Showing papers by "Bell Labs published in 1982"

Least squares quantization in PCM

Abstract: It has long been realized that in pulse-code modulation (PCM), with a given ensemble of signals to handle, the quantum values should be spaced more closely in the voltage regions where the signal amplitude is more likely to fall. It has been shown by Panter and Dite that, in the limit as the number of quanta becomes infinite, the asymptotic fractional density of quanta per unit voltage should vary as the one-third power of the probability density per unit voltage of signal amplitudes. In this paper the corresponding result for any finite number of quanta is derived; that is, necessary conditions are found that the quanta and associated quantization intervals of an optimum finite quantization scheme must satisfy. The optimization criterion used is that the average quantization noise power be a minimum. It is shown that the result obtained here goes over into the Panter and Dite result as the number of quanta become large. The optimum quautization schemes for 2^{b} quanta, b=1,2, \cdots, 7 , are given numerically for Gaussian and for Laplacian distribution of signal amplitudes.

Least Squares Quantization in PCM

Abstract: It has long been realized that in pulse-code modulation (PCM), with a given ensemble of signals to handle, the quantum values should be spaced more closely in the voltage regions where the signal amplitude is more likely to fall. It has been shown by Panter and Dite that, in the limit as the number of quanta becomes infinite, the asymptotic fractional density of quanta per unit voltage should vary as the one-third power of the probability density per unit voltage of signal amplitudes. In this paper the corresponding result for any finite number of quanta is derived; that is, necessary conditions are found that the quanta and associated quantization intervals of an optimum finite quantization scheme must satisfy. The optimization criterion used is that the average quantization noise power be a minimum. It is shown that the result obtained here goes over into the Panter and Dite result as the number of quanta become large. The optimum quautization schemes for 2^{b} quanta, b=1,2, \cdots, 7 , are given numerically for Gaussian and for Laplacian distribution of signal amplitudes.

Spectrum estimation and harmonic analysis

Abstract: In the choice of an estimator for the spectrum of a stationary time series from a finite sample of the process, the problems of bias control and consistency, or "smoothing," are dominant. In this paper we present a new method based on a "local" eigenexpansion to estimate the spectrum in terms of the solution of an integral equation. Computationally this method is equivalent to using the weishted average of a series of direct-spectrum estimates based on orthogonal data windows (discrete prolate spheroidal sequences) to treat both the bias and smoothing problems. Some of the attractive features of this estimate are: there are no arbitrary windows; it is a small sample theory; it is consistent; it provides an analysis-of-variance test for line components; and it has high resolution. We also show relations of this estimate to maximum-likelihood estimates, show that the estimation capacity of the estimate is high, and show applications to coherence and polyspectrum estimates.

Two-Dimensional Magnetotransport in the Extreme Quantum Limit

Abstract: A quantized Hall plateau of ${\ensuremath{\rho}}_{\mathrm{xy}}=\frac{3h}{{e}^{2}}$, accompanied by a minimum in ${\ensuremath{\rho}}_{\mathrm{xx}}$, was observed at $Tl5$ K in magnetotransport of high-mobility, two-dimensional electrons, when the lowest-energy, spin-polarized Landau level is $\frac{1}{3}$ filled. The formation of a Wigner solid or charge-density-wave state with triangular symmetry is suggested as a possible explanation.

Theory of the linewidth of semiconductor lasers

Abstract: A theory of the spectral width of a single-mode semiconductor laser is presented and used to explain the recent measurements of Fleming and Mooradian on AlGaAs lasers. They found the linewidth to be inversely proportional to power and to have a value of 114 MHz at 1 mW per facet. This value is 30 times greater than can be explained by existing theories. The enhanced linewidth is attributed to the variation of the real refractive index n' with carrier density. Spontaneous emission induces phase and intensity changes in the laser field. The restoration of the laser to its steady-state intensity results in changes in the imaginary part of the refractive index \Delta n" . These changes are accompanied by changes in the real part of the refractive index \Delta n' , which cause additional phase fluctuations and line broadening. The linewidth enhancement is shown to be 1 + \alpha^{2} , where \alpha = \Delta n'/\Delta n" . A value of \alpha \approx 5.4 , needed to explain the observed linewidth, is close to the experimental values of a of 4.6 and 6.2.

