We believe that the filtering problem for a digital local exchange is best studied by division into two parts. The quantisation noise filter design entails a trade-off between this filter and the type of coder. The complexity of the bandlimiting filter to achieve the CCITT specification is divided between the analogue prefilter and the second part of the digital filter. Several designs of digital filter are described which may be suitable for large scale integration.

Filtering for code conversion in digital telephone exchanges

A new approach to the implementation problem of digital filters is presented. This approach capitalizes on recent advances in semiconductor memory technology and is shown to offer significant reductions in cost and power consumption for the same speed of operation as that of existing realizations. Furthermore, this approach makes possible speeds of operation which cannot be achieved by existing realizations. The proposed approach yields a very flexible hardware configuration and a discussion of the various options is presented together with a comparison to existing realizations. The mean-squared error resulting from the use of finite word length is analyzed.

A new hardware realization of digital filters

In this paper a general theory of multistage decimators and interpolators for sampling rate reduction and sampling rate increase is presented. A set of curves and the necessary relations for optimally designing multistage decimators is also given. It is shown that the processes of decimation and interpolation are duals and therefore the same set of design curves applies to both problems. Further, it is shown that highly efficient implementations of narrow-band finite impulse response (FIR) filters can be obtained by cascading the processes of decimation and interpolation. Examples show that the efficiencies obtained are comparable to those of recursive elliptic filter designs.

Optimum FIR digital filter implementations for decimation, interpolation, and narrow-band filtering

Any digital filter can be decomposed into two basic subsets, an extrapolator the output of which is sampled at a frequency depending only on the filter bandwidth and an interpolator delivering the filtered signal at the imposed output sampling rate. Redundancy in extrapolator and interpolator is removed by introducing half-band nonrecursive filtering elements for which definition, performance figures and efficient implementation are supplied. They reduce significantly the necessary computation and storage at the cost of a slight group delay increase. A formula is given for the amount of multiplications to be carried out every second in a filter; it depends on the filter bandwidth, signal to distortion ratio, and input-output sampling rate. The method is extended to recursive filters and a comparison is made with existing techniques of implementing digital filters for the needs in computation and storage hardware: a specific example of design underlines the reduction in computation speed achieved in practice through this method, which brings digital filters in a most favorable position for their competition against analog filters in many application fields.

Filtering for code conversion in digital telephone exchanges

References

A new hardware realization of digital filters

Optimum FIR digital filter implementations for decimation, interpolation, and narrow-band filtering

Interpolation, extrapolation, and reduction of computation speed in digital filters

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