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At least 343 records · Page 19

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NASA NTRS1976

Investigation of television transmission using adaptive delta modulation principles

The results are presented of a study on the use of the delta modulator as a digital encoder of television signals. The computer simulation of different delta modulators was studied in order to find a satisfactory delta modulator. After finding a suitable delta modulator algorithm via computer simulation, the results were analyzed and then implemented in hardware to study its ability to encode real time motion pictures from an NTSC format television camera. The effects of channel errors on the delta modulated video signal were tested along with several error correction algorithms via computer simulation. A very high speed delta modulator was built (out of ECL logic), incorporating the most promising of the correction schemes, so that it could be tested on real time motion pictures. Delta modulators were investigated which could achieve significant bandwidth reduction without regard to complexity or speed. The first scheme investigated was a real time frame to frame encoding scheme which required the assembly of fourteen, 131,000 bit long shift registers as well as a high speed delta modulator. The other schemes involved the computer simulation of two dimensional delta modulator algorithms.

Schilling, D. L.↗

NASA NTRS1976

The fast decoding of Reed-Solomon codes using number theoretic transforms

It is shown that Reed-Solomon (RS) codes can be encoded and decoded by using a fast Fourier transform (FFT) algorithm over finite fields. The arithmetic utilized to perform these transforms requires only integer additions, circular shifts and a minimum number of integer multiplications. The computing time of this transform encoder-decoder for RS codes is less than the time of the standard method for RS codes. More generally, the field GF(q) is also considered, where q is a prime of the form K x 2 to the nth power + 1 and K and n are integers. GF(q) can be used to decode very long RS codes by an efficient FFT algorithm with an improvement in the number of symbols. It is shown that a radix-8 FFT algorithm over GF(q squared) can be utilized to encode and decode very long RS codes with a large number of symbols. For eight symbols in GF(q squared), this transform over GF(q squared) can be made simpler than any other known number theoretic transform with a similar capability. Of special interest is the decoding of a 16-tuple RS code with four errors.

Reed, I. S.↗

NASA NTRS1976

Atmospheric temperature profiles derived through the inversion of a system of first order differential equations

Generation of vertical temperatures profiles from remotely sensed atmospheric radiance data is described as an analogous communications system. The radiative transport characteristics of the atmosphere encodes the continuous temperature profile into an 'n' element vector where 'n' is the number of channels in the satellite instrument. The temperature profile is a message transmitted from station A to station B and the link is the satellite instrument. At station B the decoder reproduces a continuous function which is the best estimate of the message encoded at station A. It is shown that the decoder must operate in a tuned mode where the parameters used in the encoder precisely determine the decoder parameters, and that the characteristics of the total message block must be given by a set of decoder constraints

Gatlin, J. A.↗

NASA NTRS1977

A programmable microcomputer for real time speech processing

The implementation of a programmable microprocessor for real time speech processing is described. The design of a fast special-purpose computer operating at 150 nsec per instruction time reduces the problem of speech encoding and decoding to a software problem. Each instruction is a twelve-bit word, and the information conveyed by a word is explained. The microprocessor is controlled by two clocks - the A/D clock, or input sampling clock, which loads the digital input into the input register and presets the counter to a value set by the external switches; and the instruction clock, or system clock, which operates at 6 MHz. At this system clock rate and a sampling clock rate of 30 KHz, 200 instructions can be executed between samples, and for many speech encoding and decoding algorithms, 200 instructions are more than enough. The microcomputer is being used to test various delta modulator encoding algorithms.

Apelewicz, T.↗

NASA NTRS1979

A connection between block and convolutional codes

Convolutional codes of any rate and any constraint length give rise to a sequence of quasi-cyclic codes. Conversely, any quasi-cyclic code may be convolutionally encoded. Among the quasi-cyclic codes are the quadratic residue codes, Reed-Solomon codes and optimal BCH codes. The constraint length K for the convolutional encoding of many of these codes (Golay, (48, 24) QR, etc.) turns out to be surprisingly small. Thus using the soft decoding techniques for convolutional decoding, a new maximum likelihood decoding algorithm for many block codes is established. Conversely an optimal quasi-cyclic code will yield a convolutional encoding with optimal local properties and therefore with good infinite convolutional coding properties.

Solomon, G.↗

NASA NTRS1981

Radiographic applications of spatial frequency multiplexing

The application of spacial frequency encoding techniques which allow different regions of the X-ray spectrum to be encoded on conventional radiographs was studied. Clinical considerations were reviewed, as were experimental studies involving the encoding and decoding of X-ray images at different energies and the subsequent processing of the data to produce images of specific materials in the body.

