Function Of Signal Frequency Research Articles

Time-Intensity Trading Relations as a Function of Signal Frequency

Levels of monaural signals at behavioral threshold were determined by a psychophysical method of adjustment for seven highly trained listeners. Thresholds were studied as a function of signal frequency (octave steps) from 125 to 8 kHz, and of duration, from 16 to 1024 msec. Measurements were made in the presence of a contralateral broad-band masking noise with a spectrum level of 30 dB SPL. The time constant τ was estimated by means of Plomp and Bouman's Hypothesis III [J. Acoust. Soc. Amer. 31, 749–758 (1959)]. Average values of τ, from at least 12 replications of each measurement, are found to range systematically from those considered “normal” (125–175 msec) by some earlier investigators, at low frequencies, 125–500 Hz, to much lower values (30–70 msec) at the higher frequencies. Preliminary comparisons between values of τ for normal listeners and for listeners with sensory-neural loss show this interaction between frequency and the time constant to be similar for both populations. The data are also compared to the results of a control procedure which employed a two-alternative forced-choice method. [Work supported by the National Institutes of Health, Public Health Service, U. S. Department of Health, Education, and Welfare.]

Frequency-response characteristic of auditory observers detecting signals of a single frequency in noise: the probe-signal method.

Four experiments were conducted to develop and test a method of determining the frequency-response characteristic of the observer when he listens for single-frequency signals presented against a continuous background of wide-band noise. After observers were trained to detect primary signals of a single frequency, probe signals of various other frequencies were presented infrequently, in lieu of the primary signal. Primary signals and all probe signals were presented with very similar amplitudes that would be expected to render them all equally detectable if presented alone in single-frequency experiments. Estimates of the detectability of the signals of the various frequencies were obtained concurrently in a two-alternative forced-choice procedure. The results from 14 observers were quite similar and show differences in detection as a function of signal frequency when the primary signal was of 1000 or of 1100 Hz. In general, the primary signal was correctly detected 75%–90% of the time while signals with frequencies at approximately 150 to 200 Hz on either side of the primary-signal frequency were detected at the chance level, 50% correct. In as few as three experimental sessions, the observer's frequency-response characteristic was obtained using the probe-signal method.

Calculating delta modulator performance

The various parameters which affect the performance of a single integration exponential delta modulator are discussed, viz., line rate, signal channel bandwidth and the time constant of the RC combination ("integrator") used in the feedback path of the coder (and in the forward path of the decoder). The performance of the delta modulator is described quantitatively in terms of its overload characteristic (level at which harmonic distortion occurs versus signal frequency), its coding threshold, and the level of quantizing noise. A novel form of diagram is constructed, showing these performance boundaries in a manner which enables the dynamic range of the coder as a function of signal frequency to be readily appreciated. At the same time, values of signal to quantizing noise ratio are easily ascertainable. The shape of the overload characteristic is discussed, and attention is drawn to the importance of relating this to the long-term spectrum of the input signal. A formula is derived for the effect of random errors occurring in the digital transmission path, and it is shown how this may be minimized. The delta-sigma modulator is treated in a similar way.

Observing behavior in the squirrel monkey in a situation analogous to human monitoring.

Observing and detection behavior were examined in four squirrel monkeys under schedule conditions similar to those employed in human monitoring experiments. Observing responses produced either a visual signal ( Sd) indicating availability of food reinforcement for an instrumental (detection) response or an SΔ indicating non-availability of reinforcement. Signals were terminated by a detection response and followed by reinforcement. SΔ exposures in the absence of signals were 0.5 sec. in duration. Detection responses in SΔ had no consequence. Signal availabilities were programmed according to random interval schedules. Mean intersignal availability times of 1, 2, 4, and 8 min. corresponding to average signal rates of 60, 30, 15, and 7.5 per hour were examined in that order. All schedules used generated a constant high rate of observing responses with short detection latencies in the presence of signals and very few detection responses in the absence of signals. No decrease in observing rate or increase in detection time was obtained within 2-hr. sessions. Neither observing rate nor detection latency varied systematically as a function of signal frequency. Distributions of observing inter-response times in the absence of stimuli were random except in one animal. Results are discussed in relation to data from other experiments on operant behavior.

