Multimode Speech Coding - Digital Speech: Coding for Low Bit Rate Communication Systems - page 321

even at the stationary voiced segments. Therefore the harmonic quantization
can be included in the closed-loop mode decision without worrying about
the quality of ACELP coded frames between the harmonic frames (except, of
course, the bit rate will be higher). However when the bit rate of ACELPmode
is low, the quantization noise becomes audible; hence, trying to eliminate
the quantization noise of the harmonic mode by switching to ACELP mode
does not improve the perceptual quality. Therefore, in all the tests described
here, harmonic parameter quantization is not included in the transition
detection loop.
The sensitivity of AbS transition detection is different for each parameter.
The sensitivity is high for the pitch period and PPL. Changes in these
parameters dramatically reduce the cross-correlation of the original and the
synthesized speech, due to the resulting time shifts. The spectral amplitudes
and the LPCparameters are least sensitive. In fact, quantized and unquantized
LPC parameters both produced the same classification decisions for the
test speech material. The LPC memory locations of the transition detection
algorithm are initialized for each frame with the memory locations of the LPC
synthesis filter. This avoids drifting the LPC synthesis filter of the transition
detection algorithm from the synthesized speech.

Pitch Quantization
The pitch period, τ , is quantized using a nonlinear scalar quantizer, reflecting
the high sensitivity of the human ear to the pitch deviations at shorter pitch
periods. A logarithmic scale is used for the pitch values from 16 to 60 samples
and a linear scale is used for the pitch values from 60 to 160 samples (see
Figure 9.28). The quantized pitch τ i is given by,

τ min τ 0
τ min

i
N 0 − 1

τ i = for i = 0 , 1 , 2 , ... ,N 0 − 1 (9.57)

τ max − τ 0

τ i = τ 0 + (i − N 0 + 1 ) for i = N 0 ,N 0 + 1 , ... ,N − 1 (9.58)

−

N N 0

where τ min is 16, τ max is 160, τ 0 is 60, N 0 is 156, and N is 256. Therefore eight
bits are required to transmit the quantized pitch period.

Pitch Pulse Location Quantization
The pitch pulse location (PPL) is the location of the pitch pulse closest to the
centre of the analysis frame. PPL may be defined as the distance to the pitch
pulse concerned from the centre of the analysis frame, measured in samples.
Assuming that the maximum possible pitch is 160 samples, PPL varies
between − 80 and 80. However the pitch pulse location may be normalized