Showing papers by "Bell Labs published in 1974"
TL;DR: The authors compare the Fourier algorithm and a search algorithm using a simulated phantom to speed the search algorithm by using fewer interactions leaves decreased resolution in the region just inside the skull which could mask a subdural hematoma.
Abstract: The Fourier reconstruction may be viewed simply in the spatial domain as the sum of each line integral times a weighting function of the distance from the line to the point of reconstruction A modified weighting function simultaneously achieves accuracy, simplicity, low computation time, as well as low sensitivity to noise Using a simulated phantom, the authors compare the Fourier algorithm and a search algorithm The search algorithm required 12 iterations to obtain a reconstruction of accuracy and resolution comparable to that of the Fourier reconstruction, and was more sensitive to noise To speed the search algorithm by using fewer interactions leaves decreased resolution in the region just inside the skull which could mask a subdural hematoma
2,100 citations
TL;DR: Estimation of the parameters of a single-frequency complex tone from a finite number of noisy discrete-time observations is discussed and appropriate Cramer-Rao bounds and maximum-likelihood estimation algorithms are derived.
Abstract: Estimation of the parameters of a single-frequency complex tone from a finite number of noisy discrete-time observations is discussed. The appropriate Cramer-Rao bounds and maximum-likelihood (MI.) estimation algorithms are derived. Some properties of the ML estimators are proved. The relationship of ML estimation to the discrete Fourier transform is exploited to obtain practical algorithms. The threshold effect of one algorithm is analyzed and compared to simulation results. Other simulation results verify other aspects of the analysis.
1,878 citations
TL;DR: The system has been designed to optimize the collection of data to give higher than usual accuracy in measuring the individual elements and final performance of assembled diffraction limited optical systems, and furthermore, the short loop time of a few minutes makes the system an attractive alternative to constraints imposed by test glasses in the optical shop.
Abstract: A self-scanned 1024 element photodiode array and minicomputer are used to measure the phase (wavefront) in the interference pattern of an interferometer to lambda/100. The photodiode array samples intensities over a 32 x 32 matrix in the interference pattern as the length of the reference arm is varied piezoelectrically. Using these data the minicomputer synchronously detects the phase at each of the 1024 points by a Fourier series method and displays the wavefront in contour and perspective plot on a storage oscilloscope in less than 1 min (Bruning et al. Paper WE16, OSA Annual Meeting, Oct. 1972). The array of intensities is sampled and averaged many times in a random fashion so that the effects of air turbulence, vibrations, and thermal drifts are minimized. Very significant is the fact that wavefront errors in the interferometer are easily determined and may be automatically subtracted from current or subsequent wavefrots. Various programs supporting the measurement system include software for determining the aperture boundary, sum and difference of wavefronts, removal or insertion of tilt and focus errors, and routines for spatial manipulation of wavefronts. FFT programs transform wavefront data into point spread function and modulus and phase of the optical transfer function of lenses. Display programs plot these functions in contour and perspective. The system has been designed to optimize the collection of data to give higher than usual accuracy in measuring the individual elements and final performance of assembled diffraction limited optical systems, and furthermore, the short loop time of a few minutes makes the system an attractive alternative to constraints imposed by test glasses in the optical shop.
1,206 citations
TL;DR: It is concluded that in artificial copper proteins as well as in the naturally occurring nonblue copper proteins copper is ligated to oxygen and nitrogen but not to sulfur.
1,180 citations
TL;DR: The nature and implementation of the file system and of the user command interface are discussed, including the ability to initiate asynchronous processes and over 100 subsystems including a dozen languages.
Abstract: UNIX is a general-purpose, multi-user, interactive operating system for the Digital Equipment Corporation PDP-11/40 and 11/45 computers. It offers a number of features seldom found even in a larger operating systems, including: (1) a hierarchical file system incorporating demountable volumes; (2) compatible file, device, and inter-process I/O; (3) the ability to initiate asynchronous processes; (4) system command language selectable on a per-user basis; and (5) over 100 subsystems including a dozen languages. This paper discusses the nature and implementation of the file system and of the user command interface.
1,140 citations
TL;DR: In this article, a simple rectangular potential well with a depth of ≈088ΔE g, where ΔE g is the difference in the semiconductor energy gaps is defined.
