2 Perez-Meana & Nakano-Miyatake
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Introduction
The advances of the VLSI technology have allowed the development of high performance
digital signal processing (DSP) devices, enabling the implementation of very efcient and
sophisticated algorithms, which have been successfully used in the solution of a large amount
of practical problems in several elds of science and engineering. Thus, signal processing
techniques have been used with great success in telecommunications to solve the echo prob-
lem in telecommunications and teleconference systems (Amano, Perez-Meana, De Luca, &
Duchen, 1995), to solve the inter-symbol interference in high speed data communications
systems (Proakis, 1985), as well as to develop efcient coders that allow the storage and
transmission of speech and audio signals with a low bit rate keeping at the same time a high
sound quality (Bosi & Golberg, 2002; Kondoz, 1994). Signal processing algorithms have
also been used for speech and audio signal enhancement and restoration (Childers, 2000;
Davis, 2002) to reduce the noise produced by air conditioning equipment and motors (Kuo &
Morgan, 1996), and so forth, and to develop electronic mufers (Kuo & Morgan, 1996) and
headsets with active noise control (Davis, 2002). In the educational eld, signal processing
algorithms that allow the time scale modication of speech signals have been used to assist
the foreign language students during their learning process (Childers, 2000). These systems
have also been used to improve the hearing capability of elder people (Davis, 2002).
The digital technology allows an easy and error free reproduction of any digital material,
allowing the illegal reproduction of audio and video material. Because this fact represents
a huge economical loss for the entertainment industry, many efforts have been carried out
to solve this problem. Among the several possible solutions, the watermarking technology
appears to be a desirable alternative for copyright protection (Bassia, Pitas, & Nikoladis,
2001; Bender, Gruhl, Marimoto, & Lu, 1996). As a result, several audio and speech water-
marking algorithms have been proposed during the last decade, and this has been a subject
of active research during the last several years. Some of these applications are analyzed in
the remaining chapters of this book.
This chapter presents an overview of signal processing systems to storage, transmission,
enhancement, protection, and reproduction of speech and audio signals that have been suc-
cessfully used in telecommunications, audio, access control, and so forth.
Adaptive.Echo.Cancellation
A very successful speech signal processing application is the adaptive echo cancellation
used to reduce a common but undesirable phenomenon in most telecommunications sys-
tems, called echo. Here, when mismatch impedance is present in any telecommunications
system, a portion of the transmitted signal is reected to the transmitter as an echo, which
represents an impairment that degrades the system quality (Messershmitt, 1984). In most
telecommunications systems, such as a telephone circuit, the echo is generated when the
long distant portion consisting of two one-directional channels (four wires) is connected
with a bidirectional channel (two wires) by means of a hybrid transformer, as shown in
Figure 1. If the hybrid impedance is perfectly balanced, the two one-directional channels are