pitris
Abstract The subtle tissue changes associated with the early stages of malignancies, such as cancer, are not clearly discernible even at the current, improved, resolution of optical coherence tomography (OCT) systems. However, these changes directly affect the spectral content of the OCT image that contains information regarding these unresolvable features. Spectral analysis of OCT signals has recently been shown to provide additional information, resulting in improved contrast, directly related to scatterer size changes. Amplitude modulation,frequency modulation (AM,FM) analysis, a fast and accurate technique for the estimation of the instantaneous frequency, phase, and amplitude of a signal, can also be applied to OCT images to extract scatterer-size information. The proposed technique could make available an extremely valuable tool for the investigation of disease characteristics that now remain below the resolution of OCT and could significantly improve the technology's diagnostic capabilities. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Prolonged maturation of auditory perception and learning in gerbils

DEVELOPMENTAL NEUROBIOLOGY, Issue 9 2010
Emma C. Sarro
Abstract In humans, auditory perception reaches maturity over a broad age range, extending through adolescence. Despite this slow maturation, children are considered to be outstanding learners, suggesting that immature perceptual skills might actually be advantageous to improvement on an acoustic task as a result of training (perceptual learning). Previous non-human studies have not employed an identical task when comparing perceptual performance of young and mature subjects, making it difficult to assess learning. Here, we used an identical procedure on juvenile and adult gerbils to examine the perception of amplitude modulation (AM), a stimulus feature that is an important component of most natural sounds. On average, Adult animals could detect smaller fluctuations in amplitude (i.e., smaller modulation depths) than Juveniles, indicating immature perceptual skills in Juveniles. However, the population variance was much greater for Juveniles, a few animals displaying adult-like AM detection. To determine whether immature perceptual skills facilitated learning, we compared naïve performance on the AM detection task with the amount of improvement following additional training. The amount of improvement in Adults correlated with naïve performance: those with the poorest naïve performance improved the most. In contrast, the naïve performance of Juveniles did not predict the amount of learning. Those Juveniles with immature AM detection thresholds did not display greater learning than Adults. Furthermore, for several of the Juveniles with adult-like thresholds, AM detection deteriorated with repeated testing. Thus, immature perceptual skills in young animals were not associated with greater learning. © 2010 Wiley Periodicals, Inc. Develop Neurobiol 70: 636,648, 2010 [source]

The magnocellular theory of developmental dyslexia

DYSLEXIA, Issue 1 2001
John Stein
Abstract Low literacy is termed ,developmental dyslexia' when reading is significantly behind that expected from the intelligence quotient (IQ) in the presence of other symptoms,incoordination, left,right confusions, poor sequencing,that characterize it as a neurological syndrome. 5,10% of children, particularly boys, are found to be dyslexic. Reading requires the acquisition of good orthographic skills for recognising the visual form of words which allows one to access their meaning directly. It also requires the development of good phonological skills for sounding out unfamiliar words using knowledge of letter sound conversion rules. In the dyslexic brain, temporoparietal language areas on the two sides are symmetrical without the normal left-sided advantage. Also brain ,warts' (ectopias) are found, particularly clustered round the left temporoparietal language areas. The visual magnocellular system is responsible for timing visual events when reading. It therefore signals any visual motion that occurs if unintended movements lead to images moving off the fovea (,retinal slip'). These signals are then used to bring the eyes back on target. Thus, sensitivity to visual motion seems to help determine how well orthographic skill can develop in both good and bad readers. In dyslexics, the development of the visual magnocellular system is impaired: development of the magnocellular layers of the dyslexic lateral geniculate nucleus (LGN) is abnormal; their motion sensitivity is reduced; many dyslexics show unsteady binocular fixation; hence poor visual localization, particularly on the left side (left neglect). Dyslexics' binocular instability and visual perceptual instability, therefore, can cause the letters they are trying to read to appear to move around and cross over each other. Hence, blanking one eye (monocular occlusion) can improve reading. Thus, good magnocellular function is essential for high motion sensitivity and stable binocular fixation, hence proper development of orthographic skills. Many dyslexics also have auditory/phonological problems. Distinguishing letter sounds depends on picking up the changes in sound frequency and amplitude that characterize them. Thus, high frequency (FM) and amplitude modulation (AM) sensitivity helps the development of good phonological skill, and low sensitivity impedes the acquisition of these skills. Thus dyslexics' sensitivity to FM and AM is significantly lower than that of good readers and this explains their problems with phonology. The cerebellum is the head ganglion of magnocellular systems; it contributes to binocular fixation and to inner speech for sounding out words, and it is clearly defective in dyslexics. Thus, there is evidence that most reading problems have a fundamental sensorimotor cause. But why do magnocellular systems fail to develop properly? There is a clear genetic basis for impaired development of magnocells throughout the brain. The best understood linkage is to the region of the Major Histocompatibility Complex (MHC) Class 1 on the short arm of chromosome 6 which helps to control the production of antibodies. The development of magnocells may be impaired by autoantibodies affecting the developing brain. Magnocells also need high amounts of polyunsaturated fatty acids to preserve the membrane flexibility that permits the rapid conformational changes of channel proteins which underlie their transient sensitivity. But the genes that underlie magnocellular weakness would not be so common unless there were compensating advantages to dyslexia. In developmental dyslexics there may be heightened development of parvocellular systems that underlie their holistic, artistic, ,seeing the whole picture' and entrepreneurial talents. Copyright © 2001 John Wiley & Sons, Ltd. [source]