Abstract: Quantum levels associated with the confinement of carriers in very thin, molecular-beam—grown AlxGa1−xAs−GaAs-Alx Ga1−xAs heterostructures result in pronounced structure in the GaAs optical absorption spectrum Up to eight resolved exciton transitions, associated with different bound-electron and bound-hole states, have been observed The heterostructure behaves as a simple rectangular potential well with a depth of ≈088ΔE g , for confining electrons and ≈012ΔE g for confining holes, where ΔE g is the difference in the semiconductor energy gaps
976 citations
TL;DR: In this paper, necessary and sufficient conditions under which a random variable X may be generated as the ratio ZI V where Z and V are independent and Z has a standard normal distribution are presented.
Abstract: SUMMARY This paper presents necessary and sufficient conditions under which a random variable X may be generated as the ratio ZI V where Z and V are independent and Z has a standard normal distribution. This representation is useful in Monte Carlo calculations. It is established that when 7 V2 is exponential, X is double exponential; and that when WV has the asymptotic distribution of the Kolmogorov distance statistic, X is logistic.
936 citations
TL;DR: An algorithm and related theory is described for a general number K of node-disjoint paths with minimum total length, which applies shortest path labeling algorithms familiar in the literature.
Abstract: Routes between two given nodes of a network are called diversified if they are node-disjoint, except at the terminals. Diversified routes are required for reliability in communication, and an additional criterion is that their total cost, assumed to be the sum of individual arc lengths or costs, is minimum. An algorithm and related theory is described for a general number K of node-disjoint paths with minimum total length. The algorithm applies shortest path labeling algorithms familiar in the literature. K node-disjoint paths are found in K iterations of a single shortest path algorithm.
662 citations
TL;DR: A technique for producing bandpass linear amplification with nonlinear components (LINC) is described, which produces an amplified and/or translated replica of the original signal.
Abstract: A technique for producing bandpass linear amplification with nonlinear components (LINC) is described. The bandpass signal first is separated into two constant envelope component signals. All of the amplitude and phase information of the original bandpass signal is contained in phase modulation on the component signals. These constant envelope signals can be amplified or translated in frequency by amplifiers or mixers which have nonlinear input-output amplitude transfer characteristics. Passive linear combining of the amplified and/or translated component signals produces an amplified and/or translated replica of the original signal.
654 citations
TL;DR: The superconducting phase transition is predicted to be weakly first order, because of effects of the intrinsic fluctuating magnetic field, according to a Wilson-Fisher $\ensuremath{\epsilon}$expansion analysis, as well as a generalized mean-field calculation appropriate to a type-I superconductor.
Abstract: The superconducting phase transition is predicted to be weakly first order, because of effects of the intrinsic fluctuating magnetic field, according to a Wilson-Fisher $\ensuremath{\epsilon}$-expansion analysis, as well as a generalized mean-field calculation appropriate to a type-I superconductor. Similar results hold for the phase transition from a smectic-$A$ to a nematic liquid crystal.
653 citations
TL;DR: In this article, the weak nonexponential relaxation of alloys was explained in terms of a cluster reaction and diffusion process, and the experimental observability of this slowing down was discussed.
Abstract: The weak nonexponential relaxation $\ensuremath{\propto}{t}^{\ensuremath{-}{a}^{\ensuremath{'}}}$ recently found in computer experiments on the phase separation of alloys is explained in terms of a cluster reaction and diffusion process. The nonlinear features of this process can be accounted for by a time-dependent diffusion constant. Estimates for the resulting exponents [${a}^{\ensuremath{'}}=\frac{1}{(3+d)}$] are consistent with the computer simulations. The experimental observability of this slowing down is discussed.
TL;DR: In this paper, the authors show that techniques of fitting are robust of efficiency when their statistical efficiency remains high for conditions more realistic than the utopian cases of Gaussian distributions with errors of equal variance.
Abstract: Techniques of fitting are said to be resistant when the result is not greatly altered in the case a small fraction of the data is altered: techniques of fitting are said to be robust of efficiency when their statistical efficiency remains high for conditions more realistic than the utopian cases of Gaussian distributions with errors of equal variance. These properties are particularly important in the formative stages of model building when the form of the response is not known exactly. Techniques with these properties are proposed and discussed.
TL;DR: In this article, the formation and stability of glassy alloys of compositions (Pd1−xMx)0.835Si0.165, Pd 1−xTx)1−xpPxp and (Pt 1 −xNix) 1−expPxp, have been investigated.
TL;DR: In this paper, the anomalous properties of polytypes of charge-density waves, their periodic structure distortions, and the superlattices they induce are attributed to charge density waves.
