Multimode Speech Coding - Digital Speech: Coding for Low Bit Rate Communication Systems - page 323

1

0.8

0.6

0.4

0.2

0

− 0.2

0.4

−

− 0.6

− 0.8

− 1

0 50 100 150

quantizer index (i)

Figure 9.29 PPL quantizer levels

received PPL value accurately. Figure 9.29 depicts a plot of the normalized
PPL quantizer levels.

Pitch Pulse Shape Quantization
Large variations in the PPS introduces a reverberant character into the
synthesized speech, regardless of the PPS value. Therefore, in terms of the
perceptual quality, all the PPS values are equally important and a linear
quantizer is employed to quantize the PPS using 16 values. The quantized
PPS, θ i , is given by,

2 π
N i

θ i = − π for i = 0 , 1 , ... ,N − 1and − π ≤ θ i <π (9.61)

where N , the number of quantizer levels, is 16 and four bits are required to
quantize PPS.

Harmonic Amplitude Quantization
Harmonic amplitudes of the LPC residual are quantized using Switched
Predictive Mel-scale-based Vector Quantization (SP-MVQ) [51]. SP-MVQ
(see block diagram in Figure 9.30) converts the variable-dimension spectral-
amplitude vectors into fixed-dimension vectors by warping the frequency
axis using a logarithmic scale. The warping process emphasizes the low