On the effects of interpolator window shape and symbol location on the BER of diversity PSAM 16-QAM

EUROPEAN TRANSACTIONS ON TELECOMMUNICATIONS, Issue 4 2007
Lingzhi Cao
We consider pilot symbol assisted modulation (PSAM) implemented with a sinc interpolator. The sensitivity of the error rate performance to different window functions applied to the sinc interpolator is investigated. The bit error rates (BERs) for diversity M -ary quadrature amplitude modulation (MQAM) with different window functions applied to the sinc interpolator are compared. The Kaiser window is found superior to the other windows considered. A thorough discussion of the dependency of the BER performance on the symbol location is also given. Copyright © 2007 John Wiley & Sons, Ltd. [source]

Angle differential-QAM scheme for resolving phase ambiguity in continuous transmission system

INTERNATIONAL JOURNAL OF COMMUNICATION SYSTEMS, Issue 6 2008
Jeng-Kuang Hwang
Abstract An angle differential quadrature amplitude modulation (ADQAM) scheme is proposed to solve phase ambiguity problem in non-data-aided continuous transmission system with square QAM constellation. Starting from the 16-ADQAM case, we derive differential encoding and decoding schemes in terms of two differential angles and use a solar system analogy for explanation. The 16-ADQAM system incurs only about 0.5-dB performance degradation compared with the coherent 16-QAM system under AWGN channel. Generalization of flat fading channel and higher-level ADQAM is straightforward. Copyright © 2007 John Wiley & Sons, Ltd. [source]

A simple approach for phase-modulated single-scan 2D NMR spectroscopy

MAGNETIC RESONANCE IN CHEMISTRY, Issue 10 2005
Nikolas Salisbury Andersen
Abstract Conventional NMR spectroscopy techniques require long acquisition times due to the recovery time between the repeated excitations necessary for each increment of the evolution times in the indirectly detected dimensions. Here we outline a pulse sequence element for gradient-assisted ultrafast multidimensional NMR spectroscopy using frequency-modulated ,chirp' pulses to generate phase-modulated magnetization in an indirectly detected spectral dimension. The potential of this sequence element is demonstrated by acquiring a correlation spectroscopy (COSY) spectrum in 96 ms. This new pulse sequence element is an extension of ultrafast spectroscopy techniques based on the generation of amplitude modulation of the NMR signal in the indirectly detected spectral dimensions. The use of phase modulation instead of amplitude modulation helps broaden the applicability and may provide an increase of sensitivity in some experiments due to the ability to distinguish between positive and negative frequency offsets relative to the carrier frequency of the sequence element. Copyright © 2005 John Wiley & Sons, Ltd. [source]

Use of Cell-Specific PAM-Fluorometry to Characterize Host Shading in the Epiphytic Dinoflagellate Gambierdiscus toxicus

MARINE ECOLOGY, Issue 2 2002
Tracy A. Villareal
Abstract. Cell-specific fluorescence characteristics were used to characterize the light tolerance of the toxic benthic dinoflagellate Gambierdiscus toxicus. The fluorescence parameter Fv : Fm was measured using pulse amplitude modulation (PAM) fluorometry on individual cells collected from foliose red algae growing in the sub-tidal margin of South Water Cay, Belize. Samples were collected over several days during sunny and cloudy conditions and compared to samples incubated in situ. The data from individual cells were used to generate both Fv : Fm frequency histograms and averages. Maximum individual cell values of Fv : Fm reached 0.81 in pre-dawn samples, a value near the theoretical maximum for PAM fluorometry. In field samples from macroalgal hosts, average Fv : Fm values declined only slightly during the day, but cells incubated in bottles under 47 % incident sunlight showed a significant mid-day depression. In freshly collected samples, near-maximum Fv : Fm values could be found in individual cells during the entire day; however, the frequency histograms indicated a greater range in Fv : Fm values during the afternoon than in the morning. In contrast, cultures of G. toxicus showed a tight distribution around a mean. Field samples showed a rapid recovery to near-maximum Fv : Fm within 2 min when assayed using a standardized actinic light series. Similar results were obtained in laboratory cultures of G. toxicus grown at 73 µmol photons · m -2 · s -1, but not at 383 µmol photons · m -2 · s -1. These data provide empirical support for suggestions that G. toxicus exploits the three-dimensional structure of the algal host thallus to minimize light exposure. This strategy permits G. toxicus, a high-light intolerant species in culture, to thrive in shallow, well-lit tropical seas. It may also partially explain the observed preference of G. toxicus for complex, foliose macroalgae as hosts. [source]

Performance evaluations of microwave power amplifiers with single- and multi-carrier quadrature amplitude modulations

