Microwave photonic transversal filter for intermodal dispersion equalisation

An experimental demonstration is given of the use of a transversal RF photonic filter to accomplish intermodal dispersion equalisation in a 10 Gbit/s optical fibre link using a Fabry-Perot laser. The structure operates in an equivalent fashion to an electronic equaliser but in the photonic domain. A record distance of 24.2 km using the equalising filter is obtained.     

High-performance digitally programmable analogue transversal filter

An electrically programmable transversal filter is presented, based on a tapped analogue delay line. The experimental results presented here highlight the large dynamic range capability of the filter structure. The approach is highly suitable for monolithic implementation using standard c.t.d. technology.     

Efficient adaptive complex filtering algorithm with application tochannel equalisation

The paper describes a means of efficiently implementing an adaptive complex transversal filter. Three real-coefficient filter sections are used to realise the transversal filter section of a complex equaliser, thus using one less filter than a conventional realisation. An adaptive algorithm is developed, in a manner similar to the least mean square algorithm, which allows the three filters to be trained independently and in parallel using real valued arithmetic. In this way, the throughput can be maintained, whilst reducing the number of multipliers in both the filter and coefficient update sections by 25%. Using independence theory, it is demonstrated that the three filters converge to a solution consistent with the optimal Wiener-Hopf solution. The convergence speed is characterised in terms of the complex input data stream. The transient behaviour of the algorithm is examined using a simulation of a channel equaliser and is supported by analysis     

Dynamic gain equalisation filter based on integrated optical transversal filter with asymmetric combiner

A compact dynamic gain equalisation filter on a silica-based planar lightwave circuit has been fabricated. The filter is based on the authors' newly developed optical transversal filter that uses an asymmetric combiner. This device flattened the amplified spontaneous emission spectrum of an erbium-doped fibre amplifier to a ripple of /spl sim/1 dB over a wavelength range of 30 nm.     

Circuit noise reduction by analogue lowpass filtering and dataaveraging

All electrical signals are, to some extent, contaminated by noise. Two popular techniques for eliminating noise are analogue lowpass filtering and data averaging. Here, consideration is given to the cutoff frequency of analogue lowpass filters, and the minimum number of data samples needed to be averaged in order to reduce noise with known autocorrelation function within a predefined limit. The time delays introduced by both analogue lowpass filtering and data averaging are given. Finally, the signal-to-noise ratio, the averaging technique used in a capacitance/resistance spectroscopic tomography system design and their effect on measurement accuracy and data acquisition rate are analysed     

Integrated MOS offset error canceller for analogue adaptive transversal filter

An offset error canceller based on the switched-capacitor technique for realisation of analogue LSIs is developed using a short-channel MOSFET process. Excellent characteristics, such as exceptional capability of suppression of the DC offset component ranging from -3 V to +3 V, large dynamic range of 83 dB and fast operation speed of 2 MHz clock rate, are obtained.     

Monolithic, programmable analogue c.c.d. transversal filter

A 64-point monolithic, fully programmable analogue c.c.d. transversal filter has been produced. The design principles and limitations of this approach are discussed, and results are presented for the filter acting as a chirp-waveform correlator and for other filtering functions.     

Multi-component filtering of magnetic signals

This paper presents two treatments of real signals recorded by an immersed passive array during an underwater magnetic detection experiment. In this situation, the signal-to-noise ratio is very low, and the noise characteristics are close to the characteristics of the signal that is to be detected. Methods that estimate the signal by minimizing the mean-square distance between the signal and its estimate (using a Wiener filter) are optimal, but some a priori knowledge must first be available before these methods can be used. In this specific application, this a priori knowledge can only be acquired by analyzing the data, and a straightforward application of the optimal method may be misleading. Two filtering methods are therefore implemented to estimate a vector valued signal in a noisy observation. These filtering methods are simple and robust, and they give good results on real data. One method is an adaptation of Wiener filtering. The other consists of global filtering using the eigen-structure of the spectral density matrix of the received signals. Results for each method are presented and compared.     

Phase filtering with time reversal

Correction of phase shift by time-reversal recording is discussed from a unified viewpoint, including considerations of physical realisability, time delay and a systematic approach to practical application.     