Abstract: The previously termed "anomalous" properties of the various polytypes of ${d}^{1}$ Ta${\mathrm{S}}_{2}$, Ta${\mathrm{Se}}_{2}$, etc. are attributed to charge-density waves, their periodic structure distortions, and the superlattices they induce.
TL;DR: In this article, a semi-empirical method for calculating the room temperature refractive index of Ga 1− x Al x As at energies below the direct band edge is presented.
TL;DR: Results suggest that phonological representations play a significant role in visual word recognition and that there is a dependence between successive phonemic-encoding operations.
Abstract: Previous investigators have argued that printed words are recognized directly from visual representations and/or phonological representations obtained through phonemic recoding. The present research tested these hypotheses by manipulating graphemic and phonemic relations within various pairs of letter strings. Ss in two experiments classified the pairs as words or nonwords. Reaction times and error rates were relatively small for word pairs (e.g., BRIBE-TRIBE) that were both graphemically, and phonemically similar. Graphemic similarity alone inhibited performance on other word pairs (e.g., COUCH-TOUCH). These and other results suggest that phonological representations play a significant role in visual word recognition and that there is a dependence between successive phonemic-encoding operations. An encoding-bias model is proposed to explain the data.
TL;DR: In this paper, the surface scaling theory previously presented by the authors is derived further, and derived heuristically from a cluster model, and Monte Carlo calculations are carried out to obtain the spatial and temperature dependence of the magnetization in Ising and Heisenberg systems with free surfaces.
Abstract: The surface scaling theory previously presented by the authors is developed further, and derived heuristically from a cluster model. Monte Carlo calculations are carried out to obtain the spatial and temperature dependence of the magnetization in Ising and Heisenberg systems with free surfaces. The exponent ${\ensuremath{\beta}}_{1}$ of the (surface) layer magnetization is shown to agree with the scaling value (${\ensuremath{\beta}}_{1}\ensuremath{\approx}\frac{2}{3}$) previously derived. In the Heisenberg system, the results at low temperature agree with a spin-wave calculation by Mills and Maradudin. Ising models with modified exchange ${J}_{s}=J(1+\ensuremath{\Delta})\ensuremath{
e}J$ on the surface are considered, both in mean-field theory and by means of high-temperature-series expansions. The critical value ${\ensuremath{\Delta}}_{c}$ for surface ordering is found from the series to be 0.6, compared to the mean-field value of 0.25. For $\ensuremath{\Delta}g{\ensuremath{\Delta}}_{c}$ there is a temperature region in which the surface behaves like a bulk two-dimensional Ising model near its phase transition. The critical exponents experience a crossover at $\ensuremath{\Delta}={\ensuremath{\Delta}}_{c}$, which is reflected in poorly behaved series, and effective exponents differing from the true ones for $\ensuremath{\Delta}\ensuremath{\lesssim}{\ensuremath{\Delta}}_{c}$. In the case of weakened surface exchange ($0l{J}_{s}lJ$), the layer magnetization is shown to fit a linear temperature dependence over a large temperature range below ${T}_{c}$, thus providing a possible explanation for previous experiments. For sufficiently strong negative ${J}_{s}$, mean-field theory predicts that the surface will order antiferromagnetically while the bulk is ferromagnetic.
TL;DR: A simple expression for the threshold voltage of an IGFET is derived from a charge conservation principle which geometrically takes into account two-dimensional edge effects in this paper, which is valid for short and long-channel lengths.
Abstract: A simple expression for the threshold voltage of an IGFET is derived from a charge conservation principle which geometrically takes into account two-dimensional edge effects. The expression is derived for zero drain voltage and is valid for short and long-channel lengths. The dependence of the threshold voltage on the source and drain diffusion depth, r j , and channel length, L , is explicitly given. In the limit, L / r j → ∞, the threshold voltage equation reduces to the familiar expression for the long-channel case. The theory is compared with the measured threshold voltages on IGFET's fabricated with 1·4, 3·8 and 7·4 μm channel lengths. The dependence of the threshold voltage under backgate bias voltages ranging from zero to breakdown agrees closely with the theory.
TL;DR: In this article, the dielectric properties of single-phase ceramics in the TiO2-rich region of the BaO-TiO2 system were investigated.