Macovski, A.↗

NASA NTRS1982

Modeling of the Mode S tracking system in support of aircraft safety research

This report collects, documents, and models data relating the expected accuracies of tracking variables to be obtained from the FAA's Mode S Secondary Surveillance Radar system. The data include measured range and azimuth to the tracked aircraft plus the encoded altitude transmitted via the Mode S data link. A brief summary is made of the Mode S system status and its potential applications for aircraft safety improvement including accident analysis. FAA flight test results are presented demonstrating Mode S range and azimuth accuracy and error characteristics and comparing Mode S to the current ATCRBS radar tracking system. Data are also presented that describe the expected accuracy and error characteristics of encoded altitude. These data are used to formulate mathematical error models of the Mode S variables and encoded altitude. A brief analytical assessment is made of the real-time tracking accuracy available from using Mode S and how it could be improved with down-linked velocity.

Sorensen, J. A.↗

NASA NTRS1983

Effects of ISI and satellite channel nonlinearity on Viterbi demodulation/decoding

It is shown that, for convolutionally coded transmission over bandwidth-constrained channels, a mere reversal of the switching direction at the encoder output produces a change in the system bit error probability performance. This change is significant when the Viterbi demodulator/decoder is matched to the total channel memory and is more significant for linear channels than for nonlinear ones. The reversal of switching direction is a simple demonstration of the fact that the well-known optimum codes for the linear AGWN channel are no longer necessarily optimum for a bandwidth-constrained channel with or without the addition of channel nonlinearity. It is concluded that potentially significant performance improvement can be obtained by matching the encoder (through the appropriate choice of tap weights and modulo-2 summers) to the channel in addition to matching the demodulator/decoder to the channel for a given encoder.

Divsalar, D.↗

NASA NTRS1984

A high-accuracy optical linear algebra processor for finite element applications

Optical linear processors are computationally efficient computers for solving matrix-matrix and matrix-vector oriented problems. Optical system errors limit their dynamic range to 30-40 dB, which limits their accuray to 9-12 bits. Large problems, such as the finite element problem in structural mechanics (with tens or hundreds of thousands of variables) which can exploit the speed of optical processors, require the 32 bit accuracy obtainable from digital machines. To obtain this required 32 bit accuracy with an optical processor, the data can be digitally encoded, thereby reducing the dynamic range requirements of the optical system (i.e., decreasing the effect of optical errors on the data) while providing increased accuracy. This report describes a new digitally encoded optical linear algebra processor architecture for solving finite element and banded matrix-vector problems. A linear static plate bending case study is described which quantities the processor requirements. Multiplication by digital convolution is explained, and the digitally encoded optical processor architecture is advanced.

Casasent, D.↗

NASA NTRS1987

Synchronization trigger control system for flow visualization

The use of cinematography or holographic interferometry for dynamic flow visualization in an internal combustion engine requires a control device that globally synchronizes camera and light source timing at a predefined shaft encoder angle. The device is capable of 0.35 deg resolution for rotational speeds of up to 73 240 rpm. This was achieved by implementing the shaft encoder signal addressed look-up table (LUT) and appropriate latches. The developed digital signal processing technique achieves 25 nsec of high speed triggering angle detection by using direct parallel bit comparison of the shaft encoder digital code with a simulated angle reference code, instead of using angle value comparison which involves more complicated computation steps. In order to establish synchronization to an AC reference signal whose magnitude is variant with the rotating speed, a dynamic peak followup synchronization technique has been devised. This method scrutinizes the reference signal and provides the right timing within 40 nsec. Two application examples are described.

Chun, K. S.↗

NASA NTRS1988

Quantile Vocoder

Parameters of spectral envelope obtained statistically. Algorithm for digital compression of speech signals encodes power spectral density of each short interval of speech by use of quantiles or order statistics. Purpose to reduce bit rate and bandwidth required for transmission. When fully developed, quantile vocoder - speech-encoding system based on new algorithm - expected moderately complicated compared with other speech-encoding systems and reproduce high-quality speech from code transmitted at relatively low bit rates. Speech signal treated mathematically as though amplitude spectrum stationary during short coding intervals, called "windows". Window duration of 20 or 35 ms, chosen as compromise between frequency resolution and time resolution. During each window, short-time amplitude and power spectra found by sampling at high rate (typically 10 kHz) and taking fast Fourier transforms (FFT's).

