Showing papers on "Signal-to-noise ratio published in 1968"

PCM Data Reliability Monitoring Through Estimation of Signal-to-Noise Ratio

Abstract: Reliability monitoring of digital pulse-code modulation (PCM) data in coherent transmission systems can be based upon online estimation of signal-to-noise ratio (SNR) at the receiver. Maximum likelihood SNR estimation is analyzed to determine the structure of the estimator and its ability to monitor error rates with a given degree of confidence. In particular, relationships between estimation confidence intervals, observation time, word error rates, and PCM word structure are developed.

Optimum Signal-Processing for Pulse-Amplitude Spectrometry in the Presence of High-Rate Effects and Noise

Abstract: In this paper the idea of a general signal processing system which should satisfy various pulse rate and noise requirements is explored. Optimum processing functions (weighting functions) are considered for an ideal system, and for real conditions where effects like imperfect pole-zero cancellation are present. Time-variant filters of the gain-varying class are used to realize the required optimum weighting functions of finite width. It is shown how nonfinite-width weighting functions of some time-invariant filters can be modified into finite-width functions by switching. These switched-gain time-variant filters are somewhat limited in choice of weighting functions. A general processing system can be realized employing filters with continuously time-variant elements. In particular, a gain-varying element (i.e., an analog multiplier) can be used in conjunction with an integrator to realize arbitrary weighting functions, and therefore the theoretically maximum signal-to-noise ratio. The system is time-variant only for the noise and not for the signal, so that it does not require high precision of the time-variant element. The system output is independent of the gating interval, and does not require precise timing. A method for evaluation of such systems in terms of noise, ballistic deficit and sensitivity to parameter variations is given.

Time-of-flight velocity analysis of atomic and molecular beams

Abstract: Conditions required for resolving a given beam speed distribution were evaluated by calculating the time-of-flight (TOF) signal for a finite open time of the beam shutter. Design criteria for a beam chopper and detection system are discussed in terms of the resolution, the range of speeds to be measured, and the optimum signal to noise ratio. A TOF system for detection of high intensity molecular beams with large speed ratios, as well as for low intensity scattered beams, is described. Experimental results are presented.

Adaptive Combining of Wideband Array Data for Optimal Reception

Abstract: A study of several closely related adaptive processors for array data has been completed. Processors were designed to converge approximately to a minimum-variance linear unbiased estimator of an unknown signal common to all elements of the array. Any spatial structure of the background noise is used by such a system to enhance the output signal-to-noise ratio. Possible areas of application include sonar, underwater communication, space communication, and seismology. The basic linear adaptive processor has variable coefficients adjusted by a rule similar to that for the minimization by the projection gradient method of a quadratic form which is subject to a linear constraint. Modifications of the basic adjustment procedure have been introduced to reduce system sensitivity to data anomalies, decrease computational requirements, and decrease memory requirements. Experimental evaluation of the adaptive array processors has been completed using data from an actual array of seismometers. Both transient recovery from initially poor processor coefficients and steady-state operation have been found quite satisfactory.

The clipping loss in correlation detectors for arbitrary input signal-to-noise ratios

Abstract: The detection of stationary Gaussian signals in a background of stationary Gaussian noise by the analog multiplier correlator, the polarity coincidence correlator (PCC), and the sample polarity coincidence correlator (SPCC) is considered. It is assumed that signal and noise have identical normalized autocovariance functions, and they are not cross correlated with each other. The main contributions of this paper are the exact expressions for the output signal-to-noise ratio (SNR) for the correlators mentioned for all values of the input SNR. It is shown that there exists a critical value of the input SNR, such that, whenever this value is exceeded, the PCC output SNR exceeds that of the analog correlator. A sufficient condition for this gain in output SNR is obtained in terms of the input SNR. This result is illustrated for stationary Gauss-Markov processes.

