Showing papers on "Bandwidth (signal processing) published in 1983"

Suggested formulae for calculating auditory‐filter bandwidths and excitation patterns

Abstract: Recent estimates of auditory‐filter shape are used to derive a simple formula relating the equivalent rectangular bandwidth (ERB) of the auditory filter to center frequency. The value of the auditory‐filter bandwidth continues to decrease as center frequency decreases below 500 Hz. A formula is also given relating ERB‐rate to frequency. Finally, a method is described for calculating excitation patterns from filter shapes.

Simultaneous transmission of two information signals within a band-limited communications channel

Abstract: A co-channel communications system is disclosed which permits a digital data signal to be simultaneously transmitted with a communications medium signal such as telephone voice or television video. The data signal or signals are converted into very low multifrequency signals consisting of fundamental frequencies and harmonics which span the communications bandwidth. Because a number of frequencies are used for each data signal, the signal level of the data signals can be greatly suppressed relative to the communications medium signal to a level such that there is no perceptible degradation of the telephone voice or television video. Synchronous detectors for each frequency transmitted, including harmonic frequencies, are utilized to recapture and regenerate the data signals transmitted.

Spectral consequences of photoreceptor sampling in the rhesus retina

Abstract: Optical transforms were used to compute the power spectra of rhesus cones treated as arrays of image sampling points. Spectra were obtained for the central fovea, parafovea, periphery, and far periphery. All were consistent with a novel spatial sampling principle that introduces minimal noise for spatial frequencies below the Nyquist limits implied by local receptor densities, while frequencies above the nominal Nyquist limits are not converted into conspicuous moire patterns, but instead are scattered into broadband noise. This sampling scheme allows the visual system to escape aliasing distortion despite a large mismatch between retinal image bandwidth and the Nyquist limits implied by extrafoveal cone densities.

Gap detection as a function of frequency, bandwidth, and level.

Abstract: The threshold for detection of a temporal gap in a noiseband was measured. A notched noise masker was used to restrict listening to a limited spectral region. Threshold was measured as a function of center frequency, bandwidth, and level. For a signal bandwidth of one‐half the center frequency, the gap threshold decreased from 22.5 ms for a center frequency of 0.2 kHz to 3.2 ms at 8.0 kHz: a wideband condition provided an estimate of 2.3 ms, a value in agreement with previously published estimates. Bandwidth manipulation showed that the variation with frequency was not due to changes in absolute bandwidth alone. The effect of changes in level was determined at three frequencies, 0.4, 1.0, and 6.5 kHz, using a signal bandwidth of half the center frequency. At all frequencies gap threshold decreased as the signal spectrum level was raised from 10 to 25 dB, but a further increase to 40 dB showed no additional improvement. At frequencies up to about 1.0 kHz, the variation of gap threshold with frequency matches well the reciprocal of the bandwidth of the auditory filter, as determined from masking experiments using a notched‐noise masker. This suggests that the temporal response of the auditory filter may limit gap detection at low frequencies.

Temporal gap detection in noise as a function of frequency, bandwidth, and level

Abstract: Temporal gap resolution is measured with Bekesy tracking procedure and filtered noise stimuli in the frequency range below 6000 Hz. Stimulus parameters include high‐pass and low‐pass cutoff frequency, band center frequency, bandwidth in a 2‐oct range, and signal level in the low‐to‐moderate intensity range. The pattern of results indicates that gap resolution improves with an increase in stimulus frequency in a manner that can be described by a linear function with a slope of about 2 ms/oct. This relationship applies to signal levels greater than 25–30 dB SL. A linear trend also describes gap threshold as a function of the empirical critical bandwidth within the same frequency range. Implications of the results for simple functional models of temporal processing are examined.

Switched diversity with feedback for DPSK mobile radio systems

Abstract: Switched diversity with feedback for differential phase shift keying (DPSK) mobile radio is discussed. The technique uses multiple transmit antennas at the base station but only one receive antenna at the mobile. The base station transmits with one antenna that is switched when the mobile informs the base station that the received signal has fallen below a fixed level. The implementation of switched diversity with feedback in a digital mobile radio system is first described, and then the bit error rate performance of the system is analyzed with fading as a function of several design parameters. Implementation of the system is shown to be relatively simple, yet the system is shown to reduce substantially the required received E b /N 0 for a given error rate at the mobile as compared to a system without diversity. For example, with five transmit antennas the required received E b /N 0 for a 10-3bit error rate is 13 dB less. The system capacity and availability assuming 32 kb/s audio and flat fading is then discussed. It is shown that with three-corner base station diversity and four transmit antennas at each base station, 126 two-way circuits per cell can be used in a fully loaded 40-MHz bandwidth system with a ten-percent probability that the error rate exceeds 10-3.

