Digital Signal Processing Reference
In-Depth Information
Speech Spectrum
LPC Envelope
Postfilter Response
43.0
23.0
3.0
0.0
1.0
2.0
3.0
4.0
Frequency (kHz)
Figure 7.27
Typical response of APF together with the original LPC envelope and
speech spectrum (
µ
=
0
.
3
, β
=
0
.
6
,
and
α
=
0
.
9
)
sample-by-sample normalization, block-wise normalization can be used, i.e.
sum the values of
δ
o
(n)
and
δ
p
(n)
for a block and use the average. Small
block sizes (e.g. 10 samples) generally produce indistinguishable results. The
effect of the APF of Figure 7.26 can be seen in Figure 7.27 alongside a typical
example of the original LPC envelope.
Generally, the following parameter ranges have been found to give reason-
able subjective results:
0
.
2
≤
µ
≤
0
.
4
(7.117)
0
.
5
≤
β
≤
0
.
7
(7.118)
0
.
8
≤
α
≤
0
.
9
(7.119)
The factor
µ
controls the 'brightness' of the speech, and hence larger
values tend to bring in more high-frequency background noise. The factors
β
and
α
control the degree of spectral filtering, and the difference between
the parameters determines the filtering effect. Subjectively, large differences
give quieter speech, but this is usually accompanied by an unnatural 'deep'
voice effect. Applying APF with the correct subjectively-selected control
parameters to the coders produces significant subjective noise reduction
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