B. P. Lathi, Zhi Ding - Modern Digital and Analog Communication Systems-Oxford University Press (2009)

6.8 Vocoders and Video Compression 30 1
Figure 6.34
(a) The human
speech
production
mechanism.
(b) Typical
pressure
impulses.
Nose cavity
Mouth cavity
(a)
(b)
For a low-pass source signal with finite bandwidth B Hz, even if we apply the minimum
Nyquist sampling rate 2B Hz and I -bit encoding, the bit rate cannot be lower than 2B bit/s.
There have been many successful methods introduced to drastically reduce the source coding
rates of speech and video signals, very important to our daily communication needs. Unlike
waveform encoders, the most successful speech and video encoders are based on the human
physiological models involved in speech generation and in video perception. Here we describe
the basic principles of the linear prediction voice coders (known as vocoders) and the video
compression method proposed by the Moving Picture Experts Group (MPEG).
6.8.1 Linear Prediction Coding Vocoders
Voice Models and Model-Based Vocoders
Linear prediction coding (LPC) vocoders are model-based systems. The model, in tum, is
based on a good understanding of the human voice mechanism. Fig. 6.34a provides a crosssectional
illustration of the human speech apparatus. Briefly, human speech is produced by
the joint interaction of lungs, vocal cords, and the articulation tract, consisting of the mouth
and the nose cavity. Based on this physiological speech model, human voices can be divided
into voiced and the unvoiced sound categories. Voiced sounds are those made while the vocal
cords are vibrating. Put a finger on your Adam's apple* while speaking, and you can feel the
vibration the vocal cords when you pronounce all the vowels and some consonants, such as g
as in gut, b as in but, and n as in nut. Unvoiced sounds are made while the vocal cords are not
vibrating. Several consonants such as k, p, and t are unvoiced. Examples of unvoiced sounds
include h in hut, c in cut, and p in put.
For the production of voiced sounds, the lungs expel air through the epiglottis, causing
the vocal cords to vibrate. The vibrating vocal cords interrupt the airstream and produce a
quasi-periodic pressure wave consisting of impulses. The pressure wave impulses are commonly
called pitch impulses, and the frequency of the pressure signal is the pitch frequency or
fundamental frequency as shown in Fig. 6.34b. This is the part of the voice signal that defines
the speech tone. Speech that is uttered in a constant pitch frequency sounds monotonous. In
ordinary cases, the pitch frequency of a speaker varies almost constantly, often from syllable
to syllable.
* The slight projection at the front of the throat formed by the largest cartilage of the larynx, usually more prominent
in men than in women.