Abstract: The dielectric properties of ceramics in the TiO2-rich region of the BaO-TiO2 system were investigated. In the composition range between BaTi4O9 and TiO2, another compound, Ba2Ti9O20, can be obtained when calcining and sintering conditions are controlled carefully. When dense and single-phase, this ceramic has excellent dielectric and physical properties. At 4 GHz, the dielectric K= 39.8, Q= 8000, and τK (temperature coefficient of dielectric constant) =−24 ± 2 ppm/°C.
TL;DR: In this paper an asymptotic (large signal-to-noise ratio) expression is derived for the error rate and it is rigorously proved in the Appendix that the optimum constellations tend toward an equilateral structure, and become uniformly distributed in a circle.
Abstract: A considerable amount of literature exists on the problem of selecting an efficient set of N digital signals with in-phase and quadrature components for use in a suppressed carrier data transmission system. However, the signal constellation which minimizes the probability of error in Gaussian noise, under an average power constraint, has not been determined when the number of signals is greater than two. In this paper an asymptotic (large signal-to-noise ratio) expression, of the minimum distance type, is derived for the error rate. Using this expression, a gradient-search procedure, which is initiated from several randomly chosen N -point arrays, converges in each case to a locally optimum constellation. The algorithm incorporates a radial contraction technique to meet the average signal power constraint. The best solutions are described for several values of N and compared with well-known signal formats. As an example, the best locally optimum 16-point constellation shows an advantage of about 0.5 dB in signal-signal-to-noise ratio over quadrature amplitude modulation. The locally optimum constellations are the vertices of a trellis of (almost) equilateral triangles. As N \rightarrow \infty , it is rigorously proved in the Appendix that the optimum constellations tend toward an equilateral structure, and become uniformly distributed in a circle.
TL;DR: In this paper, the authors generalized the renormalization-group method for studying critical phenomena to a class of dynamical systems, the time-dependent Ginzburg-Landau models.
Abstract: The renormalization-group method for studying critical phenomena is generalized to a class of dynamical systems---the time-dependent Ginzburg-Landau models. The effects of conservation laws on the critical dynamics are investigated through the study of models with different conservation properties for the energy and the space integral of the order parameter. Dynamic critical exponents near four dimensions ($d\ensuremath{\approx}4$) are obtained from recursion relations, analogous to those of Wilson and Fisher. The physical significance of the time-dependent Ginzburg-Landau models is explored and the applicability of the results to experiments on the NMR linewidth of Fe${\mathrm{F}}_{2}$ is discussed.
TL;DR: In this article, the stimulated-emission cross sections and fluorescence branching ratios for the principal fluorescence transitions of YAG:Nd emanating from the $^{4}F_{\frac{3}{2}}$ state have been measured at room temperature.
Abstract: The stimulated-emission cross sections and fluorescence branching ratios for the principal fluorescence transitions of YAG:Nd emanating from the $^{4}F_{\frac{3}{2}}$ state have been measured at room temperature. The peak cross section of the well-known 1064.2-nm laser line is here found to have a value of 4.6\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}19}$ ${\mathrm{cm}}^{2}$. Since the cross sections reported here are inconsistent with the generally accepted value of near unity for the radiative quantum efficiency $\ensuremath{\eta}$ of the $^{4}F_{\frac{3}{2}}$ state the quantity $\ensuremath{\eta}$ has been measured directly. A value of 0.56\ifmmode\pm\else\textpm\fi{}0.11 is found for $\ensuremath{\eta}$ which is consistent with the observed fluorescence lifetime of the $^{4}F_{\frac{3}{2}}$ state and the measured cross sections.
TL;DR: A computer program was devised to provide the phase and attenuation constants and wavefunctions for TE and TM modes in symmetric and asymmetric guides, illustrating the mismatch of modes at transitions between unclad and metal-clad waveguides.
Abstract: Planar optical waveguides consisting of thin dielectric films with metal cladding have been investigated theoretically and experimentally. A computer program was devised to provide the phase and attenuation constants and wavefunctions for TE and TM modes in symmetric and asymmetric guides. Approximate expressions suitable for slide-rule calculation were also derived. Numerical results and illustrations are given for films of photoresist with Al, Ag, and Au cladding. Direct measurements of the attenuation and phase constants at 0.633 microm of numerous experimental waveguides are in reasonable agreement with theory. Attenuations <1 dB/cm, which is sufficiently small for application in devices, were measured. Calculated wavefunctions illustrate the mismatch of modes at transitions between unclad and metal-clad waveguides. Experimentally, we find substantial losses at such abrupt junctions. They can be overcome by simple tapered transitions.
