The noise existence is inevitable in real applications of speech processing. It is well known that the additive noise affects negatively the performance of the speech codecs designed to work with noise-free speech especially codecs based on linear prediction coefficients (LPC).
Another application strongly influenced by noise is related to the hands free phones where the background noise reduces the signal to noise ratio (S/N) and the speech intelligibility.Last but not least, is the problem of speech recognition in a noisy environment.
Description of Adaptive Blind Noise Suppression
A system that works well in noise-free conditions, usually shows considerable degradation in performance when background noise is present It is clear that a strong demand for reliable noise cancellation methods exists that efficiently separate the noise from speech signal. The endeavors in designing of such systems can be followed some 20 years ago.
The core of the problem is that in most situations the characteristics of the noise are not known a priori and moreover they may change in time. This implies the use of adaptive systems capable of identifying and tracking the noise characteristics. This is why the application of adaptive filtering for noise cancellation is widely used.The classical systems for noise suppression rely on the usage of adaptive linear filtering and the application of digital filters with finite impulse response (FIR).
The strong points of this approach are the simple analysis of the linear systems in the process of adaptation and the guaranteed stability of FIR structures. It is worth mentioning the existence of relatively simple and well investigated adaptive algorithms for such kind of systems as least mean squares (LMS) and recursive least squares (RLS) algorithms. The investigations in the area of noise cancellation reveal that in some applications the nonlinear filters outperform their linear counterparts.
That fact is a good motivation for a shift towards the usage of nonlinear systems in noise reduction .Another approach is based on a microphone array instead of the two microphones, reference and primary, that are used in the classical noise cancellation scheme.