Swaminathan, Kumar↗

NASA NTRS1988

Monolithic GaAs digitizer for space-based laser pulse spreading effect

A 6-bit 1-GHz digitizer was designed to analyze the 1-ns pulse spreading effects in a space based altimeter. The digitizer consisted of four 4-bit flash A/D converters and a 6-bit encoder. Also, the converter utilized four 4-bit converters and a 4-to-6 bit encoder to achieve 6 bit resolution at the 1 GHz sample rate. The design was unique because it utilized only the inverters and NOR gates for the converters and encoder, hence it could be fabricated using the existing state-of-the-art GaAs processing techniques. This GHz digitizer has many commercial applications. It could be applicable to: (1) digital microwave transmission system for the telecommunication industries, (2) pulse monitoring in high kinetic chemical reactions, (3) transient signals in the medical field, and (4) microwave signals in astronomy.

Lao, Bennig↗

NASA NTRS1991

Hidden-Markov-Model Analysis Of Telemanipulator Data

Mathematical model and procedure based on hidden-Markov-model concept undergoing development for use in analysis and prediction of outputs of force and torque sensors of telerobotic manipulators. In model, overall task broken down into subgoals, and transition probabilities encode ease with which operator completes each subgoal. Process portion of model encodes task-sequence/subgoal structure, and probability-density functions for forces and torques associated with each state of manipulation encode sensor signals that one expects to observe at subgoal. Parameters of model constructed from engineering knowledge of task.

Hannaford, Blake↗

NASA NTRS1990

The Cortex Transform as an image preprocessor for sparse distributed memory: An initial study

An experiment is described which was designed to evaluate the use of the Cortex Transform as an image processor for Sparse Distributed Memory (SDM). In the experiment, a set of images were injected with Gaussian noise, preprocessed with the Cortex Transform, and then encoded into bit patterns. The various spatial frequency bands of the Cortex Transform were encoded separately so that they could be evaluated based on their ability to properly cluster patterns belonging to the same class. The results of this study indicate that by simply encoding the low pass band of the Cortex Transform, a very suitable input representation for the SDM can be achieved.

Olshausen, Bruno↗

NASA NTRS1991

An edge preserving differential image coding scheme

Differential encoding techniques are fast and easy to implement. However, a major problem with the use of differential encoding for images is the rapid edge degradation encountered when using such systems. This makes differential encoding techniques of limited utility especially when coding medical or scientific images, where edge preservation is of utmost importance. We present a simple, easy to implement differential image coding system with excellent edge preservation properties. The coding system can be used over variable rate channels which makes it especially attractive for use in the packet network environment.

Rost, Martin C.↗

NASA NTRS1992

Nonlinear, nonbinary cyclic group codes

New cyclic group codes of length 2(exp m) - 1 over (m - j)-bit symbols are introduced. These codes can be systematically encoded and decoded algebraically. The code rates are very close to Reed-Solomon (RS) codes and are much better than Bose-Chaudhuri-Hocquenghem (BCH) codes (a former alternative). The binary (m - j)-tuples are identified with a subgroup of the binary m-tuples which represents the field GF(2 exp m). Encoding is systematic and involves a two-stage procedure consisting of the usual linear feedback register (using the division or check polynomial) and a small table lookup. For low rates, a second shift-register encoding operation may be invoked. Decoding uses the RS error-correcting procedures for the m-tuple codes for m = 4, 5, and 6.

Solomon, G.↗

NASA NTRS1990

Real-time optical correlator using computer-generated holographic filter on a liquid crystal light valve

Limitations associated with the binary phase-only filter often used in optical correlators are presently circumvented in the writing of complex-valued data on a gray-scale spatial light modulator through the use of a computer-generated hologram (CGH) algorithm. The CGH encodes complex-valued data into nonnegative real CGH data in such a way that it may be encoded in any of the available gray-scale spatial light modulators. A CdS liquid-crystal light valve is used for the complex-valued CGH encoding; computer simulations and experimental results are compared, and the use of such a CGH filter as the synapse hologram in a holographic optical neural net is discussed.

Chao, Tien-Hsin↗

NASA NTRS1992

Bandwidth compression of the digitized HDTV images for transmission via satellites

This paper investigates a subband coding scheme to reduce the transmission bandwidth of the digitized HDTV images. The HDTV signals are decomposed into seven bands. Each band is then independently encoded. The based band is DPCM encoded and the high bands are encoded by using nonuniform Laplacian quantizers with a dead zone. By selecting the dead zone on the basis of energy in the high bands an acceptable image quality is achieved at an average of 45 Mbits/sec (Mbps) rate. This rate is comparable to some very hardware intensive schemes of transform compression or vector quantization proposed in the literature. The subband coding scheme used in this study is considered to be of medium complexity. The 45 Mbps rate is suitable for transmission of HDTV signals via satellites.

Al-Asmari, A. KH.↗