This paper presents an accurate estimation formula for the minimum filter length of optimum (the minimax criterion based) FIR low-pass digital filters when ripples in passband and those in stopband are identical. Two estimation formulae have been already proposed, but they have some problems which lead to wrong estimation. Discussing the problems based on both experimental results and theoretical considerations, we propose a new estimation formula. The accuracy of the proposed formula, is evaluated for some design examples and compared with that of the conventional formulae.
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