Digital Signal Processing Reference
In-Depth Information
Shape
Look-up
in a
table
Nearest
neighbor
rule
x
(
m
)
ˆ
(
m
)
Gain
...
f
L
...
f
1
f
1
f
L
Coefficients
1
A
(
z
)
1
A
(
z
)
LPC
...
...
c
1
c
L
c
1
c
L
Transmitter
Channel
Receiver
Figure 6.4.
Alternative CELP coder principle diagram
from
N
samples of the signal in the current analysis frame. This filtering operation
aims to partition the space
R
N
finely where the vector
x
has a strong probability of
appearing and coarsely elsewhere.
Two other characteristics are introduced in this chapter. The first is the use of
a weighting function in the frequency domain so that the quantization noise is put
into shape. This shaping of the noise is very different from the case of music coders
where hearing models are used as we will see later on because, for the target bit rates
(4.8-16 kb/s), the quantization noise is too high and the useful band (less than 4 kHz),
is far too reduced to justify a precise model. The second characteristic (not shown
in the diagram) is the need to introduce a
long-term predictor
which has an ADPCM
structure.
6.3.2.
Determining the synthesis filter coefficients
The first process involves determining and then coding the coefficients used
in the synthesis filter. Linear predictive modeling is quasi-systematically used.
A compromise is made in choosing the prediction order
P
.
P
must be sufficiently high
to reproduce the speech spectrum correctly: an order of 8 is necessary to create four
peaks in the spectrum and we know that speech signals generally have four formants.
However, the prediction order must be as low as possible to economize the bit rate.
Therefore, a value between 8 and 16 is usually chosen for
P
.
a
P
of the filter
A
(
z
) are not quantized directly because they have poor properties for
coding, their dynamics are especially poorly controlled. Quantization, whether scalar
or vector, of the coefficients
k
1
···
The filter coefficients must be coded.
In practice, the coefficients
a
1
···
k
P
of the corresponding trellis filter is preferred.
