Showing papers on "Bandwidth (signal processing) published in 1980"

Analytical expressions for critical‐band rate and critical bandwidth as a function of frequency

Abstract: The critical‐band rate as well as the critical bandwidth are functions of frequency. These dependencies have been given in table form. For effective use of these values in computers, relatively simple equations are given to express the dependence of critical‐band rate on frequency with an accuracy better than 0.2 Bark and that of critical bandwidth on frequency with an accuracy better than 10% over the whole auditory range.

Frequency-hopped multilevel FSK for mobile Radio

Abstract: A multiple-access modulation technique that uses multilevel frequency shift keying (FSK) to modulate frequency-hopped, spread-spectrum carriers is examined for possible application to digital mobile radiotelephony. This technique, in which all users employ the full system bandwidth simultaneously, would be resistant to the frequency-selective fading so troublesome in mobile radio. We have studied base-to-mobile communication of 32 kb/s per user in the 20-MHz (one-way) bandwidth of the 850-MHz mobile radio band. The number of users that can be served within a given bit error rate criterion depends on the quality of the radio channel. For perfect transmission, where the only degradation is mutual interference, an error rate less than 10−3 can be maintained with up to 209 simultaneous users. Transmission impairments, consisting of white Gaussian noise and frequency-selective Rayleigh fading with an average rf signal-to-noise ratio of 25 dB, reduce the number of simultaneous users to about 170. This capacity is roughly three times that of a phase-shift-keying spread-spectrum system recently proposed for mobile radio. For mobile-to-base transmission of FH-fsk, we have yet to study impairments resulting from delay spread in a synchronous system or, alternatively, the penalty for operating asynchronously. These effects would reduce the number of possible users from the estimates we have given for base-to-mobile transmission.

Off‐frequency listening and auditory‐filter asymmetry

Abstract: The phenomenon of off-frequency listening, and the asymmetry of the auditory filter, were investigated by performing a masking experiment in which a 2.0-kHz tonal signal (0.4 sec in duration) was masked by a pair of noise bands, one below and the other above the tone. The noise bands were 0.8-hKz wide. The edges of the bands were very sharp, the spectrum level in the band was 40 dB SPL, and the masker was on continuously throughout the experiment. Tone threshold was measured as a function of the distances from the tone to the nearer edge of each noise band. It was assumed that conditions in which one noise band was near the tone and the other remote from the tone would encourage the observer to listen off frequency, that is, to center his auditory filter, not at the tone frequency, but at the frequency that optimizes the signal-to-noise ratio at the output of the filter. The threshold data were analysed with a power spectrum model of masking in which it was assumed that the general form of the filter shape was a rounded exponential (a pair of back-to-back, negative exponentials with the peak smoothed and the tails raised). The specific filter shape obtained by applying this model to the threshold data has a broad passband (a 200-Hz, 3-dB bandwidth), steep skirts (slopes of 100 dB/octave) and shallower tails (slopes of 30-50 dB/octave) that take over 30-35 dB down from the peak of sensitivity. The filter is asymmetric, with the lower branch slightly broader than the upper. The filter is shifted off frequency by more than half its bandwidth in some cases, and the shift can improve the signal-to-noise ratio by up to 5.0 dB.

International digital facsimile coding standards

Abstract: Recently Study Group XIV of CCITT has drafted a new Recommendation (T.4) with the aim of achieving compatibility between digital facsimile apparatus connected to general switched telephone networks. A one-dimensional coding scheme is used in which run lengths are encoded using a modified Huffman code. This allows typical A4 size documents in the form of black and white images scanned at normal resolution (3.85 lines/mm, 1728 pels/line) to be transmitted in an average time of about a minute at a rate of 4800 bit/s. The Recommendation also includes a two-dimensional code, known as the modified relative element address designate (READ) code, which is in the form of an optional extension to the one-dimensional code. This extension allows typical documents scanned at high (twice normal) resolution (with every fourth line one dimensionally coded) to be transmitted in an average time of about 75 s at 4800 bit/s. This paper describes the coding schemes in detail and discusses the factors which led to their choice. In addition, this paper assesses the performance of the codes, particularly in relation to their compression efficiency and vulnerability to transmission errors, making use of 8 CCITT reference documents.

Improved bandwidth of microstrip antennas using parasitic elements

Abstract: A method is described of doubling the bandwidth of rectangular microstrip patch antennas by locating capacitively excited λ m /4 short circuit parasitic elements at their radiating edges. The antenna characteristics are explained in terms of an antiphase mode of a pair of coupled resonators, and it is shown that the bandwidth improvement is independent of the coupling capacitance. Experimental results are presented for both linearly and circularly polarised antennas which support the theoretical predictions.

The theory, design, and applications of surface acoustic wave Fourier-transform processors

Abstract: This paper addresses the theory, design, and applications of surface acoustic wave (SAW) Fourier-transform processors. These analog processors are shown to perform several sophisticated real-time signal-processing functions at wide bandwidth (tens of megahertz) making them attractive for use in radar, sonar, and communication equipments. Theoretical results show how specific arrangements of physically realizable SAW chirp filters permit Fourier transformation of both baseband and IF input signals. The operation of SAW Fourier-transform processors, with separate real- and imaginary-baseband inputs, highlights their use as wide-band "FFT-type" processors. The application of individual SAW-based processors to spectrum analysis, network analysis, beamforming, and frequency-hopped wave-form synthesis is discussed and demonstrated. Finally, the use of combinations of two or more SAW Fourier-transform processors for cepstrum analysis and programmable correlation is described.

Finite amplifier gain and bandwidth effects in switched-capacitor filters

Abstract: Discusses the effects of finite operational-amplifier gain and bandwidth on the response of the most widely used switched-capacitor filter section. Formulas are derived for the minimum acceptable values of the DC amplifier gain and the unity-gain frequency under specified conditions.

The bandwidth of digital data signal

Abstract: The engineering of data communications systems invites involvement with a number of rather refined parametric concepts, such as bit error rate (BER), antenna gain, radiated power, communication efficiency, and bandwidth. Of these, none has been the subject of more lively discussion and revision than bandwidth. The implications of bandwidth can vary considerably from context to context, as the profusion of definitions of bandwidth will attest. The purpose of the present article is to explore the subject of data transmission bandwidth through an examination of its various definitions. The spectrum is finite, yet as technology manipulates it, a dazzling increase in bandwidth capability emerges.

The critical band coder--Digital encoding of speech signals based on the perceptual requirements of the auditory system

Abstract: The development of a digital encoding system designed to exploit the limited detection ability of the auditory system is described. By dynamically shaping the encoding error spectrum as a function of the input speech signal, the error is masked by the speech. Psychoacoustic experiments and results from the literature provide a basis for determining the system parameters that ensure that the error is inaudible. The encoder is a multi-channel system, each channel approximately of critical bandwidth. The input signal is filtered into 17 frequency channels via the quadrature mirror filter technique. Each channel is then coded using block-companding adaptive PCM. For 4.1 kHz bandwidth speech, the differential threshold of the encoding degradation occurs at a bit rate of 34.4 kbps. At 16 kbps, the encoder produces toll quality speech output.

Adaptive HF communication system

Abstract: A high frequency communication system has at least two stations with each station having a discrete address, a transmitter, a receiver and a means for selecting the preferred frequency for transmitting at least a preselected format between the station includes a means for sequentially tuning the receiver through a group of channels with each channel having a preselected frequency and bandwidth, a means for measuring the link quality of each channel that is received according to the discrete address of the station transmitting the message. After the link quality is measured it is stored in a link quality matrix according to the channel and address of the station transmitting the message, so that when transmission is initiated by the station receiving the message it may select the channel with the best link quality and initiate transmission on that channel if it is available.

Gain enhancement in Raman amplifiers with broadband pumping.

Abstract: We have observed near-monochromatic gains in a Raman amplifier pumped by a laser with a bandwidth up to 30 times wider than the molecular linewidth. This gain enhancement is obtained when the corresponding temporal fine structure of the pump and Stokes waveforms are superimposed in the amplifier.

The use of current amplifiers for high performance voltage applications

Abstract: A current amplifier is used to realize a voltage amplifier having an improved high frequency response and slew rate capability. It is shown that the closed loop bandwidth is independent of the closed loop voltage gain. The design and application of a unity gain and a high gain current amplifier to voltage signal processing circuits are given. The results demonstrate an efficient use of the inherent frequency response capabilities of the active devices in the circuit to achieve the amplification of high frequency and large amplitude voltage signals.

Ultrasound Phased Array Delay Lines Based on Quadrature Sampling Techniques

Abstract: Two signal processing techniques are presented for applica- tion to ultrasonic phased array delay line systems. By using bandwidth considerations, it is shown that the signals may be either mixed to a lower frequency range or sampled at a rate commensurate with the signal bandwidth, rather than the highest frequency component, prior to delay. Both techniques lower the bandwidth requirements of the delay lines and neither require reconstructing a delayed version of the original radio frequency (W) waveform. The latter approach requires no mixers, however, as samples of the in-phase and quadrature compo- nents are obtained directly from the RF signal. The required focused signal is produced by delaying and summing the samples. It is shown that RF signals with 60 percent fractional bandwidth may be processed in this way by sampling in quadrature at only 5 of the center fre- quency. (Quadrature sampling requires that two samples be taken 90' apart with respect to the center frequency every sampling interval.) This was implemented in a four-element 5-MHz annular array system, and the results, including beam plots and tissue scans, are shown.

New wideband microstrip antenna using log-periodic technique

Abstract: A novel wideband microstrip antenna using a series-fed linear array of patch resonators in a log-periodic arrangement is described. A 9-element example gives good input v.s.w.r. and radiation control over a 30% bandwidth with better than 70% efficiency. The new array configuration gives wider bandwidth than single-layer or stacked microstrip patches combined with better efficiency than patches on lossy substrates or microstrip spirals.

Helium-speech communication

Abstract: The invention relates to a novel helium-speech unscrambler which can be located at a diver's location, and enables the helium-speech voiced by the diver to be subjected to waveform time expansion to reduce the bandwidth of the helium speech (e.g. to 2 to 3 KHZ) prior to transmitting the speech signals to a distant location on a carrier wave selected for optimum transmission through the water.

Improved decoding Scheme for frequency-hopped multilevel FSK system

Abstract: We have recently examined, for possible application to digital mobile radio telephony, a digital spread-spectrum technique employing multiple frequency-shift keying (MFSK) modulation with code-division-multiple access (CDMA) by frequency-hopping over a common bandwidth. The system uses the cellular approach where all mobiles within a cell communicate with a fixed base station in the cell. An analysis of base-to-mobile transmission shows that mutual interference limits the number of users which the system can accommodate at a given error rate. This paper describes a new decoding scheme to reduce mutual interference which makes use of the well-defined algebraic structure of the users' addresses. Analysis of the new decoder at high signal to noise (s/n) ratio shows it to outperform conventional decoding, allowing a 50 to 60 percent increase in the number of users who can simultaneously share the system at a given error rate. We describe a simple implementation of the decoder using shift registers.

Fading channel communications

Abstract: Adaptive processing can reduce the effects of fading on beyond-the-horizon digital radio links. Two radio propagation channels for beyond-thehorizon communications, troposcatter, and HF are currently being reexamined. In the past, transmission over these radio channels had been considered unreliable due to fading effects. Recently, conversion from analog to digital transmission and the use of new adaptive signal processing techniques have offered promise of acceptable network communication quality. In addition, over-the-horizon radio provides economic and/or secu-· rity advantages relative to satellite, cable, or line-of-sight terrestrial microwave links. There is renewed interest in . the use of high frequency (HF) and troposcatter communications for networks carrying digital traffic.

