BANDWIDTH EXTENSION OF TELEPHONY SPEECH - Adaptive Signal Processing: Next Generation Solutions - page 355

information that is suitable for this task most of the algorithms employ the so-called
source-filter model of speech generation [7, 11, 33].
7.4.1 Source-Filter Model
This model is motivated by the anatomical analysis of the human speech apparatus
(see Fig. 7.6). A flow of air coming from the lungs is pressed through the vocal
cords. At this point two scenarios can be distinguished.
† In the first scenario the vocal cords are loose causing a turbulent (noise-like)
air flow.
† In the second scenario the vocal cords are tense and closed. The pressure of the
air coming from the lungs increases until it causes the vocal cords to open. Now
the pressure decreases rapidly and the vocal cords close once again. This scen-
ario results in a periodic signal.
The signal that could be observed directly behind the vocal cords is called an exci-
tation signal. This excitation signal has the property of being spectrally flat. After pas-
sing the vocal cords the air flow goes through several cavities such as the pharynx
cavity, the nasal cavity, and the mouth cavity. In all these cavities the air flow under-
goes frequency dependent reflections and resonances depending on the geometry of
the cavity.
Figure 7.6 Human speech apparatus.