Automatic video level control employing iris and amplifier gain adjustments

Abstract: The amplitude of a signal generated by a video pickup tube is controlled by employing an automatic iris at high illumination levels and an automatic gain control circuit at low illumination levels. The crossover point between iris and gain control is selected so that the gain circuit operates as long as increased illumination and decreased gain improve the signal to noise ratio and picture quality. Above this point, signal level is maintained by an automatic iris which improves the depth of field as illumination increases. Both functions are controlled by sampling a selected field of the output from the pickup tube.

Influence of Bandwidth Restriction on the Signal-to-Noise Performance of a PCM/NRZ Signal

Abstract: The influence of bandwidth restriction on the performance of a PCM transmission system is treated. In particular, the relationship is investigated between signal-to-noise, bandwidth, and bit-error probability of an NRZ (non-return-to-zero) signal. The detector used in the investigation contains a device that integrates the signal over the bit period. Theoretical results were obtained by a Fourier analysis of bandwidth-restricted signals and by an autocorrelation analysis of the bandwidth-restricted noise. Theoretical and experimental results are in good agreement.

Elimination of speckle noise in holograms with redundancy

Abstract: Holograms made of diffusely reflecting or diffusely illuminated objects can be scratched, spotted with dirt, and even broken into pieces without serious loss of information. This remarkable property is due to the redundancy introduced by diffuse illumination which, in effect, spreads information all over the hologram. Unfortunately, an unwanted by-product of diffuse illumination is speckle noise. This noise becomes more severe as hologram size is reduced. We show that, in order to obtain an acceptably high signal-to-noise ratio, the area of this type hologram must be more than 100 times the area needed to achieve a desired image resolution, making it prohibitively large for most data storage applications. In this paper we described a multibeam recording technique that produces redundant holograms that yield speckle free images of transparencies. We show that (1) achievable redundancy is equal to the ratio of actual hologram area to that area which just satisfies resolution requirements, (2) a two-dimensional phase grating provides a simple, efficient means for generating multiple beams, an optimized grating producing nine equally intense beams with 81% efficiency, (3) an optimized phase grating having a spatial period just equal to desired image resolution yields fourfold redundancy in holograms that are just large enough to encompass both side bands of the central (or zero order) beam, and (4) holograms having fourfold redundancy give reasonably good immunity to dust and scratches.

Variance Analysis of the Angle Output Voltage for an Amplitude Monopulse Receiver

Abstract: The amplitude-comparison monopulse receiver utilizes two overlapping antenna patterns to determine the angle of arrival of an incoming RF signal for use in a direction-finding application. The thermal noise introduced by the receiver distorts the signal, causing an error in determining the exact angle of arrival of the signal. The analysis derives an expression for the deviation of the angle output voltage, or the angular error, due to receiver noise as a function of 1) the angle of arrival and 2) the signal-to-noise ratios of the two channels of the receiver. A receiver using a square-law detector and one using a linear detector are analyzed.

Common Bandwidth Tranmission of Information Signals and Pseudonoise Synchronization Waveforms

Abstract: This paper describes methods for the simultaneous transmission of information signals and pseudonoise synchronization waveforms in a common bandwidth. For a relatively low level of the pseudonoise signal, a satisfactorily high synchronization accuracy is obtained by means of correlation techniques. Conversely, the information signal is detected with negligible interference due to the synchronization waveform. This permits a new approach to synchronization in speech systems and data transmission. The application to speech systems is quite straightforward. Emphasis is placed on design considerations for narrow-band systems with regard to output signal-to-noise ratio and acquisition performance. In data transmission the method is applied for bit and word synchronization. Thus, no restrictions are imposed on the transmitted data patterns and no inclusion of special synchronization words is required. In addition, the bandwidth of the system is not increased. The high accuracy required for synchronization in data systems is obtained here by a pseudoternary precoding of the transmitted data which is adapted to the spectral properties of the synchronization cross correlator. In this way the influence of the data signal on the received synchronization signal is minimized. Calculations of synchronization accuracy are given which include the effect of transmission noise.

