Data Communications Networking Devices - 4th Ed.pdf

662 _________________________ WIDE AREA NETWORK DATA CONCENTRATION EQUIPMENTFigure 6.24 Speech-producing elements of the human vocal tract.Reprinted withpermission from Data Communications Management, # 1988 Auerbach Publishers, NewYork)Under linear predictive coding speech is ®rst sampled by a 12-bit analog to digitalconverter. Next, the output of the converter is used as input to four parametricdetectors. These parametric detectors include a pitch detector, a voice/unvoiceddetector, a power detector and a spectral data decoder.To appreciate the employment of parametric detectors, consider Figure 6.24which illustrates the speech-producing elements of the human vocal tract. Thepitch detector analyzes data to obtain the fundamental pitch frequency at whichhuman vocal cords vibrate. Next, the voice/unvoiced detector senses whether soundis caused by the vibration of vocal cords voice) or by sounds such as `shhh'unvoiced), which do not vibrate. The power detector determines the amplitude orloudness of the sound, while the spectral data detector models the resonant cavityformed by the throat and the mouth.Through the use of parametric detectors, a parametric model of human speech isconstructed, resulting in the generation of a series of speech coef®cients whichrepresent human voice. These speech coef®cients developed through the use oflinear predictive coding are then transmitted instead of the actual amplitude of thesignal which is used by PCM and other waveform coding techniques. As a result ofthe modeling of human speech instead of actual speech, a very low data rate isobtained to represent a voice conversation. The use of linear predictive codingallows digitized voice to be transmitted at data rates as low as 2400 or 4800 bps,enabling the communications manager to consider a variety of strategies to integratevoice into low-speed data networks or to obtain multiple voice channels on onephysical line.One of the most popular implementations of linear predictive coding is LPC10which obtains its name as a result of a linear prediction of 10 speech components.Under LPC10 speech is sampled at an 8 kHz rate. Ten speech parameters arepredicted and encoded into 42 bits while seven bits are used to represent pitch andvoicing and ®ve bits for gain. This results in 54 bits per frame using a framing ratethat repeats 44.44 times per second. Thus, the resulting data rate becomes 54 bits/frame 44.44 frames/second, or 2400 bps.