Pseudopotentials that work: From H to Pu

Abstract: Recent developments have enabled pseudopotential methods to reproduce accurately the results of all-electron calculations for the self-consistent electronic structure of atoms, molecules, and solids. The properties of these potentials are discussed in the context of earlier approaches, and their numerous recent successful applications are summarized. While the generation of these pseudopotentials from all-electron atom calculations is straightforward in principle, detailed consideration of the differences in physics of various groups of atoms is necessary to achieve pseudopotentials with the most desirable attributes. One important attribute developed here is optimum transferability to various systems. Another is the ability to be fitted with a small set of analytic functions useful with a variety of wave-function representations. On the basis of these considerations, a consistent set of pseudopotentials has been developed for the entire Periodic Table. Relativistic effects are included in a way that enables the potentials to be used in nonrelativistic formulations. The scheme used to generate the numerical potentials, the fitting procedure, and the testing of the fit are discussed. Representative examples of potentials are shown that display attributes spanning the set. A complete tabulation of the fitted potentials is given along with a guide to its use.

Hidden structure in liquids

Abstract: The canonical partition function for classical many-body systems is transformed so that the temperature-independent packing statistics and the thermal excitations are uniquely separated. This requires classification of particle configurations according to multidimensional potential-energy minima that can be reached by steepest-descent paths ("quenches"). Such classifications have been constructed for several starting configurations in the solid, fluid, and coexistence phases of the two-dimensional Gaussian core model. These quenches reveal a remarkable degree of polycrystalline order hidden within the fluid phase by "vibrational" distortion, and that order appears to have a large correlation length. The results suggest that melting hinges upon defect softening in the quenched packings, and a crude "theory" of melting for the Gaussian core model is developed in the Appendix.

Parametric amplification and frequency conversion in optical fibers

Abstract: We find that the parametric four-photon gain for light pulses decreases for fibers longer than a characteristic length. This length is related to the common experimental observation that stimulated parametric emission is usually prominent only in short fibers while in long fibers stimulated Raman scattering dominates. Despite the fact that the actual process involves an intensity dependent bandwidth and broadening of the pump linewidth from self-phase modulation, it is possible to develop a simple expression for the characteristic length which requires only the initial pump linewidth and the low-power parametric bandwidth. This bandwidth can often be estimated from the pump wavelength and the measured frequency shift between the pump and the generated waves. Expressions for gain and amplification are derived from coupled wave equations and in the Appendixes it is shown that these are of the same form as the planewave equations, but modified by coupling coefficients called overlap integrals.

A new model of LPC excitation for producing natural-sounding speech at low bit rates

Abstract: The excitation for LPC speech synthesis usually consists of two separate signals - a delta-function pulse once every pitch period for voiced speech and white noise for unvoiced speech. This manner of representing excitation requires that speech segments be classified accurately into voiced and unvoiced categories and the pitch period of voiced segments be known. It is now well recognized that such a rigid idealization of the vocal excitation is often responsible for the unnatural quality associated with synthesized speech. This paper describes a new approach to the excitation problem that does not require a priori knowledge of either the voiced-unvoiced decision or the pitch period. All classes of sounds are generated by exciting the LPC filter with a sequence of pulses; the amplitudes and locations of the pulses are determined using a non-iterative analysis-by-synthesis procedure. This procedure minimizes a perceptual-distance metric representing subjectively-important differences between the waveforms of the original and the synthetic speech signals. The distance metric takes account of the finite-frequency resolution as well as the differential sensitivity of the human ear to errors in the formant and inter-formant regions of the speech spectrum.

Approximating a Point Process by a Renewal Process, I: Two Basic Methods

Abstract: This paper initiates an investigation of simple approximations for stochastic point processes. The goal is to develop methods for approximately describing complex models such as networks of queues and multiechelon inventory systems. The proposed approach is to decouple or decompose the model by replacing all the component flows point processes by independent renewal processes. Here attention is focused on ways to approximate a single point process by a renewal process. This is done in two steps: First, properties of the point process are used to specify a few moments of the interval between renewals; then a convenient distribution is fit to these moments. Two different methods are suggested for specifying the moments of the renewal interval. The stationary-interval method equates the moments of the renewal interval with the moments of the stationary interval in the point process to be approximated. The asymptotic method, in an attempt to account for the dependence among successive intervals, determines the moments of the renewal interval by matching the asymptotic behavior of the moments of the sums of successive intervals. These two procedures are applied to approximate the superposition merging of point processes. The purpose here is to provide a better understanding of these procedures and a general framework for making new approximations. In particular, the two basic procedures can be used as building blocks to construct refined composite procedures. Composite procedures for the ∑Gi/G/1 queue with a superposition arrival process are discussed by Albin in Part II. Albin has developed a hybrid procedure for approximating the mean sequence length and other characteristics in the ∑Gi/G/1 queue for which the average error when compared with simulated values was 3% over a large number of test systems.