Communication at Low Date Rates--Spectral Analysis Receivers

Abstract: The purpose of this paper is to introduce a multiple frequency shift keying spectral analysis receiver based on the fast Fourier transform. A novel decision-directed automatic frequency control (AFC) for use with the fast Fourier receiver is mentioned. Not only is the system to be described better than a truncated autocorrelation receiver, the analysis required is unorthodox, being based on the fact that the spectral coefficients of the incoming waveform are chi-square (central or noncentral) distributed. The fast Fourier receiver, under typical conditions, is 1.5 dB better than the autocorrelation receiver, is more versatile (e.g., tone spacing to multiples of 1/T Hz, spectral window shaping very easy), easier to implement, less subject to signal-to-noise ratio degradation due to frequency offset, and provides fine frequency information to operate a decision-directed AFC.

On the Optimum Design of PCM Signals with System Constraints

Abstract: This paper considers the design of PCM signals with system constraints using Pontryagin's maximum principle. The transmitter signal as well as the correlation signal are determined while maximizing the output signal-to-noise ratio. Both the single and three-pole transmitter filters are considered. An upper bound has been obtained on the performance of PCM signals with constraints.

Methods for improving the signal-to-noise ratio of photon and electron beam accessed magnetic-film memory systems

Abstract: Methods are described for reducing shot noise in magnetic-film memories which are to be read magnetooptically with the auxiliary aid of an electron beam. In such memories the principal source of shot noise is from the light which is used to illuminate the memory array. The following methods of reducing array shot noise are considered: 1) selective background, 2) magnetooptical balance, 3) temperature control of magnetooptical spectra in rare-earth iron garnets (REIG). It is concluded that the use of REIG spectra appears to offer not only the best but also a satisfactory solution to the array shot-noise problem.

A recursive approach to signal detection

Abstract: The classical problem of detecting known signals in additive nonstationary nonwhite noise is treated using state-variable methods. The noise is assumed to be generated as the solution of a linear time-varying differential equation driven by white noise. The Markov property of the noise is used to derive a difference equation for the likelihood ratio. The terms in the difference equation are given directly in terms of the coefficients of the differential equation defining the noise. These results, being in recursive form, reduce considerably the computational effort when discrete samples are used. The signal-to-noise ratio is also derived. Explicit expressions for the likelihood ratio are also obtained for continuous processing. These results are given directly in terms of the signals and the noise-generating equation. Thus, there are no integral equations to solve. The signal-to-noise ratio is calculated explicitly for both stationary and nonstationary noise.

PSK Transmissions Through Ideal Bandpass Limiters

Abstract: Two problems related to passing signals through an ideal bandpass filter are treated in this paper. The transmission of a biphase modulated signal in Gaussian noise is solved for the asymptotically high and low values of output signal-to-noise ratio (SNR). The asymptotic solutions for high and low values of output signal to distortion ratio are given for the case in which two independent biphase modulated signals (on separate carriers) are transmitted in a noiseless environment.

Backscatter Limitations in Active Night-Vision Systems

Abstract: : This study explores atmospheric backscatter as a fundamental limitation in using an artificial light source for night viewing. A mathematical model of an active night-vision system is constructed, and formulas are derived for the signal received from a target and the backscatter caused by the intervening atmosphere. Signal-to-noise ratio is defined, and its use as a measure of system performance is discussed. Several hypothetical systems are evaluated, and graphs of the signal-to-noise ratio as a function of range are presented. Additional graphs show the power that would be required by the systems to achieve a given signal-to-noise ratio as a function of range. (Author)

An evaluation of the use of high-resolution wavenumber spectra for ambient-noise analysis

Abstract: : A principal objective of the network studies is to characterize the worldwide ambient-noise field. An excellent tool for such studies is the frequency-wavenumber spectrum of the ambient noise recorded by an array station. Since all network array stations were of small aperture, conventional techniques for computing wavenumber spectra could not achieve the wavenumber resolution required to resolve and accurately locate multiple noise sources, especially bodywave noise sources. To establish the procedure to use for computing the spectra, the experiment described in this report was performed prior to the computation of ambient-noise wavenumber spectra for the network studies. Analysis of ambient seismic noise using the high-resolution technique is shown to be valid. Wavenumber spectral estimates made from unsmoothed crosspower spectra appear more stable, less sensitive to SNR, and computationally much simpler than those made from smoothed crosspower spectra. The summing of several MCF wavenumber power responses, each designed with respect to a different reference sensor, stabilizes the wavenumber estimate. The summing of individual spectral estimates, i.e., the reciprocals of the MCF wavenumber responses, yields better resolution of coherent peaks relative to background noise levels.

