Multimode Speech Coding - Digital Speech: Coding for Low Bit Rate Communication Systems - page 299

2

(c)

(a)

1

0

(d)

(b)

− 1

2

−

0 10 20 30

0 10 20 30

samples DFT samples

Figure 9.14 PPS at a dispersed pitch period: (a) a complete pitch cycle of the LPC
residual, (b) the pitch pulse synthesized using PPS, (c) the positive half of the phase
spectrum obtained from the DFT, and (d) estimated PPS

original speech

LPC residual

SWPM SWPM

SB-LPC

SB-LPC

0 50 100 150 200 0 50 100 150 200

samples samples

(a) LPC residual signals (b) Speech signals

Figure 9.15 Speech synthesized using PPS

zero-phase excitation, even if the pitch pulse locations are synchronized. This
is because SWPM has the additional flexibility of choosing the most suitable
phase value (PPS) for pitch pulses, such that the phase discontinuities are
minimized. Figure 9.15 illustrates the effect of PPS on the LPC excitation
and the synthesized speech signals. For comparison, it includes the original
signals and the signals synthesized using the SB-LPC coder [4] which assumes
a zero-phase excitation.
The absence of audible switching artifacts at the onsets is an interesting
issue. There are two basic reasons for the differences between switching
artifacts at the onsets and at the offsets: the nature of the excitation signal