Showing papers on "Bandwidth (signal processing) published in 1974"

Cooperative broadcasting

Abstract: This paper shows that several transmitters operating in an additive white Gaussian noise environment can send at rates strictly dominating time-multiplex and frequency-multiplex rates by use of a superposition scheme that pools the time, bandwidth, and power allocations of the transmitters. This pooling can be achieved without cooperative action, except for agreement on the actual rate of transmission each transmitter will allow itself. The superposition scheme involves subtraction from the received signal of the estimated signals sent by the other transmitters, followed by decoding of the intended signal. This scheme has been shown to be optimal. We conclude that present methods of allocating different frequency bands to different transmitters are necessarily suboptimal.

Design of digital notch filters

Abstract: It is shown that a second-order digital notch filter is uniquely characterized by two distinct parameters, the notch frequency and the 3-dB rejection bandwidth. As a result, such a filter can be realized using only two multipliers. Methods are outlined to design a notch filter for a prescribed notch frequency and a prescribed 3-dB rejection bandwidth, along with procedures for postdesign adjustment of these parameters. All two-multiplier, canonic and noncanonic, notch filter configurations are developed using the multiplier extraction approach. These networks are then compared with regard to the effect of internal multiplication roundoff errors. Results of computer simulation of the notch filter configurations are also included.

Coherent optical processing

Abstract: The principles of communication theory were applied in the 1950's to optical imaging systems and to the analysis of images. Optical systems were analyzed in much the same way as linear systems (modulation transfer functions and channel capacities) and images were characterized in ways analogous to time signals (space-bandwidth products, spatial frequency content, etc.). Both coherently and incoherently illuminated optical systems can be treated using these concepts. Coherently illuminated systems are most useful for performing operation such as convolution, cross correlation, and spectral analysis because the Fourier transform of an optical signal physically exists and can, therefore, be measured or modified. The basic Fourier transform relationship for coherently illuminated systems is developed in this paper. It can be detected directly and used to estimate the distribution of spatial frequencies contained in the signal. Methods for constructing complex-valued spatial filters are described; these filters can be used to realize such operations as convolution or cross correlation, addition or subtraction, and differentiation or integration. Experimental results are given to illustrate the concepts and to susgest potential applications. To extend the range of applications, interface devices are needed to allow optical processing of two-dimensional raster-scanned time signals, wide bandwidth electrical signals, and incoherent optical signals. Interface devices are often needed to convert the output optical signal to an electrical signal for post-processing by a digital computer. For some applications, interface devices are needed to construct spatial filters in real time, so different operations can be performed on a given signal. The desired characteristics of these three interface devices and the current state of their development are briefly reviewed.

A versatile active RC building block with inherent compensation for the finite bandwidth of the amplifier

Abstract: An earlier paper by the authors describes an easily trimmed universal building block for active RC filters which possesses the valuable characteristic that, with suitable design, the Q -value can be made approximately independent of the gain-bandwidth product of the operational amplifiers This makes the filter usable for high frequencies, while at the same time the dependence of the Q -value on temperature variations in the operational amplifiers is drastically reduced Design formulas are presented, as well as comparative measurements which verify the theory The building block is shown to have excellent characteristics both as a universal second-order building block and as a standard block for active ladder synthesis of bandpass filters

Slow switch for bandwidth change in phase-locked loop

Abstract: The invention is directed to a phase-locked loop circuit which utilizes a voltage controlled oscillator controlled by a pair of loop filters one of which passes a narrow range of frequencies and the other of which passes a wide range of frequencies. A variable impedance connects the outputs of the filters, with the output of the narrow loop filter connected to the input of the voltage controlled oscillator. Means are provided for controlling the variable impedance so as to smoothly switch control of the voltage controlled oscillator from the wide loop filter to the narrow loop filter for accurate and smooth phase locking.

Digital communications system with immunity to frequency selective fading

Abstract: A digital communications system with increased resistance to frequency selective fading. Digital signals to be transmitted are phase shift modulated upon a spectrum spreaded carrier. The carrier is generated by mixing the output of a band spread oscillator with a locally generated IF oscillator signal. The signal thus obtained is divided into transmission periods, each of which is longer than the maximum spreading time of the nonlinear transmission medium. The increased bandwidth occupancy thus obtained increases the immunity of the transmitted signals to frequency selective fades moving across the allocated bandwidth. The signals are received by a differentially adaptive receiver which automatically adjusts its response to the envelope of received signals.