Linear Pulse Propagation in an Absorbing Medium

Abstract: The pulse velocity in the linear regime in samples of GaP: N with a laser tuned to the bound $A$-exciton line is measured with use of a picosecond time-of-flight technique. The pulse is seen to propagate through the material with little pulse-shape distortion, and with an envelope velocity given by the group velocity even when the group velocity exceeds 3\ifmmode\times\else\texttimes\fi{}${10}^{10}$ cm/sec, $\mathrm{equals}\ifmmode\pm\else\textpm\fi{}\ensuremath{\infty}$, or becomes negative. The results verify the predictions of Garrett and McCumber.

Smectic liquid crystals

Abstract: The use of a certain class of liquid crystal materials that exhibit a smectic C phase allows the production of a bistable liquid crystal display element. Such bistable display elements promote the use of matrix addressing for liquid crystal based elements in a display.

High-speed compact circuits with CMOS

Abstract: Characteristics of various CMOS and NMOS circuit techniques are described, along with the shortcomings of each. Then a new circuit type, the CMOS domino circuit is described. This involves the connection of dynamic CMOS gates in such a way that a single clock edge can be used to turn on all gates in the circuit at once. As a result, complex clocking schemes are not needed and the full inherent speed of the dynamic gate can be utilized. The circuit is most valuable where gates are complex and have high fan-out such as in arithmetic units. Examples are shown of the use of domino circuits in an 8-bit ALU, where simulations indicate a speed advantage of 1.5 to 2 over traditional circuits, and in a 32-bit ALU where a worst case add in 124 ns was projected and a time less than 100 ns was achieved.

Synthesis of communicating processes from temporal logic specifications

Abstract: In this paper, we apply Propositional Temporal Logic (PTL) to the specification and synthesis of the synchronization part of communicating processes. To specify a process, we give a PTL formula that describes its sequence of communications. The synthesis is done by constructing a model of the given specifications using a tableau-like satisfiability algorithm for PTL. This model can then be interpreted as a program.

Fast quantizing and decoding and algorithms for lattice quantizers and codes

Abstract: For each of the lattices A_{n}(n \geq 1), D_{n}(n \geq 2), E_{6}, E_{7}, E_{8} , and their duals a very fast algorithm is given for finding the closest lattice point to an arbitrary point. If these lattices are used for vector quantizing of uniformly distributed data, the algorithm finds the minimum distortion lattice point. If the lattices are used as codes for a Gaussian channel, the algorithm performs maximum likelihood decoding.

Radiation Damping in Surface-Enhanced Raman Scattering

Abstract: Theoretical calculations are presented which show that the enhancement predicted by the particle plasmon model of surface-enhanced Raman scattering is limited by radiation damping. The damping becomes more severe as particle size increases, while the enhancement produced by small particles is limited by surface scattering. Good agreement between theory and experimental measurements of the wavelength dependence of surface-enhanced Raman scattering on lithographically produced microstructures is found when radiation damping is taken into account.

Dynamic generation and overlaying of graphic windows for multiple active program storage areas

Abstract: A graphic terminal is disclosed using bitmaps to represent plural overlapping displays. Graphics software is also disclosed in which the overlapping asynchronous windows or layers are manipulated by manipulating the bitmaps. With this software, the physical screen becomes several logical screens (layers) all running simultaneously, any one of which may be interacted with at any time.

Measurement of the density of gap states in hydrogenated amorphous silicon by space charge spectroscopy

Abstract: We report a comprehensive study of the dynamic response of junction space-charge layers in undoped and P${\mathrm{H}}_{3}$-doped $a$-Si:H films grown by the rf-glow-discharge technique. By using the numerical analysis methods discussed in the adjoining theory paper, we are able consistently to interpret a variety of transient response and ac admittance measurements in terms of a bulk density of gap states $g(E)$ which is characteristic of each sample. While the general shape of $g(E)$ seems to be a characteristic property of $a$-Si:H, the overall concentration of gap states depends on growth conditions and doping. The density of states at approximately midgap is observed to vary between values as low as about 2\ifmmode\times\else\texttimes\fi{}${10}^{15}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ e${\mathrm{V}}^{\ensuremath{-}1}$ in undoped films and as high as 1\ifmmode\times\else\texttimes\fi{}${10}^{18}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ e${\mathrm{V}}^{\ensuremath{-}1}$ in some P${\mathrm{H}}_{3}$-doped films. The general shape of our $g(E)$ is dominated by a deep minimum (${10}^{16}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ e${\mathrm{V}}^{\ensuremath{-}1}$) between 0.3 and 0.6 eV from the conduction band and a broad shoulder of states extending from the valence band up to midgap. The significant difference between this type of bulk $g(E)$ and previous models for the density of states in $a$-Si:H may be explained by the effects of states at or near the surface of the films which strongly influence the previous types of measurements. We discuss recent transport and optical measurements and show that they provide strong support for our density of states as opposed to previous models for $g(E)$.