A Precision Frequency Measurement Technique for a Moving Signal with a Low Signal-to-Noise Ratio

Abstract: A technique for measuring the frequency of the RF carrier from a spacecraft, at a low signal to noise ratio, was developed for the Mariner V S-band occultation experiment. The technique consisted of analog recording of the noisy signal, simultaneously with a high-frequency tone. This tone was used as a clock to trigger a digitizer. After digitization, spectra were taken at several points to permit fitting a polynomial to the frequency. Digital filtering techniques, in which the signal was moved, in frequency, to stay in the filter bandpass, were then applied. The filtered data were then processed by a digital phase-locked receiver program that gave amplitude and frequency versus time as outputs. Several iterations were required to reach thelevel of -190 dBm at which point the signal was lost.

A Note on Sampling of Signal and Partially Correlated Noise

Abstract: Signal-to-noise improvement in sampled data systems is a function of the number of samples and the degree of correlation of signal and noise samples. This correspondence demonstrates the power of the autocorrelation function in establishing the effectiveness of digital filtering, and provides a method for determining signal-to-noise improvement factors.

Stereo TV enhancement study Final technical report

Abstract: Stereotelevision versus nonstereotelevision effectiveness for target acquisition in extraterrestrial environment with poor signal to noise ratio

Preliminary beamforming study of the tfo-37 array

Abstract: : Beamforming of the TFO-37 array reduces the rms of the noise up to 14 db over an average single sensor, and the signal/noise improvement approaches 14 db also depending on the band pass filter used. The signal loss after beamforming and summing is approximately 1 db. Power spectra was also performed on the noise after summation as well as on individual traces and showed a 15 db reduction at 1 cps; this is an improvement of N over an average single element. Maps, digital programs, instrument frequency response, etc. are included.

Large-Array Signal and Noise Analysis. Location Statistics for Frequency-Wavenumber Processing

Abstract: : Results of a computer study which theoretically determined the detection and location capabilities of various wavenumber spectral techniques are presented in this report. Two distinct short-period array configurations were simulated. The first configuration simulated was a 13-element array employing the outputs of the inner 13 subarrays of the Montana LASA. In this case, the ambient noise was considered to be completely uncorrelated. The second configuration simulated consisted of all or a portion of a standard LASA subarray. Various noise models, made up of mostly organized noise, were used.

Hyperfine beamsteering using a signal crosscorrelation technique

Abstract: : A method is developed to eliminate automatically the loss in signal amplitude of LASA phased sums with travel-time anomaly corrections due to small mis-alignments in the signal arrivals. This method is a hyper-fine or vernier adjustment of the trace alignments, making use of a least-squares estimate of the misalignments made from the time lags of all possible crosscorrelations between the signals. A mathematical description of the procedure and its errors is given. Three events with different signal-to-noise ratios recorded by 11 LASA subarrays were chosen as examples of the method. The results show improvements ranging from 0.0 to 0.5 db in the phased sum of the subarray sums. The results seem to depend on the signal-to-noise ratio of the event, the method performing best when that ratio is high.

Target detection performance with sonar displays of different sizes.

Abstract: : Target detection performance was determined for sonar PPI displays having diameters of 3 in., 6 in., 9 in., 12 in., and 14 in. It was found that performance is equivalent with 6 in. and 9 in. displays; performance with either is superior to that with the smaller (3 in.) display and the larger (12 in. and 14 in.) displays. (Author)