Data Communications Networking Devices - 4th Ed.pdf

6.2 T1/ E1 MULTIPLEXERS _________________________________________________________ 659examining how T1 multiplexers can be employed in a networkand their operationalfeatures and capabilities, a discussion of the various voice digitization techniquescommonly available is warranted to obtain an understanding of the bene®ts andlimitations of use of modules performing voice digitization in multiplexers.Voice digitization techniquesThe ®rst series of voice digitization techniques developed were based upondifferent methods used to sample a speech waveform and are collectively referred toas waveform coding. Waveform coding voice digitization techniques include pulsecode modulation PCM), Adaptive Differential PCM ADPCM) and continuouslyvariable slope delta CVSD) modulation. Although waveform coding techniquesnormally result in a relatively high bit rate for encoded analog speech, they alsoresult in a very good quality of reconstructed speech.The fact that more simultaneous voice conversations could be transported at lowdata rates than at higher data rates per digitized signal resulted in the developmentof additional techniques used to digitize voice. Those techniques can be classi®edinto two additional classesÐvoice coders and hybrid coders. Voice coders examinethe spectral components of speech such as pitch, tone, and energy level, codingvalues for key speech parameters instead of samples of actual speech. At thereceiver, the values of those parameters are used to synthesize speech. Voice codingis performed by a device commonly referred to as a vocoder and results in thegeneration of a low bit rate for digitized speech. The resulting reconstructedspeech, however, may sound at best `hollow'. Popular examples of vocoding includea family of linear prediction coding products developed during the 1970s and1980s.Recognizing the limitations associated with waveform coding and vocodingresulted in a new series of voice digitization methods that combines the best featuresof both. This series of digitization methods is commonly classi®ed as hybrid coding.Under hybrid coding speech is sampled and speech parameters are extracted in asimilar way to the vocoding process. However, those parameters are then used in aniterative process to reconstruct the sample, compare it to the original sample, andadjust the speech parameters until there is a minimum difference between the two.Once this is accomplished, the revised speech parameters are used to represent ashort period of speech. Examples of hybrid coders include a family of code-excitedlinear predictive coders CLEP) that are being used in applications ranging in scopefrom voice over frame relay and Internet telephony to inclusion in T1 and E1multiplexers to enable additional simultaneous voice conversations to be transportedover T1 and E1 lines.Waveform codingThe development of PCM was followed by a number of additional waveformcoding techniques. Each of these new techniques was developed as a mechanism toreduce the data rate required to transport a digitized voice conversation, enabling