Noise reduction combining time-domain ε-filter and time-frequency ε-filter

A time-domain ε-filter (TD ε-filter) is a nonlinear filter that can reduce noise while preserving a signal that varies drastically, such as a speech signal. Although the filter design is simple, it can effectively reduce noise. It is applicable not only to stationary noise but also to nonstationary noise. It cannot, however, be applied when the amplitude of noise is relatively large. This paper introduces an advanced method for noise reduction that applies an ε-filter to complex spectra, namely a time-frequency ε-filter (TF ε-filter). This paper also introduces noise reduction combining a TD ε-filter and a TF ε-filter. An advanced method called a variable time-frequency ε-filter is also proposed. First, the algorithm of the TD ε-filter is explained to clarify the problem. Then, the algorithms of the proposed methods are explained. By utilizing an ε-filter in the frequency domain, the proposed method can reduce not only noise that has a relatively small amplitude but also noise that has a relatively large amplitude. Experimental results are also given to demonstrate the performance of the proposed methods in comparison to the results of some conventional methods.