Digital Signal Processing Reference
In-Depth Information
Combined Approaches
Prosecuting consequently the idea presented at
the end of the section on neural network approaches of classifying the current
frame of speech signal first and then providing specific matrices leads to the
awareness that linear mapping can be applied as a postprocessing stage of codebook
approaches. In this case the nonlinear mapping between bandlimited and wideband
feature vectors is modeled as a piecewise linear mapping. Figure 7.17 shows such
a design.
First the narrowband codebook is searched for the narrowband entry or its index
i
opt
(
n
), respectively, producing minimum distance to the actual input feature vector
considering a specific distance measure
i
opt
(
n
)
¼
argmin
i
{
d
(
n
,
i
)}
:
(7
:
54)
Here the cepstral distance measure as defined in (7.30)
2
d
(
n
,
i
)
¼ d
ceps
(
n
,
i
)
¼kc
nb
(
n
)
c
i
,nb
k
(7
:
55)
Figure 7.17 Combined approach using codebook preclassification and linear
mapping.
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