Method for superimposing data on amplitude-modulated signals

The principles of a new modulation scheme are described which permit an additional data channel to be superimposed on conventional DSBAM signals without increasing band-width or envelope distortion. This scheme, named zero synchronous frequency modulation (ZSFM), requires additional signal processing and provision for combined AM/FM at the transmitter. At the receiver, the data signal is detected by a simple frequency demodulator, while envelope detection of the AM signal is performed in the usual way.     

Design and performance of robust embedded ADPCM coder

An embedded adaptive DPCM (EADPCM) speech coder is described which allows bit rate reductions to be achieved by progressive deletion of bits from output codewords. Optimised step size multipliers are given for a robust implementation using an improved algorithm for adaptive quantisation. Simulation shows that a graceful reduction in speech quality with bit rate is achieved in the range 16-48 kbit/s.<>     

Selective discarding procedure for improved tolerance to missing voice packets

A selective packet discarding procedure for voice communication is presented in which packets are labelled at the transmitter and, if necessary, discarded by network nodes according to their significance to reconstructed speech quality. Simulation and subjective tests show improved tolerance to missing packets in comparison with unselective methods where packets are discarded at random.<>     

Nonhierarchical communications networks: an application ofcompartmental modeling

Compartmental modeling is applied to nonhierarchical circuit-switched networks allowing a formulation of network dynamics in terms of a system of deterministic equations among global variables. Network dynamics under stress conditions of overload and damage are examined, and steady-state results demonstrate the expected bistability property. Network controls are implemented with the result that bistable behavior disappears. The model equations are sufficiently complex as to permit investigation of the relative merits of various network management controls and to capture time-sensitive effects, such as user calling behavior and time-varying network stress. Optimal control parameter setting is addressed for network steady-state conditions. While the model assumes a deterministic environment and trunk group blocking only, the model can be extended to include stochastic environments and switch related effects, such as queue overflow and delay     

Design and simulation of an efficient adaptive delta modulationembedded coder

An embedded coding version of hybrid companding delta modulation (HCDM) is described that operates from 16 to 48 kb/s in 8 kb/s steps. The embedded HCDM coder employs the explicit noise coding technique to transmit an adaptive PCM (APCM) coded version of the HCDM reconstruction error signal as a supplementary bit stream that may be partly or wholly deleted in transmission. SNR performance with speech input depends critically on the design of the supplemental APCM code and two new coding algorithms are investigated. In algorithm 1, the basic cue for step size adaptation is obtained from the RMS slope energy of the HCDM output whereas in algorithm 2, the HCDM reconstruction error is logarithmically compressed before quantisation and the basic step size is derived from peak input magnitudes. Instantaneous adaptation for both algorithms is achieved by using step size multipliers which are optimised for operation at single fixed bit rates and also for decoding with an unknown number of input bit deletions. Simulation results show that SNR performance is significantly enhanced using either algorithm and a graceful reduction of reconstructed speech quality with progressive bit deletion is achieved over the range from 48 kb/s to 16 kb/s. On the whole, the SNR performance of the embedded HCDM system is superior in comparison with conventional HCDM     

A convenient synthesis of copper(I) carboxylates

A simple procedure is described for the synthesis of aliphatic and aromatic copper(I) carboxylates by the reduction of copper(II) carboxylates with commercially available tin(II) 2-ethylhexanoate. In this way copper(I) acetate, benzoate and stearate have been prepared in good yield and purity from the corresponding copper(II) carboxylates.     

Conformationally Programmable Chiral Foldamers with Compact and Extended Domains Controlled by Monomer Structure

Foldamers are an important class of abiotic macromolecules, with potential therapeutic applications in the disruption of protein–protein interactions. The majority adopt a single conformational motif such as a helix. A class of foldamer is now introduced where the choice of heterocycle within each monomer, coupled with a strong conformation‐determining dipole repulsion effect, allows both helical and extended conformations to be selected. Combining these monomers into hetero‐oligomers enables highly controlled exploration of conformational space and projection of side‐chains along multiple vectors. The foldamers were rapidly constructed via an iterative deprotection‐cross‐coupling sequence, and their solid‐ and solution‐phase conformations were analysed by X‐ray crystallography and NMR and CD spectroscopy. These molecules may find applications in protein surface recognition where the interface does not involve canonical peptide secondary structures.     

Ligand-exchange chemistry of Cr13-polyacrylamide gels

Evidence that ligand-induced de-gelation of Cr(+3)/polyacrylamide (PAAm) gels involves simple extraction of Cr(+3) from the gel matrix has been obtained from rheological studies. Kinetic experiments provide evidence that de-gelation involves dissociative ligand exchange at Cr(+3). On the basis of de-gelation results acetate is suggested to be a poor thermodynamic model for the bonding of PAAm to Cr(+3) and it is hypothesized that Cr(+3)/PAAm complexation may involve bonding to both a polymer carboxylate and a neighboring amide group.     

Kinetic study of the OH + ethylene reaction using frequency-modulated laser absorption spectroscopy

The pressure dependence of the OH + C₂H₄ addition reaction has been investigated using frequency-modulated laser absorption spectroscopy to monitor OH kinetics. Bimolecular rate coefficients for the title reaction are reported in argon bath gas at room temperature and total pressures ranging from 2 to 361 Torr. The pressure-dependent rate coefficients measured here agree well with the majority of published kinetic studies under similar conditions. Previous high-level ab initio calculations have identified a prereaction complex on the OH + C₂H₄ potential energy surface. The influence of this complex on the OH + C₂H₄ kinetics has been investigated using one-dimensional master equation analyses of the current and previous experimental measurements.     

Rheological studies of the gelation kinetics of Cr+3-polyacrylamide solutions

A new rheological approach for the study of the gelation kinetics of aqueous Cr⁺³ polyacrylamide solutions has been investigated which relies on the assumption that at the gel point, GP, a constant amount of Cr⁺³ will have reacted, independent of the initial Cr⁺³ concentration. From this, a kinetic expression relating GP to the initial concentrations of Cr⁺³ and reactive groups on the polymer has been derived for the case of a crosslinking reaction first order in Cr⁺³ and polymer. Experimental evidence that the amount of Cr⁺³ reacted at the GP is constant for a given polymer and solution pH has been obtained. The good fit of the kinetic expression to the rheological data provides new evidence that the gelation reaction is first order in Cr⁺³ rather than second order as claimed previously.