Linear FM Signal Formats for Beacon and Communication Systems

Abstract: This paper examines the capabilities of the class of linear FM spread-spectrum signals within the context of potential communications systems usage in order to establish some performance criteria and bounds that permit comparison with other spread-spectrum formats. A systematic basis is provided for parameter selection for this class of signals by examining the interaction a mong the frequency-modulation indices, time-bandwidth product, and cross-talk criteria that determine the number of effective linear FM signals (or channels) that can be used within the constraints of a bounded time-frequency region. A general expression is derived relating N, the number of useful signals, R2, a cross-talk parameter, ToWo, the mean time-bandwidth product, and ?max and ?min, the maximum and minimum FM rates of the signal set. Canonic signal processor structures are described for ensembles of linear FM signals that have either constant duration or constant bandwidth. It is then shown that the signal modulation format can be modified in accordance with classical paired-echo theory to expand the utility of this class of signals in both synchronous and nonsynchronous operations to yield the equivalent of time-division and code multiplexing. Possible applications for this signal format are discussed.

New Theory and Design for Hairpin-Line Filters

Abstract: Hairpin-line and hybrid hairpin-line/half-wave parallel-coupled-line filters are preferred filters for microstrip and TEM printed-circuit realizations. This class of filters offers small size and, in general, needs no ground connections for resonators. A new design theory is presented that is based on a sparse capacitance matrix for the array of coupled lines that constitute the filter, as opposed to a sparse-inductance-matrix assumption in previous theories that is much harder to satisfy. It is shown that to a good approximation, hairpin-line filters result from frequency-scaling half-wave parallel-coupled-line filters. Because of this; the bandwidth can be accurately predicted. Design procedures are given for Type-A filters, which are useful up to 20-percent bandwidth. A variety of hybrid hairpin-line/half-wave parallel-coupled-line filters is possible, and their design is explained. Numerical results for a number of designs and experimental results for a 5-percent bandwidth filter are included.

Unequal bandwidth spectral analysis using digital frequency warping

Abstract: The application to unequal bandwidth and vernier spectrum analysis of a technique referred to as digital frequency warping is discussed. In this technique a sequence is transformed in such a way that the Fourier transforms of the original and transformed sequences are related by a nonlinear transformation of the frequency axis. An equal bandwidth analysis carried out on the transformed sequence then corresponds to an unequal bandwidth analysis of the original sequence. A comparison is presented between the bandwidth as a function of frequency for the digital warping technique and proportional bandwidth analysis. An analysis of the effects of finite register length in implementing digital frequency warping is also presented.

Error Performance for Adaptive Transmission on Fading Channels

Abstract: A communication system is analyzed in which the transmission power and/or rate are adjusted to the observed state (attenuation) of a slowly fading channel. The optimum control of power and rate with channel attenuation is determined. Some suboptimum control rules are also investigated. The average probability of error is evaluated for various types of adaptive transmission, imposing different constraints on power, rate, and bandwidth. It is shown that rate control is superior to power control and that simultaneous power and rate control further improves the performance. The most effective power and rate control presupposes a high peakpower-average-power ratio and a high peak-rate-peak-rate-average-rate ratio; with no peak power and bandwidth limitation, arbitrary small error probability is attainable for a given power and average rate.

Computer-Aided Design of Waveguide Multiplexer (Short Papers)

Abstract: A procedure based on an analysis algorithm and practical rules is described for the design of waveguide multiplexer. Simple rules, which enable the designer to quickly find a near-optimum solution in a small number of iteration steps, are given. An example of a 6-channel communications multiplexer, which utilizes narrow-bandpass elliptic function waveguide filters, is also included.

Adaptive low pass filter

Abstract: An adaptive low-pass recursive filter adaptively changes its bandwidth filtering characteristics in accordance with the bandwidth of the incoming signal. This results in an input signal w(k) changed into output signal x(k) in accordance with the equation x(k) a(k) x(k-1) + (1-a(k)) w(k), where k is a sampling-interval and a is the adaptive value representing ratio of known noise power to average error between the filter input and output.

