t, which give speech its characteristic timbre and sound. This sound, referred to by speech pathologists as the "buzz," can be described by both its pitch and its intensity. LPC, therefore, models the behavior of the vocal cords and the vocal tract itself. To create the digitized voice samples, the "buzz" is passed through an inverse filter that is selected based upon the value of the coefficients. The remaining signal, after the buzz has been removed, is called the residue. In the most common form of LPC, the residue is encoded as either a voiced or unvoiced sound. Voiced sounds are those that require vocal cord vibration, such as the "g" in "glare," the "b" in "boy," the "d" and "g" in "dog." Unvoiced sounds -such as the "h" in "how," the "sh" in "shoe," the "f" in "frog"-require no vocal cord vibration. The transmitter creates and sends the prediction coefficients, which include measures of pitch, intensity and whatever voiced and unvoiced coefficients are required. The receiver undoes the process: It converts the voice residue, pitch, and intensity coef- ficients into a representation of the source signal, using a filter similar to the one used by the transmitter to synthesize the original signal. Digital Speech Interpolation (DSI) Human speech has many measurable (and therefore predictable) char- acteristics, one of which is a tendency to have embedded pauses. As a rule, people do not spew out a series of uninterrupted sounds; they tend to pause for emphasis, to collect their thoughts, to reword a phrase while the other person listens quietly on the other end of the line. When speech technicians monitor these pauses, they discover that during considerably more than half of the total connect time, the line is silent. Digital Speech Interpolation (DSI) takes advantage of this character- istic silence to drastically reduce the bandwidth required for a single Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserv