Digital Signal Processing Reference
In-Depth Information
2000.0
(a)
0.0
-2000.0
(b)
-4000.0
-6000.0
(c)
-8000.0
-10000.0
-12000.0
0.0
500.0
1000.0
1500.0
Time (Samples)
Figure 7.24
Speech waveforms of (a) original, (b) overlapping centre-clipped
Gaussian excited CELP output and, (c) PAME excited CELP output
candidate excitation is derived as follows:
1
n
=
iP < L,
i
=
0
,
1
,
2
, ...
x
j
(n)
=
(7.93)
0
otherwise
In order to form all possible phase positions, for each primary vector
x
j
,
P
−
1 further vectors
x
j
+
k
,
k
=
1
,
2
, ... ,P
−
1, are derived by shifting as,
0
n
=
0
,
1
,
2
, ... ,k
−
1
x
j
+
k
(n)
=
(7.94)
x
j
(n
−
k) n
=
k, k
+
1
, ... ,L
−
1
It should be noted that the number of candidate excitation vectors
C
a
depends on
L
and
D
min
such that,
L/
2
L/
2
−
1
C
a
=
L
+
I
+
I
(7.95)
I
=
D
min
I
=
1
Thus the number of bits required by the adaptive excitation index range is
I
=
log
2
C
a
.If
C
a
does not correspond to an integer power of 2, a further
2
I
−
C
a
vectors are then searched in the fixed codebook. As with all algebraic
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