Videophone system synchronizer

Abstract: In a videophone system sound, video, synchronizing and signalling signals are to be transmitted through a single pair of telephone cables of existing telephone communications. In that case sound, synchronizing and signalling signals are transmitted in a digital form and the video signals are transmitted in an analog form. It is then possible to achieve optimum picture quality at the given bandwidth of the existing telephone communications. If for the given bandwidth the video signal were also digitalized, this would be at the expense of the picture quality. However, it is then necessary to transmit information regarding clock pulses for decoding at the receiver end. To be able to synchronize at the receiver end only on two frequencies instead of on clock pulse frequency, line frequency and field frequency, the information regarding the clock pulses is cotransmitted in such a manner that a clock pulse generator at the receiver end can be synchronized therewith which generator supplies clock pulses. A field code word is also transmitted during each field flyback period. In the receiver the regenerated clock pulses are divided in frequency in a divider stage and the divided signal is compared with the detected code word in a synchronizing verification circuit. As long as there is no phase equality between component signals and detected code word, an output signal from the verification circuit resets the divider stage.

Design of Digital Notch Filters

Abstract: It is shown that a second-order digital notch filter is uniquely characterized by two distinct parameters, the notch frequency and the 3-dB rejection bandwidth. As a result, such a filter can be realized using only two multipliers. Methods are outlined to design a notch filter for a prescribed notch frequency and a prescribed 3-dB rejection bandwidth, along with procedures for postdesign adjustment of these parameters. All two-multiplier, canonic and noncanonic, notch filter configurations are developed using the multiplier extraction approach. These networks are then compared with regard to the effect of internal multiplication roundoff errors. Results of computer simulation of the notch filter configurations are also included.

Quantized Feedback in an Experimental 280-Mb/s Digital Repeater for Coaxial Transmission

Abstract: A 280-Mb/s digital coaxial repeater employing two-level transmission with quantized feedback for dc-wander control has been designed and constructed. The linear channel forward path includes a 10-MHz low-frequency cutoff, and the missing low frequencies are supplied by a filtered signal from the repeater output. This approach effectively removes dc wander without use of code restriction, making it possible to use two-level rather than three-level transmission without loss of information transmission. Eye margins are consequently significantly greater than for previously designed 280-Mb/s three-level repeaters. The bandwidth ratio, or ratio of upper to lower 3-dB down points of the signal transmission path, is only 10 for this approach compared to several hundred for the three-level bipolar approach (and more for codes with higher transmission efficiency). This results in greatly reduced susceptibility to interference from lightning and power surges, elimination of preamplifier overload, and simplified repeater design.

Measurement of plasma wave spectral density from the cross-power density spectrum

Abstract: Determination of the spectral density of plasma waves from the cross‐power density spectrum is shown to be simpler than the alternative determination from the cross‐correlation function. A device for measuring the cross‐power spectral density is described in detail. A model with a bandwidth of 30 MHz has been constructed using integrated circuits, and applied to the study of ion acoustic wave propagation on a plasma positive column. Since it is an analog device, it should be possible to extend the bandwidth for operation into the gigahertz range. Only a fixed delay is required to measure the quadrature component of the cross‐power spectral density. These features are in contrast to other correlators and cross‐power spectrum analyzers described to date, which are either limited to the low megahertz range, or require a variable delay line.

Design of RC-active networks using generalized-immittance converters

Abstract: A systematic RC-active network synthesis procedure for the realization of second-order transfer functions is proposed. The realization employs a single generalized impedance converter which can be implemented by using only two operational amplifiers. The sensitivity to passive element variations are found to be low. The sensitivity to variations in the amplifier d.c. gain is minimized and found comparable to that in other low sensitivity structures. A design procedure is developed in which tuning can be achieved by trimming only resistors. With the exception of one case, second-order sections can be cascaded without additional isolating amplifiers. The stability properties as well as the influence of the finite bandwidth of the amplifiers are examined. The design procedure is used to obtain a sixth-order Chebychev low-pass filter, and a sixth-order elliptic band-pass filter. Experimental results show that these realizations are insensitive to temperature and power supply variations.

Asymptotic Behavior of Digital FM Spectra

Abstract: An asymptotic form for the spectral density of a continuous-phase digital FM signal is derived. This expression is valid at frequencies far from the carrier and shows how the rate at which the spectral density falls away depends on the baseband pulse shape.

Fixed-point truncation arithmetic implementation of a linear prediction autocorrelation vocoder

Abstract: The implementation of the Linear Prediction Auto-correlation Vocoder (LPAV) with fixed-point truncation arithmetic is presented. For a speech bandwidth of up to 3.3 kHz, it is possible to implement LPAV with a maximum of 16-bit fixed-point arithmetic, thus allowing for very efficient software simulation on minicomputers or economical hardware implementation on a special purpose computer. The resultant degradation with respect to a floating-point implementation is negligible. The three most critical system components are discussed in detail along with various tradeoffs such as effects on system stability as a function of preemphasis and sampling frequency. The results are also applicable to general finite word length digital filter implementation problems.

