4.9 Modeling the speech signal  (Page 2/4)

Singers modify vocal cord tension to change the pitch to produce the desired musical note. Vocal cord tension is governed by acontrol input to the musculature; in system's models we represent control inputs as signals coming into the top orbottom of the system. Certainly in the case of speech and in many other cases as well, it is the control input that carriesinformation, impressing it on the system's output. The change of signal structure resulting from varying the control inputenables information to be conveyed by the signal, a processgenerically known as modulation . In singing, musicality is largely conveyed by pitch; in western speech, pitchis much less important.A sentence can be read in a monotone fashion without completely destroying theinformation expressed by the sentence. However, the difference between a statement and a question is frequently expressed bypitch changes. For example, note the sound differences between "Let's go to the park." and "Let's go to the park?";

For some consonants, the vocal cords vibrate just as in vowels. For example, the so-called nasal sounds "n" and "m" havethis property. For others, the vocal cords do not produce a periodic output. Going back to mechanism, when consonants suchas "f" are produced, the vocal cords are placed under much less tension, which results in turbulent flow.The resulting output airflow is quite erratic, so much so that we describe it as being noise . We define noise carefully later when we delve into communication problems.

The vocal cords' periodic output can be well described by the periodic pulse train $p_T(t)$ as shown in the periodic pulse signal , with $T()$ denoting the pitch period. The spectrum of this signal contains harmonics of the frequency $\frac{1}{T}$ , what is known as the pitch frequency or the fundamental frequency $\mathrm{F0}()$ . The primary difference between adult male and female/prepubescent speech is pitch. Before puberty, pitchfrequency for normal speech ranges between 150-400 Hz for both males and females. After puberty, the vocal cords of malesundergo a physical change, which has the effect of lowering their pitch frequency to the range 80-160 Hz. If we couldexamine the vocal cord output, we could probably discern whether the speaker was male or female. This difference is also readilyapparent in the speech signal itself.

To simplify our speech modeling effort, we shall assume that the pitch period is constant. With this simplification, we collapsethe vocal-cord-lung system as a simple source that produces the periodic pulse signal ( [link] ). The sound pressure signal thus produced enters the mouth behind the tongue, creates acoustic disturbances, andexits primarily through the lips and to some extent through the nose. Speech specialists tend to name the mouth, tongue, teeth,lips, and nasal cavity the vocal tract . The physics governing the sound disturbances produced in the vocal tract andthose of an organ pipe are quite similar. Whereas the organ pipe has the simple physical structure of a straight tube, thecross-section of the vocal tract "tube" varies along its length because of the positions of the tongue, teeth, and lips. It is thesepositions that are controlled by the brain to produce the vowel sounds. Spreading the lips, bringing the teeth together, andbringing the tongue toward the front portion of the roof of the mouth produces the sound "ee." Rounding the lips, spreading theteeth, and positioning the tongue toward the back of the oral cavity produces the sound "oh." These variations result in alinear, time-invariant system that has a frequency response typified by several peaks, as shown in [link] .