Multimode Speech Coding - Digital Speech: Coding for Low Bit Rate Communication Systems - page 325

based on a weighted spectral-distortion measure. Therefore the quantization
scheme is called switched predictive Mel-scale-based vector quantization
(SP-MVQ). Moreover, the switching scheme restricts error propagation under
noisy channel conditions.
SP-MVQ quantizes spectral amplitudes every 10ms using 14 bits. The har-
monic analysis/synthesis scheme described estimates the harmonic param-
eters every 20ms. However there are sufficient bits for the allocation of
28 bits per 20ms frame for spectral amplitudes at 4 kb/s (see Table 9.5).
Therefore the harmonic analysis/synthesis scheme is modified to update
the spectral amplitudes every 10ms. However the pitch is transmitted only
every 20ms, and linearly interpolated to compute the number of harmonics
corresponding to the centre of the synthesis frame or the first subframe,
at the decoder. The spectral amplitude quantization uses the quantized
(second subframe) or quantized and interpolated (first subframe) pitch to
compute the number of harmonics, in order to ensure the correct dequan-
tization of the spectral amplitude vectors. In the spectral amplitude quan-
tization of the first subframe, if the actual number of harmonics is greater
than the computed number of harmonics by interpolation, the higher har-
monics are ignored. If the actual number of harmonics is less than the
computed number of harmonics by interpolation, the amplitude vector is
zero-padded. Usually the pitch values of the stationary voiced segments are
fairly unchanged and linear interpolation of the number of harmonics gives
a good approximation.

Harmonic Gain Quantization
The spectral amplitude vectors are normalized before the quantization, in
order to improve the dynamic range. The shape components of the vectors
are quantized using SP-MVQ, as described above, and the gain component is
scalar quantized.
The normalized amplitude, a k n ,ofthe k th harmonic is given by,

a k
g

a k n = (9.64)

where a k is the spectral amplitude estimated for the k th harmonic and g is the
normalization factor, given by,

K

a k

k = 1

g = (9.65)

K