Digital filter and its designing method, desiging apparatus, and program for designing digital filter

Abstract

An original filter having first filter coefficients where the numeral sequence is symmetrical and a regulation filter having second filter coefficients realizing reverse direction frequency amplitude characteristics B in contact with the frequency amplitude characteristics A of the original filter at a maximum amplitude ''1'' are connected in the longitudinal way, and filter coefficients to be determined are designed by product-sum operation of the first and the second filter coefficients.

Description

technical field [0001] The present invention relates to a digital filter, a design method, a design device, and a program for designing a digital filter, in particular to a design method of an FIR filter. Background technique [0002] One of the forms of digital filters is FIR (Finite Impulse Response: finite impulse response) filter. This FIR filter is a type of filter that has a tapped delay line composed of a plurality of delayers, multiplies the output signal of each tap by a filter coefficient, and outputs the result of adding their products. It has the following advantages. First, the pole of the transfer function of the FIR filter is only at the origin of the Z plane, so the circuit is always stable. Second, as long as the filter coefficients are symmetrical, completely correct linear phase characteristics can be realized. [0003] In this FIR filter, an impulse response represented by a finite time length directly becomes a filter coefficient. Therefore, designin...

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Problems solved by technology

[0005] However, the frequency characteristics of the FIR filter obtained by conventional design methods depend on window functions or approximate expressions, so unless these are properly set, the desired good frequency characteristics cannot be obtained.

However, it is often not easy to properly set the window function or approximation

In addition, since truncation errors of coefficients occur when windowing is performed, it is difficult to achieve desired frequency characteristics

In particular, there is a problem that it is extremely difficult to obtain ideal frequency characteristics with a steep slope of the stop region (the frequency band between the pass region and the cutoff region).

However, no matter which method is used, the pass band of the filter tends to be smaller than before adjustment, and the bandwidth cannot be expanded to make the slope steeper.

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