Bandwidth limits of nonlinear (soliton) optical communication systems

Abstract: We have shown that interactions between pulses can lead to a significant reduction of the bandwidth of nonlinear systems. The inclusion of loss is essential and implies that solitons must be launched with a separation of at least 10 times their pulse width for short systems of ∼ 30 km and even more for longer systems.

A Learning Filter for Removing Noise Interference

Abstract: We have developed a microcomputer-based filter that removes line induced electrical interference from biopotential signals by learning one period of the noise waveform and subtracting it from the signal. Since it uses a noise template, the filter can remove noise waveforms containing several harmonics of the 60 Hz line frequency.

Adaptive enhancement of finite bandwidth signals in white Gaussian noise

Abstract: The steady-state behavior of the adaptive line enhancer (ALE) is analyzed for stationary inputs consisting of finite bandwidth signals embedded in a white Gaussian noise (WGN) background. Analytic expressions for the weights and output of the LMS adaptive filter are derived as functions of input signal bandwidth and SNR, as well as ALE length and bulk delay. The steady-state gain in broad-band SNR from input to output is derived as a function of these same four variables. For fixed ALE parameters and input SNR, it is shown that this gain increases as the input signal becomes narrower and approaches the sinusoidal limit. It is emphasized that because the correlation time of finite bandwidth signals is limited, excessively large values of the ALE bulk delay parameter result in diminished gain. Furthermore, there is an optimal filter length, whose value depends upon signal bandwidth and SNR, for which the broad-band gain is maximized. These results demonstrate the importance of including the effects of algorithm noise in analyzing the performance of real-time adaptive processors.

Voice messaging system with pitch-congruent baseband coding

Abstract: An improved voice messaging system using LPC baseband speech coding. In standard LPC-based baseband speech coding techniques, LPC parameters plus a residual signal are transmitted. To save band width, the residual signal is filtered so that only a fraction of its full bandwidth (e.g., the bottom 1 KHz) is transmitted. At the decoding station, this fraction of the residual signal (which is known as the baseband signal) is copied up or otherwise expanded to higher frequencies, to provide the excitation signal which is filtered according to the LPC parameters to provide the reconstituted speech output. However, this tends to produce perceptually significant ringing effects and high frequency distortion in the reconstituted signal. The present invention uses a variable baseband width, which is adaptively varied, in accordance with an integral multiple of the frequency of the pitch of the input signal, to provide a more appropriate harmonic match in the reconstituted excitation signal. This eliminates the noticeable ringing effect.

Superconductive tapped delay lines for microwave analog signal processing

Abstract: Passive superconducting tapped delay lines have been fabricated for use as matched filters for multigigahertz bandwidth analog signal processing Specifically, linear frequency-modulated dispersive delay lines, also known as chirp filters, having a bandwidth of 26 GHz centered at 4 GHz and a dispersion time of 35 ns have been constructed The stripline structure consists of a 4000-A-thick patterned niobium film sandwiched between 5-cm-diameter, 125-μm-thick sapphire wafers Two parallel striplines, each 16-m long, are wound in a spiral pattern The taps are backward-wave couplers formed by bringing the two lines into and out of proximity at specified locations Pulse expansion and compression have been demonstrated with these devices and are in close agreement with a new theoretical model for this class of signal processors

Time- and frequency-domain response of directional-coupler traveling-wave optical modulators

Abstract: We analyze the time- and frequency-domain response of directional-coupler traveling-wave optical modulators (DCTWOM) in which the optical and electrical velocities are not matched. We demonstrate for traveling-wave modulators that a directional-coupler device is intrinsically faster, by nearly a factor of \sqrt{3} for the same drive, than an interferometric device of the same length. We also show, that because of nonlinearities in the directional-coupler response, the switching speed inferred through a numerical analysis in the frequency domain underestimates the impulse response derived directly in the time domain. For a directional-coupler traveling-wave modulator based in LiNbO 3 , the Fourier transformation of the impulse response (33 ps/cm FWHM) indicates a characteristic length-bandwidth product of 15 GHz. cm when microwave loss is neglected. When realistic frequency-dependent losses are included in the frequency-domain calculations, we conclude that a 1-cm-long directional coupler in LiNbO 3 has a large signal 3-dB bandwidth in excess of 10 GHz. We determine the optical response to closely spaced electrical pulses, in order to evaluate the signal encoding bit rate capabilities of the device and its relation to the frequency bandwidth.

Wideband Interference Cancellation in Adaptive Sidelobe Cancellers

Abstract: This paper presents numerical results from a study of sidelobe cancellers operating against wideband interfering signals. The study was designed to provide information that would be useful to system designers. Among other things, we address the question of whether it is best to enhance the bandwidth capability of the canceller by using tapped delay lines or simply by using more auxiliary elements. We also develop guidance rules to help the designer determine the number of adaptive loops required for a given environment.

