h coders are essential components in telecommunications and in the multimedia infrastructure. This section describes the speech signal properties with regard to the human auditory system. It summarizes current capabilities in speech coding and describes how the field has evolved to reach these capabilities. Speech Signal Properties There are three basic types of speech sound-voiced, unvoiced, and plosive-which produce different spectra because of their sound formation. Voiced speech requires air pressure from the lungs to force open the normally closed vocal cords, which then vibrate. The pitch or vibration frequency varies from about 50 to 400 Hz (as determined by the speaker's age and gender); the vocal tract then resonates at odd harmonics. These resonant peaks are known as formants. All vowels (and some of the consonants) are considered voiced sound. On the other hand, unvoiced sounds, or fricatives (derived from the word friction; e.g., S, f, sh), are formed by forcing air through an aperture without causing the vocal cords to vibrate. This lack of resonant characteristics produces a largely aperiodic structure (as might be expected). Figure 1.1(a) shows the time domain presentation of a sample voiced sound, while Figure 1.1(b) shows the autocorrelation. In general, autocorrelation is considered as a robust indicator of periodicity. Similarly, Figure 1.1(c) and Figure 1.1(d) show the time domain and autocorrelation of a sample unvoiced sound. Chapter 1 Voice Transmission -0.04 -0.08 X 10 -3 -0.04 -0.08 Fig. 1.1 Example of (a) voiced sound segment, (b) autocorrelation of voiced sound segment, (c) an unvoiced sound segment, and (d) autocorrelation of unvoiced sound segment. The third category of sounds is plosive sound. Plosive sounds start with a closure of the mouth and/ or nose and therefore contain a high degree of low-frequency energy. The terms "plosive" and "stop" are usually used interchangeably, but they are not exact syn- onyms. Notice that some sounds still cannot be categorized as in any of the thr