Fast subsampled-updating stabilized fast transversal filter (FSUSFTF) RLS algorithm for adaptive filtering

We present a new, doubly fast algorithm for recursive least-squares (RLS) adaptive filtering that uses displacement structure and subsampled-updating. The fast subsampled-updating stabilized fast transversal filter (FSU SFTF) algorithm is mathematically equivalent to the classical fast transversal filter (FTF) algorithm. The FTF algorithm exploits the shift invariance that is present in the RLS adaptation of an FIR filter. The FTF algorithm is in essence the application of a rotation matrix to a set of filters and in that respect resembles the Levinson (1947) algorithm. In the subsampled-updating approach, we accumulate the rotation matrices over some time interval before applying them to the filters. It turns out that the successive rotation matrices themselves can be obtained from a Schur-type algorithm that, once properly initialized, does not require inner products. The various convolutions that appear In the algorithm are done using the fast Fourier transform (FFT). The resulting algorithm is doubly fast since it exploits FTF and FFTs. The roundoff error propagation in the FSU SFTF algorithm is identical to that in the SFTF algorithm: a numerically stabilized version of the classical FTF algorithm. The roundoff error generation, on the other hand, seems somewhat smaller. For relatively long filters, the computational complexity of the new algorithm is smaller than that of the well-known LMS algorithm, rendering it especially suitable for applications such as acoustic echo cancellation     

Phase errors in transversal and recursive filters realised with charge-transfer devices

It is shown that nonideal amplitude transfer in charge-transfer devices can cause large phase errors in transversal and recursive filters using these elements.     

Two new equalisation schemes for high-density digital magnetic recording systems with quad-phase modulation code

Quad-phase has desirable features for use in digital magnetic recording. The authors propose two novel equalisation schemes that seem attractive at high information densities. One scheme estimates the even and odd data bits in reverse order, while the other uses two equalisers to estimate even and odd bits separately.<>     

Voltage-transfer equalisation of general dissipative loads

This letter is concerned with the solution of the problem of synthetising equaliser networks for general dissipative loads when the voltage-transfer function is given as a function of the complex-frequency variable p=?+j?. The necessary and sufficient conditions are given for both lossless and dissipative equaliser networks.     

Hilbert-transformer side-chain phase equaliser for analogue magnetic recording

A substantial amount of phase distortion occurring in analogue magnetic recording can be eliminated by using a simple equalisation scheme which removes the Hilbert-transformed portion of the signal that is added in a normal record/reproduce process. In the phase equaliser, the performance of an ideal Hilbert transformer can be approximated by using a quadrature-phase-shift circuit. The methods of phase equalisation is explained theoretically and oscilloscope photographs of experimental results are presented.     

Colour histogram equalisation of multichannel images

Histogram equalisation works well on single-channel images; however, the technique used is ineffective on multiple channels because there are an infinite number of ways to perform the equalisation. A technique is developed that works on images with any number of channels, and the inadequacy of a direct extension of histogram equalisation to multiple dimensions is shown. A logarithmic metric is developed to express how well the available histogram volume is utilised and how well equalisation is working. Results of using the technique and using it in reverse, i.e. to reduce the histogram utilisation, are shown. Extensions of the technique are used to measure the difference between and conform one multichannel histogram to another, and uses of these techniques are discussed.     

Bilateral filtering of diffusion tensor magnetic resonance images.

We extend the well-known scalar image bilateral filtering technique to diffusion tensor magnetic resonance images (DTMRI). The scalar version of bilateral image filtering is extended to perform edge-preserving smoothing of DT field data. The bilateral DT filtering is performed in the Log-Euclidean framework which guarantees valid output tensors. Smoothing is achieved by weighted averaging of neighboring tensors. Analogous to bilateral filtering of scalar images, the weights are chosen to be inversely proportional to two distance measures: The geometrical Euclidean distance between the spatial locations of tensors and the dissimilarity of tensors. We describe the noniterative DT smoothing equation in closed form and show how interpolation of DT data is treated as a special case of bilateral filtering where only spatial distance is used. We evaluate different recent DT tensor dissimilarity metrics including the Log-Euclidean, the similarity-invariant Log-Euclidean, the square root of the J-divergence, and the distance scaled mutual diffusion coefficient. We present qualitative and quantitative smoothing and interpolation results and show their effect on segmentation, for both synthetic DT field data, as well as real cardiac and brain DTMRI data.     

Cutoff wavelength determination in single-mode optical fibres by measurement of equalisation wavelength

A simple interferometric method for indirectly determining a value for the cutoff wavelength of single-mode optical fibres is described. The technique is accurate and no stringent requirements are placed on the experimental equipment.     

Delay equalisation of vestigialsideband waveform transmission systems

The well known conditions for distortionless transmission through a vestigial-sideband (v.s.b.) system, i.e. that the v.s.b. shaping network should have amplitude and phase characteristics that are skew symmetric about the carrier frequency, form only one set from an infinite number of possible sets of conditions. A practical example shows that use of an alternative set of conditions proposed by Saraga considerably reduces the complexity of delay equalisation necessary when television or similar waveforms are to be transmitted. However, consideration of the effect of phase errors in the demodulating carrier signal and of noise in a practical system shows that, in these respects at least, conventional v.s.b. transmission is superior.     

Adaptive non-linear equalisation of digital communications channels

Non-linear distortion introduced by communications channels increases the probability of error. In this paper, an adaptive non-linear canceller is presented which attempts to remove non-linear interference from the output of a feedforward or decision feedback equaliser, improving the error rate. It may be implemented using a memory look-up table architecture, making it suitable for high speed real time operation.