The kinetic chemistry of dense interstellar clouds

Abstract: A model of the time-dependent chemistry of dense interstellar clouds is formulated to study the dominant chemical processes in carbon and oxygen isotope fractionation, the formation of nitrogen-containing molecules, and the evolution of product molecules as a function of cloud density and temperature. The abundances of the dominant isotopes of the carbon- and oxygen-bearing molecules are calculated. The chemical abundances are found to be quite sensitive to electron concentration since the electron concentration determines the ratio of H3(+) to He(+), and the electron density is strongly influenced by the metals abundance. For typical metal abundances and for H2 cloud density not less than 10,000 molecules/cu cm, nearly all carbon exists as CO at late cloud ages. At high cloud density, many aspects of the chemistry are strongly time dependent. Finally, model calculations agree well with abundances deduced from observations of molecular line emission in cold dense clouds.

Voronoi regions of lattices, second moments of polytopes, and quantization

Abstract: If a point is picked at random inside a regular simplex, octahedron, 600 -cell, or other polytope, what is its average squared distance from the centroid? In n -dimensional space, what is the average squared distance of a random point from the closest point of the lattice A_{n} (or D_{n}, E_{n}, A_{n}^{\ast} or D_{n}^{\ast})? The answers are given here, together with a description of the Voronoi (or nearest neighbor) regions of these lattices. The results have applications to quantization and to the design of signals for the Gaussian channel. For example, a quantizer based on the eight-dimensional lattice E8 has a mean-squared error per symbol of 0.0717 \cdots when applied to uniformly distributed data, compared with 0.08333 \cdots for the best one-dimensional quantizer.

Predictive Coding of Speech at Low Bit Rates

Abstract: Predictive coding is a promising approach for speech coding. In this paper, we review the recent work on adaptive predictive coding of speech signals, with particular emphasis on achieving high speech quality at low bit rates (less than 10 kbits/s). Efficient prediction of the redundant structure in speech signals is obviously important for proper functioning of a predictive coder. It is equally important to ensure that the distortion in the coded speech signal be perceptually small. The subjective loudness of quantization noise depends both on the short-time spectrum of the noise and its relation to the short-time spectrum of the Speech signal. The noise in the formant regions is partially masked by the speech signal itself. This masking of quantization noise by speech signal allows one to use low bit rates while maintaining high speech quality. This paper will present generalizations of predictive coding for minimizing subjective distortion in the reconstructed speech signal at the receiver. The quantizer in predictive coders quantizes its input on a sample-by-sample basis. Such sample-by-sample (instantaneous) quantization creates difficulty in realizing an arbitrary noise spectrum, particularly at low bit rates. We will describe a new class of speech coders in this paper which could be considered to be a generalization of the predictive coder. These new coders not only allow one to realize the precise optimum noise spectrum which is crucial to achieving very low bit rates, but also represent the important first step in bridging the gap between waveform coders and vocoders without suffering from their limitations.

Intensity discrimination of optical pulses with birefringent fibers.

Abstract: An intensity discriminator for optical pulses can be made with a birefringent fiber. Such a discriminator would be useful for separating the intense subpicosecond pulses formed by solitonlike compression from the weaker uncompressed background. The discriminator utilizes an intensity-dependent state of polarization out of the fiber.

An Operational Approach to Requirements Specification for Embedded Systems

Abstract: The approach to requirements specification for embedded systems described in this paper is called "operational" because a requirements specification is an executable model of the proposed system interacting with its environment. The approach is embodied by the language PAISLey, which is motivated and defined herein. Embedded systems are characterized by asynchronous parallelism, even at the requirements level; PAISLey specifications are constructed by interacting processes so that this can be represented directly. Embedded systems are also characterized by urgent performance requirements, and PAISLey offers a formal, but intuitive, treatment of performance.

Crystallization Rates of a Lennard-Jones Liquid

Abstract: The crystallization of a Lennard-Jones liquid has been studied with molecular-dynamics techniques. In particular, the motion of the fcc (100) interface has been measured for a range of temperatures below the melting point. The crystallization rates are not limited by the mobility of atoms in the bulk liquid; measurable rates are even observed below the glass transition temperature. This result suggests the existence of a class of materials which is qualitatively different from conventional glass-forming materials.