Lateralization of an Auditory Signal in Correlated Noise and in Uncorrelated Noise as a Function of Signal Frequency

Listeners lateralized a monaural signal presented against a continuous background of perfectly correlated noise (NO) and of uncorrelated noise (NU). Measures of signal detection were also secured in separate tests. Psychometric functions (percent correct versus signal energy) were determined for each task. For a tonal signal of either low or high frequency, the listener requires only slightly greater signal energy (1–2 dB) in order to lateralize as well as he can detect when the noise is uncorrelated (NU). When the noise is perfectly correlated (NO), the slope of the psychometric function for lateralization depends upon signal frequency. With 250 cps, the slope of the function for lateralization is much smaller than that for detection. With 1000 cps, the function for lateralization is steeper than that for 250, but the slope is still less than that for the function for detection of 1000. With 2000 cps, the function has about the same slope as that for detection. [Research supported by U. S. Air Force Office of Scientific Research.]

Frequency selectivity of the ear in forward masking.

In a stimulus paradigm in which the masker terminates prior to the signal presentation (forward masking), threshold shifts as a function signal frequency closely resemble those observed for simultaneous masking. The purpose of the present study was to determine if a “critical band” could be identified for forward masking. Two-component masker (primary) stimuli centered in the regions of 800 and 3200 Hz were presented to 10 listeners. The amount of forward masking was measured as a function of signal frequency, while the frequency separation of the sinusoids comprising the primary stimulus was varied systematically. Previously reported simultaneous-masking data suggested that a critical bandwidth could be defined as the frequency separation of masker components coincident with a change in the shape of the function relating amount of masking to signal frequency. For the conditions employed in the present study, no changes in this function were observed even at wide separation of the primary components.

Human vigilance as a function of signal frequency and stimulus density.

Human vigilance as a function of signal frequency and stimulus density.

VIGILANCE AS A FUNCTION OF SIGNAL FREQUENCY AND FLASH RATE.

Signal frequency (1, 10, and 20 signals per hour) and signal flash rate (one signal every 2 and 1¼ sec.) were varied in two vigilance experiments using a “noisy” simulated radar display as the sensory input. The results indicated small changes in vigilance over the course of 1 hr. The data suggest that vigilance will decrease, stay level, or improve as a function of signal frequency and the inherent detectability of the critical signal. It is suggested that vigilance behavior is related to the extent to which S can comprehend (and hence predict) the temporal pattern of critical signals. It is concluded that not enough is known about the conditions which determine the occurrences of decrement in vigilance.

The Propagation of an Electromagnetic Wave through a Diffusing Plasma

The propagation of an electromagnetic wave through a fully ionized diffusing plasma slab is investigated. The slab is considered to be diffusing in an ambipolar fashion, with the electron-ion collision frequency proportional to the spatial and time varying ion density. The propagation characteristics of the wave are shown to be dependent on the relationship between np and nc, the densities where the signal frequency is equal to the plasma frequency and collision frequency respectively. This relationship is temperature dependent. The reflection and transmission coefficients of the plasma are calculated as a function of signal frequency, and their behavior as the slab diffuses is investigated.

Envelope transfer function analysis in A-C servo systems

IN THE ANALYSIS of a-c servo systems for stability and performance, attention is centered upon the carrier “envelope,” i.e., the signal proper. Extension of the convenient transfer function description used in direct-signal (low-pass) servo systems thus requires that the dynamic behavior of each element in the a-c system be described in terms of its effect on the carrier envelope by “envelope transfer functions” which are functions of signal frequency. In general, three types of elements are encountered in a-c servo systems: type-1 elements in which both input and output are modulated carriers; type-2 elements which have inputs at signal frequencies and outputs which are modulated carriers, and are called ”modulators” and type-3 elements which accept modulated carriers as inputs, produce outputs at signal frequencies, and are called “demodulators.” Both type-2 and type-3 elements readily lend themselves to simple envelope transfer function description if their dynamic behavior occurs on the signal side. Type-1 elements, however, have known transfer functions in terms of actual frequency, rather than signal frequency, and the envelope or signal transfer function must be derived.

Methods of Measuring the Properties of Ionized Gases at High Frequencies. IV. A Null Method of Measuring the Discharge Admittance

The admittance of a gas discharge may be obtained from measuring the ratio of the transmitted power through a microwave cavity to the incident power as a function of signal frequency near the cavity resonance. The method involves balancing the transmitted and incident signals to zero at the cavity resonance after they have passed through two separate receiving systems. When the signal frequency is changed, an attenuator is used to rebalance the two signals. The change in frequency from resonance and the corresponding change in attenuation gives the necessary data which will plot as a straight line whose slope yields the desired information.