Minimum-shift keyed modem implementations for high data rates

Abstract: IGITAL communications links capable of conveying data at hundreds of megabits per second (Mb/s) are becoming increasingly important in various applications. An example is the use of time division multiple access (TDMA) transmission through a satellite relay wherein several data channels are routed through single uplink and downlink paths (antenna beams). Separation of the channels from each other is accomplished through interleaving and deinterleaving in time, with possible rerouting taking place in the satellite. When carrying traffic from large metropolitan areas, the required data rates can exceed 100 Mb/s. One such implementation, currently under study by NASA,' involves data transmission at rates of up to 550 Mb/s. Such applications require the use of modulation schemes that use the available bandwidth efficiently. Furthermore, because power is at a premium on board a satellite, the utmost in power efficiency is desired of the chosen modulation scheme. This implies the use of a constant-envelope modulation scheme since amplifiers, such as traveling wavetube amplifiers, are most efficient when operated near saturation. Three popular constant-envelope modulation schemes for efficient transmission of digital data are quadriphase-shift keying (QPSK), offset (or staggered) quadriphase-shift keying (OQPSK), and minimum-shift keying (MSK). Two excellent recent articles [1,2] in IEEE Communications Magazine have discussed their general properties and attributes as well as those of other modulation schemes with constant (or nearly constant) envelopes. Other recent papers [3-61 have presented analytical results and computer

Analog signal translating system with automatic frequency selective signal gain adjustment

Abstract: An automatic equalizer and oscillation suppressor system utilizing phase lock loop circuits to detect oscillation candidates at identified frequencies and a low cost microprocessor to process corrective band gain action. Oscillation candidates are initially suppressed by incremental adjustment of band gains to a maximum point after which notch filters are assigned to the oscillating frequency and the band gain returned to its normal equalized value. Automatic equalization is achieved initially with a pink noise source and on an on-going real time basis with program content by use of a frequency addressable narrow band boost circuit which is selectively inserted at frequencies across the bandwidth and band gains adjusted until some oscillation has occurred in all bands or maximum master gain has been reached.

Quality of service and bandwidth efficiency of cellular mobile radio with variable bit-rate speech transmission

Abstract: The performance of an adjustable source/channel codec in a cellular mobile-radio environment is investigated. The speech transmission rate and the amount of forward error correction change in response to changing channel conditions. The channel rate is constant at 32 kb/s, and when the channel is good all of these bits are used for speech transmission. In intermediate and poor channels the speech rate is 24 or 16 kb/s, and the remaining channel symbols are used for forward error correction. Relative to conventional transmission this approach offers an improved grade of service. For example, the outage rate (the proportion of "poor or worse" communications) goes from nine percent with fixed-rate to three percent with variable-rate transmission. Alternatively, this improved grade of service can be exchanged for higher bandwidth efficiency. The fixed-rate system (with nine percent outage) has 23 users per cell. With 52 users per cell the outage of the variable-rate system is only six percent.

Method and apparatus for baseband generation of a spread spectrum reference signal for use in an LMS adaptive array processor

Abstract: A method and apparatus for baseband generation of a spread spectrum reference signal for an LMS adaptive array processor is provided. An IF summed, weighted spread spectrum signal is output from the processor and translated to baseband to produce an I (in-phase) channel spread baseband signal and a Q channel (quadrature-phase) spread baseband signal. A baseband correlator receives the I channel and Q channel baseband signals and delayed versions of in-phase and quadrature-phase PN signals and produces despread I channel and Q channel baseband output signals. A baseband linear spreader receives the despread I channel and Q channel baseband output signals and the in-phase and quadrature-phase PN signals and produces respread I channel and Q channel baseband signals. A baseband modulator modulates the respread I channel and Q channel baseband signals with a phase-shifted local oscillator to produce the reference signal. The reference signal is subtracted from the spread spectrum signal to produce an error signal consisting essentially of the undesired signal components in the spread spectrum signal. The adaptive array processor minimizes the mean-square value of the error signal to minimize these undesired signal components. If desired, multi-channel capability can be provided by a summer for summing respread I channel and Q channel baseband signals from a plurality of channels in the spread spectrum communication system.

