Optimized nested all-pass filter for digital reverberator

The reverberator simulates sound impulse response of the closed acoustic room. The digital all-pass filter is frequently used in the reverberator algorithms to increase the impulse density of their impulse response. The nested all-pass filter (NAF) is most frequently used structure. The parameters of NAF (like delays and coefficient values) found in published works were determined subjectively. In some cases of NAF setting an undesirable ringing effect can be heard and verified by short time FFT. In this paper, an optimization algorithm of NAF parameters is described. We have analyzed short time spectrum properties of NAF-s. Based on the exploring of pole arrangement in a z-plane we have designed a calculation procedure for NAF parameters determination.