Theodoor Antonius Care Claasen

Comparison of the convergence of two algorithms for adaptive FIR digital filters

The convergence properties of two different algorithms for the updating of the coefficients of an adaptive FIR digital filter are investigated and compared with one another. These algorithms are the stochastic iteration algorithm and the sign algorithm. In this latter algorithm a one-bit gradient estimation is used which makes its implementation very simple. The convergence is characterized by the residual echo variance after convergence, and a parameter that indicates the speed of the convergence. It is shown that the convergence of the sign algorithm can always be assured but is much slower than that of the stochastic iteration algorithm if the same variance of the residual echo is to be obtained.

The aliasing problem in discrete-time Wigner distributions

There is no straightforward way to proceed from the continuous-time Wigner distribution to a discrete-time version of this time-frequency signal representation. A previously given definition of such a function turned out to yield a distribution that was periodic with period π instead of 2π and this caused aliasing to occur. Various alternative definitions are considered and compared with respect to aliasing and computational complexity. From this comparison it appears that no definition leads to a function that is optimum in all respects. This is illustrated by an example.

Model for the power spectral density of quantization noise

A model is given for the spectrum of the noise produced by passing a signal through a uniform quantizer. The model requires knowledge only of the amplitude distribution of the derivative of this signal. The model is compared with experimental results for a sinusoidal input signal and the sum of two sinusoids. Conditions are given under which the quantization noise spectrum is white.

Design Considerations for a 144 kbit/s Digital Transmission Unit for the Local Telephone Network

In this paper a digital transmission unit for the forthcoming integrated services digital network (ISDN) is considered. Basic elements of such a unit are an adaptive echo canceller (EC) and an adaptive decision feedback equalizer (DFE). Each of these filters can be realized by a transversal filter structure or by a lookup table structure. An analysis and comparison of the behavior of these two structures is given, The combination of an EC and a DFE can also be realized by a single lookup table. It is shown that irrespective of the particular realization three different convergence phases can be distinguished, and that the overall convergence time is considerably longer then would be expected from the usual simple theory that ignores error propagation in the DFE. Two variants of a hardware realization of such a digital transmission unit are presented. They provide 144 kbit/s full-duplex transmission on the symmetrical cables of the existing local telephone network. A small transmission bandwidth is obtained by applying NRZ signaling. In the first circuit the EC and DFE are implemented by individual lookup tables, while in the second a combined table is used. Both circuits are to a large extent, implemented digitally. Due to the relatively low sampling frequency and the acceptably low power consumption they are very well suited for large scale integration with present day technology.

Design of Smearing Filters for Data Transmission Systems

In this paper, we introduce a new approach to design smearing and desmearing filters for data transmission systems to suppress pulse-like disturbances inserted in the communication channel. Our method is based on time domain considerations. It consists of the optimization of two merit factors defined for the impulse response functions of the filters. The first one is a measure of the smearing efficiency, and the second one is a measure of the amount of intersymbol interference caused by the insertion of the two filters in the transmission system. Our method will be applied to smearing and desmearing filters for baseband signals as well as for passband signals. In particular, we show that our method leads to filters having a better smearing efficiency and a simpler implementation than the filters obtained by the classical frequency domain approach.