BANDWIDTH EXTENSION OF TELEPHONY SPEECH - Adaptive Signal Processing: Next Generation Solutions - page 349

7
BANDWIDTH EXTENSION OF
TELEPHONY SPEECH
Bernd Iser and Gerhard Schmidt
Harman / Becker Automotive Systems, Ulm, Germany
In this chapter an introduction on bandwidth extension of telephony speech is given.
Why current telephone networks apply a limiting bandpass, what kind of bandpass is
used, and what can be done to (re)increase the bandwidth on the receiver side without
changing the transmission system is presented. Therefore, several approaches—most
of them based on the source-filter model for speech generation—are discussed. The
task of bandwidth extension algorithms that make use of this model can be divided
into two subtasks: an excitation signal extension part and a wideband envelope esti-
mation part. Different methods for accomplishing these tasks, like nonlinear proces-
sing, the use of signal and noise generators, or modulation approaches on the one
hand and codebook approaches, linear mapping schemes or neural networks on the
other, are presented.
7.1 INTRODUCTION
Speech is the most natural and convenient way of human communication. This is the
reason for the big success of the telephone system since its invention in the nineteenth
century [1]. At that time people didn't think of high quality speech but nowadays this
has changed. Often customers are not satisfied with the quality of service provided by
the telephone system especially when compared to other audio sources, such as radio
or compact disc. The degradation of speech quality using analog telephone systems is