Bark frequency transform using an arbitrary order allpass filter.

We propose an arbitrary order stable allpass filter structure for frequency transformation from Hertz to Bark scale. According to the proposed filter structure, the first order allpass filter is causal, but the second and higher order allpass filters are non-causal. We find that the accuracy of the transformation significantly improves when a second or higher order allpass filter is designed compared to a first order allpass filter. We also find that the RMS error of the transformation monotonically decreases by increasing the order of the allpass filter.