Time compression multiplexing of video signals

Abstract: Frequency and time division multiplexing arrangements are common for efficiently utilizing bandwidth in a communication system. Still another, lesser known arrangement is called time compression multiplexing. In time compression multiplexing, a signal from each input channel is stored for a short period of time. The signals from all channels are then read from the store, compressed in time and transmitted over a communication path to a receiver. Unfortunately, the art appears to be remiss in the time compression multiplexing of video signals. The subject signal processor includes a plurality of input terminals (111, . . . , 11N), each input terminal being adapted to receive a video signal, each video signal including a plurality of scan lines, each scan line having a time duration T seconds. The signal processor also includes an arrangement for compressing a video signal by a factor M (illustratively M=N). More specifically, the signal processor compressing arrangement includes apparatus (120, 131, . . . , 13N) for modulating each input video signal with a chirp signal. A first, predetermined delay (142, . . . , 14N) is introduced to the modulated signals, which may be added (150 ). The compressing arrangement also includes apparatus (160) for introducing a second, variable delay to the modulated video signals, e.g., with a dispersive filter (160). The resultant time compressed signal is extracted from the dispersive filter output by an envelope detector (170) and extended over a communication path (300) to a receiver (200) where the time compressed signal is expanded through a receiver modulator (230), distributing apparatus (240), dispersive filter (261, . . . , 26N) and envelope detector (271, . . . , 27N) for extension to a plurality of output terminals (281, . . . , 28N).

Design of Loaded-Line p-i-n Diode Phase Shifter Circuits

Abstract: The design of three types of loaded-line p-i-n diode digital phase shifter circuits is presented. The three configurations considered are 1) main line mounted, 2) stub mounted, and 3) switchable stub length. Expressions for various design parameters are derived. Calculations show that for the 22.5/spl deg/ phase bit the maximum bandwidths for three circuits are 22.7, 19.2, and 18.4 percent, respectively. Starting from the p-i-n diode parameters, optimum designs of these circuits for obtaining minimum insertion loss, maximum bandwith, and minimum size are also discussed. It is found that the switchable stub length type configuration is suitable when nearly ideal p-i-n diodes are used while stub mounted type configuration is suitable for nonideal diodes. Experimental and theoretical performance (bandwidth and insertion loss) of stub mounted type circuits are compared and found to be in good agreement.

Optimization of the Processing Gain of an FSK-FH System

Abstract: It is desired to determine the performance of a noncoherent FSK frequency-hopped spread spectrum communication system using MARK and SPACE tones which are not necessarily orthogonal, and in addition using an error-correction code to reduce the susceptibility of the system to jammer-induced errors. The overall bandwidth of the system (i.e., the spread bandwidth) will be held constant so that the processing gain of the system will always be adjusted to maintain this constant bandwidth as other factors (e.g., the bandwidth expansion due to the error correction coding) change the underlying baseband bandwidth.

Optimization of the Processing Gain of and M-ary Direct Sequence Spread Spectrum Communication System

Abstract: In most realistic communication systems, be they spread spectrum or conventional systems, some overall bandwidth constraint is usually placed upon the system. In the case of spread spectrum communications, this constraint limits the amount of processing gain that the system can employ. In addition to processing gain, factors such as alphabet size affect the total bandwidth. It is the purpose of this paper to combine these two effects to determine how one should trade off the amount of processing gain one uses relative to the underlying modulation bandwidth when one is operating under the constraint of fixed total system bandwidth.

Bandwidth requirements for video transmission of american sign language and finger spelling

Abstract: Although current video communication schemes use a bandwidth on the order of 1 megahertz, the bandwidth required for video communication of American Sign Language by a simple raster scan is only approximately 20 kilohertz.

