Single channel noise reduction is a severe challenge, especially when considering the difficulties of estimating the appropriate spectral distribution of interfering noise. Usually, all approaches, e.g. Wiener filters, Ephraim-Malah filters, and Kaiman filters, rely on the assumption that the interfering noise components are longer-term stationary when compared to desired signals.
Here, we focus on the application of noise reduction for car phones meaning that the interfering noise is car noise with its specific properties. When analyzing car noise, one observes that the assumption of long-term stationarity is true only as a first approximation. Analyzing car noise in more detail, described later in this chapter in the second section, one discovers that the different components vary in dependence of the car velocity or the engine speed.