Evoked-potential recovery during double click stimulation in a whale: a possibility of biosonar automatic gain control

False killer whale Pseudorca crassidens auditory brainstem responses (ABR) were recorded using a double-click stimulation paradigm specifically measuring the recovery of the second response (to the test click) as a function of the inter-click interval (ICI) at various levels of the conditioning and test click. At all click intensities, the slopes of recovery functions were almost constant: 0.6-0.8 microV per ICI decade. Therefore, even when the conditioning-to-test-click level ratio was kept constant, the duration of recovery was intensity-dependent: The higher intensity the longer the recovery. The conditioning-to-test-click level ratio strongly influenced the recovery time: The higher the ratio, the longer the recovery. The dependence was almost linear using a logarithmic ICI scale with a rate of 25-30 dB per ICI decade. These data were used for modeling the interaction between the emitted click and the echo during echolocation, assuming that the two clicks simulated the transmitted and echo clicks. This simulation showed that partial masking of the echo by the preceding emitted click may explain the independence of echo-response amplitude of target distance. However, the distance range where this mechanism is effective depends on the emitted click level: The higher the level, the greater the range. @ 2007 Acoustical Society of America.     

Models of auditory masking: a molecular psychophysical approach

Gilkey et al. [J. Acoust. Soc. Am. 78, 1207-1219 (1985)] measured hit proportions and false alarm proportions for detecting a 500-Hz tone at each of four starting phase angles in each of 25 reproducible noise samples. In the present paper, their results are modeled by fitting the general form of the electrical analog model of Jeffress [J. Acoust. Soc. Am. 48, 480-488 (1967)] to the diotic data. The best-fitting configurations of this model do not correspond to energy detectors or to envelope detectors. A detector composed of a 50-Hz-wide single-tuned filter, followed by a half-wave rectifier and an integrator with an integration time of 100 to 200 ms fits the data of all four subjects relatively well. Linear combinations of the outputs of several detectors that differ in center frequency or integration window provide even better fits to the data. These linear combinations assign negative weights to some frequencies or to some time intervals, suggesting that a subject's decision is based on a comparison of information in different spectral or temporal portions of the stimulus.     

An experimental study of technical fluctuations in a quartz crystal oscillator with automatic gain control

The contribution of technical fluctuations of individual circuit components of a quartz crystal oscillator with AGC to the technical characteristics of the output signal is determined. The noise of the elements studied is simulated. It is demonstrated that the AGC system significantly improves the amplitude-frequency characteristics of the oscillator output signal. Requirements are determined for the noise characteristics of individual oscillator stage components for the purpose of obtaining high short-term frequency stability.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 21, No. 12, pp. 1778–1784, December, 1978.     

Natural fluctuations in a quartz crystal oscillator with automatic gain control

A method is developed for calculation of natural fluctuations in oscillators with automatic gain control (AGC). As an example, a three-terminal capacitive transistor crystal oscillator is considered. It is shown that the AGC system permits stabilization of the amplitude and frequency of the oscillator output signal.     

Physiological mechanisms of psychophysical masking: observations from auditory-nerve fibers

Masking might be due either to the spread of the excitation produced by the masker to the place of the tone signal along the cochlea or to the suppression of the response to the signal by the masker. In order to identify the contributions of these two mechanisms to tone-on-tone masking, masked thresholds of auditory-nerve fibers were measured in anesthetized cats using the same stimulus paradigms and detection criteria as in psychophysics. Suppressive masking was identified by comparing thresholds for simultaneous masking with those for a nonsimultaneous masking technique resembling pulsation thresholds. These nonsimultaneous thresholds do not include the contribution of suppression to masking because suppression only occurs for stimuli that overlap in time. For each masker and signal frequency, the fibers with the lowest (or "best") masked thresholds had characteristic frequencies (CF) slightly on the opposite side of the masker frequency with respect to the signal frequency, consistent with the psychophysical phenomenon of off-frequency listening. Patterns of best masked thresholds against signal frequency resembled psychophysical masking patterns in that they showed a maximum for signal frequencies close to the masker, and a skew toward high frequencies. Masking was found to be both excitatory and suppressive, with the relative contribution of the two mechanisms depending on the frequency separation between signal and masker. Suppressive masking was large for signal frequencies well above the masker. For these conditions, simultaneous thresholds grew more rapidly with masker level than did nonsimultaneous thresholds, suggesting that the upward spread of masking is largely due to the growth of suppression rather than to that of excitation.     

