Introduction

Speech is a natural and therefore privileged communication modality. This is the reason for the great success of speech driven services and speech based media. Multimedia would not be imaginable without high quality audio. Today's environment is full of high quality audio sources like CD-Audio, DVD-Audio, radio broadcast, television broadcast, and so on.

One of the oldest but still most popular media based on audio is the telephone network. But since its invention in the nineteenth century capabilities and simultaneously demands on audio quality have increased [Bell 77]. Today's telephone networks still provide poor audio quality due to historical limitations (see Sect. 1.3).

This is the point where the idea of bandwidth extension comes into mind quite intuitively. Bandwidth extension in this context means the estimation of the not transmitted frequency components out of the transmitted signal by exploiting the transinformation included in speech signals and therewith increasing the speech quality (see Fig. 1.1c). This approach yields the advantage that nothing has to be changed within the network – it is simply an optional feature on the terminal side.