Multimode Speech Coding - Digital Speech: Coding for Low Bit Rate Communication Systems - page 303

(a)

(b)

(c)

(d)

0 50 100 150 200

samples

Figure 9.16 Offset target modification: (a) s(n) ,(b) s t (n) ,(c) s l (n) ,and(d) s m (n)

illustrates the waveforms of equation (9.40). It can be seen that the phases
of the low frequency components of the original speech waveform, s(n) ,are
modified in order to obtain s t (n) . The waveforms in Figures 9.16c and 9.16d
depict s l (n) and s m (n) , respectively, the low frequency components, which
have been modified. The phase relationships between the high-frequency
components account more for the perceptual quality of speech [25], and the
high-frequency phase components are unchanged in the process.
Some speech signals show rapid variations in the harmonic structure at the
offsets, which may reduce the efficiency of the phase modification process.
In order to limit those effects the spectral amplitude and phase estimation
process is not strictly confined to the harmonics of the fundamental frequency.
Instead the amplitude and phase corresponding to the spectral peak closest
to each harmonic frequency are estimated. The frequency of the selected
spectral peak is taken as the frequency of the estimated amplitude and phase.
When finding the spectral peaks closest to the harmonic frequencies, the
harmonic frequencies are determined by the fundamental frequency at the
end of the i th synthesis frame, since the pitch estimates at the transition
frame are less reliable. In fact the purpose of the offset target modification
process is to find the frequency components corresponding to the harmonics
of the harmonically-synthesized frame in the i th frame and change the phase
evolution of those components such that the discontinuities are eliminated.
Moreover, the same set of spectral peak frequencies and amplitudes are used
when synthesizing the terms s l (n) and s m (n) , hence there is no need to restrict
the synthesis process to the pitch harmonics.