Multimode Speech Coding - Digital Speech: Coding for Low Bit Rate Communication Systems - page 326

where K is the total number of harmonics. Normalization factor of the second
subframe, g 2 , is quantized using a logarithmic scale, given by,

k g max

i

− g min + k

N − 1

g 2 i = − k + g min for i = 0 , 1 , ... ,N − 1 (9.66)

k

where k is eight (which controls the gradient of the exponential function), N
(the number of quantizer levels) is 32, i.e. five bits are required to quantize
the gain of the second subframe, and g max and g min are the maximum and
minimum possible quantized normalization factors, respectively. The gain
values beyond g max and g min are clipped by the quantizer. The term g min is
introduced in equation (9.66), because only the stationary voiced segments
are synthesized using the harmonic excitation and the minimum gain is
nonzero.
The normalization factor of the first subframe, g 1 , is differentially quantized
with respect to the mean of the adjacent two quantized g 2 values, as follows:

g 2 + g 2 − 1

δ = g 1 − (9.67)

2

where g 2 − 1 is the gain of the second subframe of the previous frame, i.e. the
previous g 2 ,and δ is quantized using three bits. Finally the spectral amplitude
vectors are denormalized by multiplying with the quantized normalization
factors.

Onset Harmonic Parameter Quantization
The harmonic synthesis process interpolates the parameters between the
synthesis frame boundaries. However, at the onsets, when switching from
waveform-coding mode, the harmonic parameters of the initial synthesis
frame boundary are not directly available. The pitch, PPL, and PPS are
estimated, as described in Section 9.5.4, and quantized as described in the
preceding sections.
The spectral amplitudes of the ACELP excitation signal used before the
harmonic mode are estimated by windowing it using an asymmetric window
function given by,

0 . 46 cos π

n
n 1 −

w (n) = 0 . 54 − for 0 ≤ n < n 1 (9.68)

1

0 . 92 cos π
2

−

n n 1

w (n) = 0 . 08 + for n 1 ≤ n < n 1 + n 2 (9.69)

n 2 − 1

where n 1 is 140 and n 2 is 20. The asymmetric window function emphasizes
the excitation signal close to the switching frame boundary.