Reception Bandwidth Research Articles

Correlations between speech processing and auditory functions at 1 kHz

Even when all relevant speech information is presented above threshold, intelligibility may still be reduced for hearing-impaired listeners. An attempt is made to relate impaired speech processing to a deterioration of specific auditory functions. Performance of hearing-impaired listeners is measured on psychoacoustic tests concerning temporal and spectral resolution, frequency and intensity discrimination, and temporal and spectral integration. All auditory functions are measured at 1 kHz. Speech reception is measured with the SRBT test (speech reception bandwidth threshold). This test determines the minimum width of a speech band with a center frequency of 1 kHz required for 50% intelligibility. A wider-than-normal SRBT can simply be the consequence of inaudibility of part of the speech signal. To account for this possibility, each SRBT is converted to an SII (speech intelligibility index, the new articulation index). This SII may be interpreted as the proportion of the total speech information required by the listener for 50% intelligibility. An elevated SII is considered an indication of impaired processing of suprathreshold speech. This may be caused by the deterioration of specific auditory functions. Relations between the performance on psychoacoustic tests and the (elevated) SII will be examined. [Work supported by the Foundation ‘‘Heinsius-Houbolt Fonds.’’]

On the nature of middle-latitude F-spread

The results of a special experiment to study the statistics of short-wave signals reflected from the ionosphere during F-spread conducted in the first half of 1994 at the proving ground of the Scientific-Research Radio-Physics Institute at Zimenki of the Nizhny Novgorod region are presented. The dependence of the index of amplitude fluctuations on the reception bandwidth was studied. The experiment results are compared with existing theoretical concepts of the origin of F-spread. The experiment favors the hypothesis that middle-latitude F-spread is result of the multiple-ray nature of short-wave signals reflected from the large-scale quasi-regular inhomogeneous structure of the ionosphere.

Spatial and temporal properties of neurons of the lateral suprasylvian cortex of the cat.

Neurons in the posteromedial lateral suprasylvian cortex (PMLS) of cats were recorded extracellularly to investigate their response to stimulation by bars and by sinusoidal gratings. Two general types of cells were identified: those that modulated in synchrony with the passage of drifting bars and gratings and those that responded with an unmodulated increase in discharge. Both types responded to contrast reversed gratings with a modulation of activity: the cells that modulated to drifting gratings modulated to the first harmonic of contrast reversed gratings (at appropriate spatial phase and frequency), whereas those that did not modulate to drifting gratings always modulated to the second harmonic of contrast reversed gratings. No cell had a clear null point. Nearly all cells were selective for spatial frequency. The preferred frequency ranged from 0.1 to 1 cycles per degree (cpd), and selectivity bandwidths (full width at half height) were around two octaves. Preferred spatial frequency was not correlated with receptive field size, but bandwidth and receptive field size were positively correlated. Preferred spatial frequency decreased with eccentricity, at about 0.05 octaves/deg. The response of all cells increased as a function of grating contrast up to a saturation level. The contrast threshold for response to a grating of optimal parameters was approximately 1% for most cells and the saturation contrast approximately 10%. The contrast gain was approximately 25 spikes/s per log unit of contrast. All cells were tuned for temporal frequency, preferring frequencies from approximately 3 to 10 Hz, with a selectivity bandwidth approximately 2 octaves. For some cells, the spatial selectivity did not depend on the temporal frequency and vice versa. Others were spatiotemporally coupled, with the preferred temporal frequency being lower at high than at low spatial frequencies, and the preferred spatial frequency lower at high than at low temporal frequencies. Previous results showing broad velocity tuning to a bar were replicated and found to be predictable from the combined spatial and temporal tuning of PMLS cells and the Fourier spectrum of a bar. Preferred temporal frequency steadily decreased with eccentricity, at 0.025 octaves/deg. The results for PMLS cells are compared with those of other visual areas. Acuity and spatial preference and selectivity bandwidth is comparable to all areas except area 17, where they are a factor of about two higher. Temporal selectivity in PMLS is as fine as observed in other areas. The possibility that PMLS cells may be involved with motion detection and detection of motion in depth is discussed.

A deconvolution method for pulsed coherent radars

The effects of a necessarily limited transmission and reception bandwidth upon a pulsed coherent radar experiment can be partially overcome by the use of the deconvolution method described. Unlike ‘conventional’ techniques, an accurate knowledge of the radar system pulse response is not required to estimate the propagation delays of reflected pulses (echoes). However, a knowledge of the complex pulse response facilitates the simulation (Monte Carlo) of a ‘generalized’ echo structure model from which a deconvolution analysis can yield 95% confidence limit estimates for the calculated echo delays. Simulation results are presented showing dependence between the errors in delay estimates, the form of pulse response, and deconvolution control parameters.

Submillimeter Wave Radiometry

The sensitivities of square-law detectors and superheterodyne receivers presently available at the shorter millimeter wavelengths are reviewed and extrapolated to the submillimeter region. It is concluded that, for applications which permit wide reception bandwidths, significantly more sensitivity will result from use of a direct-detection system consisting of a chopper, square-law detector and narrow-band amplifier than will be attainable with superheterodynes in the foreseeable future. The expected sensitivity of a direct-detection system at wavelengths of the order of 1 mm is compared with that of the most sensitive centimeter radiometers.

The Navy's Portable Satellite Tracking Stations

The stations described recover doppler frequencies to an accuracy of one part in 109 and subcarrier oscillator frequencies to 15 kc bandwidths. Coherent phase detection and a tracking local oscillator to minimize required reception bandwidth maximizes signal sensitivity. Station portability accommodates rapid changes of location as dictated by satellite projects and the flexibility of the receiving and recording equipment due to unitized construction permits inexpensive modifications to accommodate present and future satellite programs.