Databases Reference
In-Depth Information
T A B L E 18 . 1
Pulses used to construct the
fixed codebook.
Pulse
Sign
Positions
i
0
s
0
=±
1
m
0
:
0
,
5
,
10
,
15
,
20
,
25
,
30
,
35
i
1
s
1
=±
1
m
1
:
1
,
6
,
11
,
16
,
21
,
26
,
31
,
36
i
2
s
2
=±
1
m
2
:
2
,
7
,
12
,
17
,
22
,
27
,
32
,
37
i
3
s
3
=±
1
m
3
:
3
,
8
,
13
,
18
,
23
,
28
,
33
,
38
4
,
9
,
14
,
19
,
24
,
29
,
34
,
39
T A B L E 18 . 2
Bit allocation per frame for
the G.729 coder.
Description
Bits
Predictor of LSP quantizer
1
First stage vector quantizer for LSP
7
Second stage for lower five coefficients
5
Second stage for upper five coefficients
5
Pitch delay for first subframe
8
Parity bit for pitch delay
1
Fixed codebook first subframe
13
Signs of fixed codebook pulses for first subframe
4
Gain codebook first subframe
7
Pitch delay for second subframe
5
Fixed codebook second subframe
13
Signs of fixed codebook pulses for first subframe
4
Gain codebook second subframe
7
The selected code vector is filtered to enhance harmonic components in order to improve the
quality of the reconstructed speech.
The number of bits used to encode the pitch information, index of the fixed code vector,
and the respective gains are given in Table
18.2
. These total to 80 bits per frame resulting in a
coding rate of 8 kbits/sec. While this is substantially less than the rate of the G.722.2 coder,
the quality is about the same [
245
] though with higher complexity.
18.5.3 SILK
The SILK coder developed by Skype is another example of how a speech coder designed for
the Internet can leverage the greater amount of processing power available to the encoder and
decoder when communicating between computers rather than between cellular phones. It uses
variable length encoding and includes the ability to adaptively respond to changing network
conditions. A block diagram of the various processes carried out in the SILK encoder is shown
in Figure
18.12
. We will use this figure to give a brief overview of some aspects of the SILK
encoder.
