Digital Signal Processing Reference
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networks—when leaving into an analog telephone network the speech signal is once
again bandlimited. Furthermore, we still have a band limitation of 4 kHz.
Thus, great efforts have been made to increase the quality of telephone speech sig-
nals in recent years. Wideband codecs are able to increase the bandwidth up to 7 kHz
or even higher at only moderate complexity [17]. Nevertheless, applying these codecs
would mean modifying the current networks. Another possibility is to increase the
bandwidth after transmission by means of bandwidth extension. The basic idea of
these enhancements is to estimate the speech signal components above 3400 Hz
and below 300 Hz and to complement the signal in the new frequency bands with
this estimate. In this case the telephone networks are left untouched. Figure 7.2
Figure 7.3 Spectrograms of (a) Wideband speech, (b) Bandlimited speech, and
(c) Reconstructed speech.
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