TL;DR: In this paper, a kinetic model involving changes in surface stoichiometry is proposed which gives quantitative agreement with experiment for both desorption rate and surface composition, assuming that As2 is adsorbed in a weakly-bound molecular precursor state from which dissociation into As atoms in As surface sites occurs Recombination of surface As atoms into the molecular state occurs at a rate whose activation energy decreases rapidly with increasing As concentration in the surface
TL;DR: An asymmetry measure is introduced to characterize thin-film optical waveguides that are asymmetric in refractive index and the minimum value of the effective guide thickness is found to be a simple function of wavelength and the film and substrate indices.
Abstract: An asymmetry measure is introduced to characterize thin-film optical waveguides that are asymmetric in refractive index. Together with the usual normalized frequency this allows the plotting of universal charts from which the guide cutoff, the effective guide index, and the effective guide thickness can be determined by the use of simple scaling rules. The minimum value of the effective guide thickness is found to be a simple function of wavelength and the film and substrate indices.
01 May 1974
TL;DR: It is pointed out that error waveforms in speech quantization cannot be regarded as additive white noise, in general, and that for finer assessments of speech coders, either relative or absolute, one needs to supplement SNR-based observations with corrections for subjective and perceptual factors.
Abstract: A study is presented on the digital coding of speech by means of a straightforward approximation of the time waveform. In particular, the closely related discrete-time discrete-amplitude signal representations that are rather well known as pulse-code modulation (PCM), differential pulse-code modulation (DPCM), and delta modulation (DM) are discussed. Speech is recognized as a nonstationary signal, and emphasis is therefore placed on "companding" and "adaptive" strategies for waveform quantization and prediction. With signal-to-quantization-error ratio SNR as a performance measure, techniques are suggested which are most likely to be appropriate for given specifications of information rate. It is pointed out that error waveforms in speech quantization cannot be regarded as additive white noise, in general. This means that for finer assessments of speech coders, either relative or absolute, one needs to supplement SNR-based observations with corrections for subjective and perceptual factors. The latter seem to defy quantification as a rule. Invaluable, therefore, are explicit preference tests for direct comparisons of coders from a perceptual standpoint, and notions such as isopreference and multidimensional scaling are naturally appropriate in interpreting the results of such tests. Final points of concern are communication questions such as multiple encodings of speech by tandem coder-decoder pairs; conversions among different digital code formats; and the effects of additive and multiplicative noise in the communication channel, as manifest in the erroneous reception of speech-carrying bits. Information on these topics tends to be heterogeneous and nontheoretical, and the present digression into the subject is cursory by intent. The gramophone record accompanying this paper demonstrates some of the manipulations of speech that are discussed.
TL;DR: In this article, a modified version of a previously studied algorithm was used to reconstruct the attenuation coefficients of a simulated head section from x-ray transmission measurements, which was then used to perform absolute measurements of attenuation coefficient at points interior to the skull.
Abstract: Several algorithms have been proposed for reconstructing the variable attenuation coefficients of tissues inside the skull from x-ray transmission measurements. We show that a simulated head section is accurately reconstructed by a modification, which is both fast and simple, of a previously studied algorithm. The possibility of using the algorithm to perform absolute measurements of the attenuation coefficient at points interior to the skull is high.
TL;DR: Observers identified a briefly flashed line segment more accurately when it was part of a drawing that looked unitary and three-dimensional than when the line was in one of several less coherent flat designs.
Abstract: Observers identified a briefly flashed line segment more accurately when it was part of a drawing that looked unitary and three-dimensional than when the line was in one of several less coherent flat designs.
TL;DR: In this paper, steady-state self-focusing, self-trapping, and self-defocusing of a cw dye laser beam in sodium vapor for frequencies within several Doppler widths of the $D$-line resonance transitions were reported.
Abstract: We report steady-state self-focusing, self-trapping, and self-defocusing of a cw dye laser beam in sodium vapor for frequencies within several Doppler widths of the $D$-line resonance transitions. We measured the variation of the beam profile as the light propagates through the vapor. We observed a 20-mW beam self-trapped in a 12-cm-long filament having a half-power diameter of 70 \ensuremath{\mu}m.
TL;DR: The results show that the principal path for the reversible thermal unfolding of tRNA 1 fMet under these solution conditions is first, transient opening of the dihydrouridine helix, followed by simultaneous melting of the diazepam helix and a “tertiary” interaction, which does not correspond to a cloverleaf helix.