The effects of selected signal processing techniques on the performance of a filter-bank-based isolated word recognizer

Abstract: To implement an isolated word recognizer based on filter bank techniques, decisions must be made as to how to condition the speech signal prior to the filter bank analysis (preprocessing), how to condition the feature vector at the output of the filter bank analysis (postprocessing), and how to perform the time alignment in the pattern comparison between an unknown test pattern and previously stored reference patterns (registration and distance computation). In the past most designers of such word recognition systems made arbitrary choices about how the various signal processing operations were to be carried out. This paper presents results of a systematic study of the effects of selected signal processing techniques on the performance of a filter bank isolated word recognizer using telephone-quality speech. In particular, the filter bank analyzer was a 13-channel, critical-band-spaced filter bank with excellent time resolution (impulse response durations of from 3 to 30 ms) and poor frequency selectivity (highly overlapping filters with ratios of center frequency to 3-dB bandwidth of about 8 for each band). Among the signal processing techniques studied were: preemphasis of the speech signal; time and frequency smoothing of the filter bank outputs; thresholding, quantization, and normalization of the feature vector; principal components analysis of the feature vector; local and global distance computations for use in the time alignment procedure; and noise analysis in both training and testing. Each of the signal processing techniques was studied individually; hence no tests were run in which several of the techniques were used together. Results showed that some fairly simple signal processing operations provided the best overall performance in the noise-free case; in noisy conditions performance degraded significantly for signal-to-noise ratios less than about 24 dB.

A Multi-Octave Frequency Selective Limiter

Abstract: A frequency selective limiter operating over more than 2 octaves bandwidth has been developed. The design uses single crystal ephaxial YIG films. Limiting threshold and leakage are about 1 milliwatt. Dynamic range approaches 20 dB. Selectivhy bandwidth for 3 dB weak signal compression is about +- 20 MHz at 100 mW input. An analytical method gives accurate predictions of limiting behavior when linear loss terms are included.

Method of transferring an additional information channel across a transmission medium

Abstract: A method of transferring a service channel over a fibre optic system line is proposed. The system line normally transmits regular data signals with a specific highfrequency (34% Mbits), which are coded according to a certain code. The power spectrum of the code is heavily suppressed for lowfrequencies. In accordance with the invention, the space at f=O is utilized to transmit a service channel with a low bandwidth. The service channel signals are frequency modulated and the frequency modulated signal is caused to pulse width modulate pulses forming the regular data signal. The pulse width modulation is done in two different ways, depending on whether the data signal is an RZ-coded bit flow or an NRZ-coded bit flow.

Four-dimensional modulation and coding: An alternate to frequency-reuse

Abstract: Four dimensional modulation as a means of improving communication efficiency on the band-limited Gaussian channel, with the four dimensions of signal space constituted by phase orthogonal carriers (cos omega sub c t and sin omega sub c t) simultaneously on space orthogonal electromagnetic waves are discussed. "Frequency reuse' techniques use such polarization orthogonality to reuse the same frequency slot, but the modulation is not treated as four dimensional, rather a product of two-d modulations, e.g., QPSK. It is well known that, higher dimensionality signalling affords possible improvements in the power bandwidth sense. Four-D modulations based upon subsets of lattice-packings in four-D, which afford simplification of encoding and decoding are described. Sets of up to 1024 signals are constructed in four-D, providing a (Nyquist) spectral efficiency of up to 10 bps/Hz. Energy gains over the reuse technique are in the one to three dB range t equal bandwidth.

Simple and effective bandwidth reduction algorithm

Abstract: An automatic algorithm is presented for reducing the bandwidth of symmetric interconnectivity matrices. The technique takes an arbitrary input scheme and reorders the nodes so as to reduce the bandwidth. A large number of examples is presented to illustrate the reliability and cost-effectiveness of the method.

Adaptive signal weighting system

Abstract: An adaptive signal weighting system is disclosed for use in transmitting an electrical information signal of a predetermined bandwidth along a signal path. The system can be used to encode or decode the signal. The system comprises filter means disposed in the signal path for varying the gain impressed on the portion of the information signal within a first select spectral region within the preselected bandwidth. The gain is varied in response to and as a function of a first control signal. Means are provided means for generating the first control signal in response to and in accordance with the signal energy of the information signal substantially within at least a part of the first select spectral region. The system also comprises gain control means disposed in the signal path and coupled to the filter means for varying the signal gain impressed on the information signal substantially throughout the predetermined bandwidth, the signal gain varying in response to and as a function of a second control signal; and means for generating the second control signal in response to and as a function of the signal energy of the information signal within at least one other select spectral region of the information signal.

Towards the fundamental limits of optical-fiber communications

Abstract: We review the theoretical limits which restrict transmission over optical fibers. The fundamental limit on channel capacity is 1 nat/photon with a coherent detection receiver or with a thermal-noise-limited receiver. With an ideal photon-counting receiver, the theoretical capacity is infinite. A practical limit of a few nats per photon for direct detection requires a bandwidth expansion consistent with monomode fibers and fast digital circuits and is 35-40 dB better than current direct detection receivers. This limit may be approached by receiver improvements (10 dB with direct detection, 17 dB with optimum coherent detection), by using digital pulse-position modulation (PPM) (10-13 dB) and by using error-correcting codes where constraints on system complexity allow.