Data synchronizing systems

Abstract: A bit synchronizer system is disclosed which is adaptive to varying input conditions such as noise and jitter of data signals particularly PCM telemetry signals. The signal to noise parameter of the input data signal is obtained from a matched filter having a response matched to signals which are orthogonal to the input data signal. This matched filter also detects transitions in the input data signal which are used to lock a phse locked loop. The loop generates a local clock which is synchronous with the incoming data and can be applied to detect the incoming data bits and reconstruct them into noise and jitter-free output data. Control signals are also extracted from the loop which represent the jitter (viz., the rate of change of the phase) of the input signal. The bandwidth of the loop is varied in accordance with the jitter and noise to signal parameters to the optimum bandwidth for the prevailing noise to signal ratio and jitter on the incoming data signal.

Digital filters with maximally flat amplitude and delay characteristics

Abstract: An analytical solution is obtained for the transfer function of a digital filter which exhibits an optimum maximally flat amplitude characteristic and a maximally flat delay characteristic simultaneously. Explicit values for the multipliers are given for the direct realization in terms of the degree of the network and an arbitrary bandwidth scaling factor. Finally, it is concluded that this type of filter is useful in the area where the degree of a non-recursive filter becomes excessive to fulfil an amplitude requirement (e.g. narrow bandwidth) and where recursive filters designed solely on an amplitude basis are too dispersive.

Relation of radar range resolution and signal-to-noise ratio to phased-array bandwidth

Abstract: Phased-array radar systems using wide-band linear-FM pulses suffer from array dispersion. This causes a loss in signal-to-noise (S/N) ratio and in radar range resolution (a broadening of the compressed-pulse width). Depending on the choice of the receiver matched filter (MF) and weighting filter (WF) one or the other of these effects may be minimized. The effects of dispersion in a square planar array with either a parallel feed or a center-fed series feed are studied. Loss in S/N ratio and in range resolution are determined for typical cases, and general curves are given for arrays with arbitrary size, scan range, and signal bandwidth. It is shown that there is an optimum or maximum-useful signal bandwidth for each array, for which the minimum S/N-loss MF-WF design is the same as the ideal-compressed-pulse-shape MF-WF design. For pulses having greater signal bandwidths, time delay steering should be used in the array.

Visibility of unpredictably flickering lights.

Abstract: We have measured the sensitivity of the visual system to temporal modulation with unpredictable, aperiodic signals. We used three kinds of stimulation, (i) a band-limited gaussian random signal, (ii) a passband-limited gaussian random signal, (iii) a periodically quenched random signal. The sensitivity to stimulation with random signals can be predicted from the sensitivity of the visual system to periodic temporal signals. The sensitivity to random signals with narrow frequency bands at high frequencies is governed by the pseudoflash phenomenon. If the bandwidth is such that the signal contains less than two independent samples per second, the psychometric curve follows from the amplitude distribution of the random signal. If the signal contains a larger number of independent samples per second, the psychometric curves are as steep as they are for sine-wave stimulation. If the De Lange characteristic is the envelope of the sensitivity characteristics of independent channels, sensitive to specific frequency bands, then these experiments permit us to estimate the bandwidth of the most-sensitive channel.

Visual Search and Image Quality

Abstract: : This report presents the results of an air-to-ground television target acquisition experiment which investigated the effects of mission profile, video system line rate and bandwidth, and video noise level The target acquisition performance data are related to these variables and to a measure of display image quality, the Modulation Transfer Function Area (MTFA), which is measured microphotometrically at the display surface The target acquisition performance results are largely as expected For a camera field of view of 188 x 142 deg, the mean ground ranges of correctly acquired targets are 28,661, 24,376, and 12,171 ft, respectively, for mission profiles of 23 deg depression angle, 500 ft/s velocity; 23 deg, 3000 ft/s; and 45 deg, 500 ft/s As depression angle decreased, there was a large decrease in acquisition range; as velocity increased, there was a smaller decrease in acquisition range Altitude was 10,000 ft

16-channel surface-acoustic-wave grating-filter bank for real-time spectral analyser

Abstract: A 16-channel surface-acoustic-wave grating-filter bank has been constructed on four y-cut LiNbO3 crystals. Each channel has a 4dB bandwidth of 6.7 MHz. The insertion loss at the peak of the passband of each channel is −32 dB, ±1.2 dB. The centre frequencies are 6.7 MHz apart and span the range 150–250 MHz. This filter has been incorporated in a realtime spectral analyser.