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Issue 12 2006
Chien-Chang Huang
Abstract This article presents the performance evaluations of microwave power amplifiers (PAs) for different kinds of quadrature amplitude modulation (QAM) schemes including ,/4-DQPSK and 16-QAM for single-carrier formats and orthogonal frequency division multiplexing (OFDM) for multi-carrier configuration, by using the time-domain signal processing technique and the hybrid Volterra analysis. The influences due to various peak-to-average power ratios (PAPRs) of the modulation signals for the PA performances such as output powers, adjacent channel power ratios (ACPRs), and error vector magnitudes (EVMs) are shown with experimental verifications. A figure-of-merit of PAs based on linearity and efficiency is presented to establish a criterion for choosing the best working condition. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 2375,2378, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22020 [source]

Electrocortical and electrodermal responses covary as a function of emotional arousal: A single-trial analysis

PSYCHOPHYSIOLOGY, Issue 4 2008
Andreas Keil
Abstract Electrophysiological studies of human visual perception typically involve averaging across trials distributed over time during an experimental session. Using an oscillatory presentation, in which affective or neutral pictures were presented for 6 s, flickering on and off at a rate of 10 Hz, the present study examined single trials of steady-state visual evoked potentials. Moving window averaging and subsequent Fourier analysis at the stimulation frequency yielded spectral amplitude measures of electrocortical activity. Cronbach's alpha reached values >.79, across electrodes. Single-trial electrocortical activation was significantly related to the size of the skin conductance response recorded during affective picture viewing. These results suggest that individual trials of steady-state potentials may yield reliable indices of electrocortical activity in visual cortex and that amplitude modulation of these indices varies with emotional engagement. [source]

Toward a chronopsychophysiology of mental rotation

PSYCHOPHYSIOLOGY, Issue 4 2002
Martin Heil
In a parity judgment task, the ERPs at parietal electrode sites become the more negative the more mental rotation has to be executed. In two experiments, it was investigated whether a temporal relationship exists between the onset of this amplitude modulation and the moment when mental rotation is executed. Therefore, the duration of processing stages located before mental rotation was manipulated. The amplitude modulation was delayed when either the perceptual quality of the stimulus was reduced (Experiment 1) or when character discrimination was more difficult (Experiment 2). The results suggest that the onset of the rotation-related negativity might be used as a chronopsychophysiological marker for the onset of the cognitive process of mental rotation. [source]

Light-curve inversions with truncated least-squares principal components: Tests and application to HD 291095 = V1355 Orionis

ASTRONOMISCHE NACHRICHTEN, Issue 4 2008
I.S. Savanov
Abstract We present a new inversion code that reconstructs the stellar surface spot configuration from the light curve of a rotating star. Our code employs a method that uses the truncated least-squares estimation of the inverse problem's objects principal components. We use spot filling factors as the unknown objects. Various test cases that represent a rapidly-rotating K subgiant are used for the forward problem. Tests are then performed to recover the artificial input map and include data errors and input-parameter errors. We demonstrate the robustness of the solution to false input parameters like photospheric temperature, spot temperature, gravity, inclination, unspotted brightness and different spot distributions and we also demonstrate the insensitivity of the solution to spot latitude. Tests with spots peppered over the entire stellar surface or with phase gaps do not produce fake active longitudes. The code is then applied to ten years of V and I -band light curve data of the spotted sub-giant HD291095. A total of 22 light curves is presented. We find that for most of the time its spots were grouped around two active longitudes separated on average by 180°. Switches of the dominant active region between these two longitudes likely occurred about every 3.15±0.23 years while the amplitude modulation of the brightness occurred with a possible period of 3.0±0.15 years. For the first time, we found evidence that the times of the activity flips coincide with times of minimum light as well as minimum photometric amplitude, i.e. maximum spottedness. From a comparison with simultaneous Doppler images we conclude that the activity flips likely take place near the rotational pole of the star. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Cytogenetic damage in human lymphocytes following GMSK phase modulated microwave exposure

BIOELECTROMAGNETICS, Issue 1 2002
Guglielmo d'Ambrosio
Abstract The present study investigated, using in vitro experiments on human lymphocytes, whether exposure to a microwave frequency used for mobile communication, either unmodulated or in presence of phase only modulation, can cause modification of cell proliferation kinetics and/or genotoxic effects, by evaluating the cytokinesis block proliferation index and the micronucleus frequency. In the GSM 1800 mobile communication systems the field is both phase (Gaussian minimum shift keying, GMSK) and amplitude (time domain multiple access, TDMA) modulated. The present study investigated only the effects of phase modulation, and no amplitude modulation was applied. Human peripheral blood cultures were exposed to 1.748 GHz, either continuous wave (CW) or phase only modulated wave (GMSK), for 15 min. The maximum specific absorption rate (,5 W/kg) was higher than that occurring in the head of mobile phone users; however, no changes were found in cell proliferation kinetics after exposure to either CW or GMSK fields. As far as genotoxicity is concerned, the micronucleus frequency result was not affected by CW exposure; however, a statistically significant micronucleus effect was found following exposure to phase modulated field. These results would suggest a genotoxic power of the phase modulation per se. Bioelectromagnetics 23:7,13, 2002. © 2002 Wiley-Liss, Inc. [source]