Vibrotactile forward masking: psychophysical evidence for a triplex theory of cutaneous mechanoreception

Threshold shifts for the detection of vibrotactile test stimuli were determined as a function of the intensity of a masker. A 50-ms sinusoidal test stimulus was applied to the thenar eminence of the hand 25 ms after the termination of a 700-ms sinusoidal masker applied to the same site. The frequency of the test stimulus and the frequency of the masker were varied. To eliminate the influence of the Pacinian receptor system, stimuli were delivered through a 0.01-cm2 contactor. The results support the hypothesis that the detection of vibration delivered through a small contactor is determined by two separate populations of non-Pacinian receptors. The study constitutes a psychophysical demonstration of the existence of three receptor systems responsible for the detection of vibration.     

Technical fluctuations in a quartz crystal oscillator with automatic gain control

Technical fluctuations in quartz crystal oscillators with automatic gain control (AGC) are studied. Using the example of a three-terminal capacitive transistor circuit, the contribution of technical fluctuations of individual components to the spectral characteristics of the output signal is determined. It is shown that an AGC system can significantly improve the oscillator technical characteristics.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 21, No. 12, pp. 1772–1777, December, 1978.The authors express their gratitude to A. V. Yakimov and A. N. Malakhov for their helpful evluation of the study.     

Forward masking of amplitude modulation: basic characteristics

In this study we demonstrate an effect for amplitude modulation (AM) that is analogous to forward making of audio frequencies, i.e., the modulation threshold for detection of AM (signal) is raised by preceding AM (masker). In the study we focused on the basic characteristics of the forward-masking effect. Functions representing recovery from AM forward masking measured with a 150- ms 40- Hz masker AM and a 50- ms signal AM of the same rate imposed on the same broadband-noise carrier, showed an exponential decay of forward masking with increasing delay from masker offset. Thresholds remained elevated by more than 2 dB over an interval of at least 150 ms following the masker. Masked-threshold patterns, measured with a fixed signal rate (20, 40, and 80 Hz) and a variable masker rate, showed tuning of the AM forward-masking effect. The tuning was approximately constant across signal modulation rates used and consistent with the idea of modulation-rate selective channels. Combining two equally effective forward maskers of different frequencies did not lead to an increase in forward masking relative to that produced by either component alone. Overall, the results are consistent with modulation-rate selective neural channels that adapt and recover from the adaptation relatively quickly.     

Forward masking as a method of measuring place specificity of neural excitation in cochlear implants: a review of methods and interpretation

This paper reviews the psychophysical forward masking methods that have been used to investigate place specificity in cochlear implantees. These experiments are relevant for investigating whether the individual variability in outcomes for people using the same device can be explained by individual variations in frequency resolution or whether place specificity is affected by different modes of stimulation (such as bipolar, monopolar or tripolar) in the same person. Unfortunately, there has been no consensus about the methods used to derive electrical forward masking functions, or in the way that they are interpreted in relation to place specificity. Here, the different methods are critically examined to provide insight into the optimal methods that should be used to measure and interpret spatial forward masking functions in electric hearing. It is shown that, in order to separate the temporal effects of masking decay from the place-specificity information, different analyses of the functions are needed depending on whether a fixed-probe or fixed-masker method is employed. The effects of unit of measurement on specificity measures and the effects of subject listening strategy on the forward masked functions are also discussed.     

Auditory brainstem response in a harbor porpoise show lack of automatic gain control for simulated echoes

The auditory brainstem response (ABR) response to simulated echolocation clicks was studied in a harbor porpoise, Phocoena phocoena, to determine the relationship between the animal's perceived echo strength and the simulated target distance. In one experiment the click level at the listening post was kept constant while delay was changed, in another, the level was varied to approximate spreading losses. Results of both experiments indicate that there is no automatic gain control in the hearing system of this harbor porpoise.     

激光对红外热成像设备自动增益控制电路的干扰效果

 为了研究激光对红外热成像设备自动增益控制（AGC）电路的干扰效果,在分析红外热成像设备AGC电路工作原理基础上,探论了激光对红外热成像设备AGC电路的干扰机理,开展了激光对某型红外热成像设备AGC电路的干扰实验,并提出了区域灰度变化测试AGC增益的分析方法。对实验结果进行了定量研究,结果表明：激光对红成像设备AGC电路持续有效干扰应满足干扰激光脉冲周期小于自动增益调整时间的条件。     

Noseleaf furrows in a horseshoe bat act as resonance cavities shaping the biosonar beam

Horseshoe bats emit their ultrasonic biosonar pulses through nostrils surrounded by intricately shaped protuberances (noseleaves). While these noseleaves have been hypothesized to affect the sonar beam, their physical function has never been analyzed. Using numerical methods, we show that conspicuous furrows in the noseleaf act as resonance cavities shaping the sonar beam. This demonstrates that (a) animals can use resonances in external, half-open cavities to direct sound emissions, (b) structural detail in the faces of bats can have acoustic effects even if it is not adjacent to the emission sites, and (c) specializations in the biosonar system of horseshoe bats allow for differential processing of subbands of the pulse in the acoustic domain.     