Coherence bandwidth loss in transionospheric radio propagation

Abstract: In this report a theoretical model is developed that predicts the single-point, two-frequency coherence function for transionospheric radio waves. The theoretical model is compared to measured complex frequency correlation coefficients using data from the seven equispaced, phase-coherent UHF signals transmitted by the Wideband satellite. The theory and data are in excellent agreement. The theory is critically dependent upon the power-law index, and the frequency coherence data clearly favor the comparatively small spectral indices that have been consistently measured from the wideband satellite phase data. A model for estimating the pulse delay jitter induced by the coherence bandwidth loss is also developed and compared with the actual delay jitter observed on synthesized pulses obtained from the Wideband UFH comb. The results are in good agreement with the theory. The results presented in this report, which are based on an asymptotic theory, are compared with the more commonly used quadratic theory. The model developed and validated in this report can be used to predict the effects of coherence bandwidth loss in disturbed nuclear environments. Simple formulas for the resultant pulse delay jitter are derived that can be used in predictive codes.

First-order sampling of bandpass signals--A new approach (Corresp.)

Abstract: The first-order sampling of a bandpass signal when the upper cutoff frequency is a multiple of the bandwidth is reduced to an application of the low-pass sampling theorem. In the general case, a simple band-imbedding procedure restores the positioning constraint and yields the expansion.

A UHF channel simulator for digital mobile radio

Abstract: A device to simulate the channel propagation characteristics of the ground mobile environment at UHF has been designed, built, and characterized. This device provides a flexible, easily changed set of simulated channel characteristics, which allows the performance of a mobile radio unit to be evaluated in the laboratory under controlled conditions. The channel simulator was designed to simulate mobile platform speeds up to 675 mi/h and multipath components having differential delays approaching 10 µs. The channel simulator can provide up to four easily selectable, independently fading, multipath components, having calculated time delay spreads of up to 3.5 µs. The adjustable fading bandwidths and the exceptional long delays were implemented using the relative new signal processing technologies of charged-coupled devices and surface wave devices. Envelope statistics such as fading distributions and level crossing rates produced by the channel simulator show excellent agreement with theoretical prediction and documented experimental data.

Self-adjusting dual mode automatic gain control circuit

Abstract: An automatic gain control circuit comprising a gain-controlled amplifier for receiving a variable amplitude, pulse-type input signal to produce a substantially constant amplitude, pulse-type output signal which is applied to a feedback circuit for comparison with a predetermined reference voltage to generate a control voltage to set the gain of the amplifier. A voltage limiter included in the feedback circuit clips the pulse peaks of the output signal whenever the peaks exceed a predetermined amplitude so that the gain of the amplifier is reduced in a prescribed manner to preclude output signal dropout. A peak detector and a filter included in the feedback circuit are capable of operating in fast and slow modes. When operating in the fast mode, the peak detector responds essentially to each pulse peak of the output signal and the filter has a wide bandwidth so that the automatic gain control circuit operates with a prescribed fast time response. When operating in the slow mode, the peak detector operates to increase the gain of the amplifier slowly in order to limit output signal overshoot and preclude output signal dropout if the input signal reappears after an absence for some period. The filter, when operating in the slow mode, has a narrow bandwidth so that the automatic gain control circuit operates with a time response substantially slower than the fast time response. A controller circuit produces, in response to the output signal, a controller signal which causes fast mode operation of the peak detector and filter whenever the output signal is present and slow mode operation thereof whenever the output signal is absent for a predetermined time period.

Digital Phase Sampling for Microcomputer Implementation of Carrier Acquisition and Coherent Tracking

Abstract: By sampling zero crossings at the Nyquist rate (i.e., the predetection bandwidth) the phase of a narrow-band signal may be extracted directly with respect to an arbitrary, stable reference. Analysis, simulation, and experimental results are presented validating this all-digital technique for acquisition and coherent tracking of a Doppler-shifted carrier at a low carrier/noise ratio and a large percentage bandwidth. The key advantage of this scheme for microcomputer-based implementation is that phase comparison is accomplished by arithmetic subtraction, without the coordinaterotation transformation which is necessary for conventional I , Q sampling. Also, the phase-sampling process is independent of the phase error, which exists only as a numerical representation in the digital tracking algorithm. The technique is applicable to Costas suppressed-carrier tracking and to BPSK bit synchronization and demodulation, by programming the appropriate digital algorithms.