A study of the gain mechanism in free electron laser systems

A detailed study of the gain mechanism in low-gain free electron laser systems is presented. The analysis is based on a recently reported analytic solution to the quasi-Bloch equations, which are assumed to govern the evolution of the system.     

Novel automatic control for the optimum optical gain and phase differences in SOA-MZI wavelength converter: Theory and experiment

We have proposed, simulated, and experimentally verified the novel automatic control method and apparatus to automatically adjust and constantly maintain the optimum optical gain and phase differences in order to achieve the automatically optimized semiconductor optical amplifier (SOA)-Mach-Zehnder interferometer (MZI) wavelength converter (WC) with wide input power dynamic range (IPDR) and maximum extinction ratio (ER). Our automatic control algorithm is proposed through the results of simulation and its validity is confirmed through the experiment.     

Examination of a linear model in an informational masking study

When multitone maskers are used in a two-interval, forced choice experiment, the amount of masking is larger when the masker is randomly chosen on each presentation interval compared to on each trial (the same masker in the two listening intervals). These conditions are referred to as having within- versus between-trial randomization. If it is assumed that an observer's ultimate detection decision depends on a single decision variable (DV), it is probable that the DV's variance will be substantially larger in the within-trial randomization condition compared to the between-trial randomization condition. The goal of the current experiment is to evaluate the degree to which this stimulus-based change in DV variance can account for the difference in thresholds in the within-versus between-trial randomization conditions. Thresholds are measured for the detection of a tone added to a six-component masker in between- and within-trial randomization conditions. The slopes of the psychometric functions provide an estimate of the variance in the DV for the between- and within-trial randomization conditions. Additionally, a channel model is fitted to the psychophysical results in the within-trial randomization condition. The resulting model is then used to predict the value of the DV for each trial, and ultimately to estimate the proportion of the total variance in the within-trial randomization condition that is attributable to changes in maskers across intervals. The variance of the DV in the between-trial randomization condition accounted for approximately 65% of the total variance in the DV in the within-trial randomization condition. Stimulus-based interval-by-interval masker randomization accounted for approximately 20% of the total variance of the within-trial randomization DV. The remaining 15% of the DV variance in the within-trial randomization condition remained unaccounted for. This result is fairly stable whether the maskers are drawn from a small versus large pool of potential maskers.     

Vibrotactile forward masking as a function of age.

Thresholds for the detection of a 50-ms test stimulus delivered to the thenar eminence were measured as a function of the time interval between the offset of a 500-ms masking stimulus and the onset of the test stimulus (delta t). The frequency of the masker and the test stimulus was the same during a particular testing session and was either 25 or 250 Hz. At all values of delta t, older subjects exhibited significantly more masking than did young subjects. The effects of age were greater for stimuli that primarily affect the Pacinian system (250 Hz) than those that primarily affect non-Pacinian systems (25 Hz). Psychophysical measurements of the apparent duration of tactile sensations suggest that both sensory persistence and adaptation are affected by aging. Since adaptation seemed to be the more dominant factor for stimuli with durations as long as 500 ms, it was concluded that the effects of aging on forward masking seen in our study were due mainly to increased amounts of adaptation produced by the masker.     

Hearing-aid automatic gain control adapting to two sound sources in the environment, using three time constants

A hearing aid AGC algorithm is presented that uses a richer representation of the sound environment than previous algorithms. The proposed algorithm is designed to (1) adapt slowly (in approximately 10 s) between different listening environments, e.g., when the user leaves a single talker lecture for a multi-babble coffee-break; (2) switch rapidly (about 100 ms) between different dominant sound sources within one listening situation, such as the change from the user's own voice to a distant speaker's voice in a quiet conference room; (3) instantly reduce gain for strong transient sounds and then quickly return to the previous gain setting; and (4) not change the gain in silent pauses but instead keep the gain setting of the previous sound source. An acoustic evaluation showed that the algorithm worked as intended. The algorithm was evaluated together with a reference algorithm in a pilot field test. When evaluated by nine users in a set of speech recognition tests, the algorithm showed similar results to the reference algorithm.