Shaped fixed codebook search for celp speech coding

Details Shaped fixed codebook search for celp speech coding Shaped fixed codebook search for celp speech coding

1999-04-28

2002-09-10

Pham, Chi (Department: 2631)

Pulse or digital communications

Systems using alternating or pulsating current

C704S219000

Reexamination Certificate

active

06449313

ABSTRACT:

BACKGROUND ART
1. Field of the Invention
The present invention relates to vocoders, and more particularly to the representation of the fixed codebook response generated thereby.
2. Description of the Background Art
FIGS. 1 and 2
illustrate transmitting and receiving units of a code excited linear prediction (CELP) vocoder, in accordance with the background art. In
FIG. 1
, the transmitting unit is a first vocoder
1
. The first vocoder
1
includes a linear predictive coding (LPC) filter
2
. The LPC filter
2
is connected a perceptual weighing filter
3
via a junction
4
. The perceptual weighing filter
3
is connected to an error minimization filter
5
. The error minimization filter
5
is connected to a first adaptive codebook
6
and a first fixed codebook
7
. The first adaptive codebook
6
is connected to a first adaptive codebook gain unit
8
. The first fixed codebook
7
is connected to a first fixed codebook gain unit
9
. The outputs of the first adaptive codebook gain unit
8
and the first fixed codebook gain unit
9
are connected at a junction
10
. The junction
10
is connected to the junction
4
.
Generally, the first vocoder
1
sequentially analyzes time segments of a digital speech input. Each time segment is referred to as a signal frame. The vocoder
1
estimates parameters characterizing each signal frame. The parameters are represented by bit patterns, which are assembled into a bit frame. The bit frames can be transmitted more quickly, or stored in less memory, than the signal frames which they represent.
Now, with reference to
FIG. 1
, a general description of the operation of a known IS127 EVRC CDMA type coder (vocoder
1
) will be given. For more detail on the operation of the vocoder
1
, reference can be made to textbooks relating to digital speech coding. The vocoder
1
is a multi-rate vocoder and has a full rate of operation corresponding to 8 kilo bits per second (kbps) and a half rate of operation corresponding to 4 kbps. The digital speech input is divided into signal frames of 20 msec. Each signal frame is further divided into first, second, and third sub-frames of approximately 6.6 msec.
When the vocoder
1
operates at full rate, a signal frame passes through the LPC filter
2
, which extracts LPC parameters characterizing the entire signal frame and outputs the LPC parameters in the form of twenty-eight LPC bits. The signal frame leaves the LPC filter, passes through the junction
4
, the perceptual weighing filter
3
, and the error minimization filter
5
. The perceptual weighing filter
3
and the error minimization filter
5
do not extract parameter bits from the signal frame, but prepare it for later processing.
Next, the signal frame is received by the first adaptive codebook
6
. The first adaptive codebook
6
estimates a pitch for the entire frame, and outputs seven ACB bits characterizing the pitch of the entire frame. Then, the first adaptive codebook gain unit
8
estimates an adaptive codebook gain of the first sub-frame, the second sub-frame, and the third sub-frame. Three ACBG bits estimate the adaptive codebook gain of the first sub-frame. Three more ACBG bits estimate the adaptive codebook gain of the second sub-frame. And, still three more ACBG bits estimate the adaptive codebook gain of the third sub-frame.
Next, the signal passes through the junction
10
, the junction
4
, the perceptual weighing filter
3
, and the error minimization filter
5
, and is received by the first fixed codebook
7
. The first fixed codebook
7
estimates the random, unvoiced characteristics of the first sub-frame, the second sub-frame, and the third sub-frame. Thirty-five FCB bits represent the fixed codebook response for the first sub-frame. Thirty-five more FCB bits represent the fixed codebook response for the second sub-frame. And, still thirty-five more FCB bits represent the fixed codebook response for the third sub-frame.
Next, the first fixed codebook gain unit
9
estimates a fixed codebook gain of the first sub-frame, the second sub-frame, and the third sub-frame. Five FCBG bits estimate the fixed codebook gain of the first sub-frame. Five more FCBG bits estimate the fixed codebook gain of the second sub-frame. And, still five more FCBG bits estimate the fixed codebook gain of the third sub-frame.
At this point, all of the bit patterns (LPC, ADC, ADCG, FCB, FCBG) are assembled into the bit frame. The bit frame, representing the signal frame, is complete and can be transmitted to a second vocoder
11
for synthesis, or stored in a memory for later retrieval. The above process sequentially repeats itself for each signal frame of the digital speech input.
FIG. 2
illustrates a decoding section of the second vocoder
11
for synthesizing the bit frames. The second vocoder
11
includes a second adaptive codebook
12
, a second fixed codebook
13
, a second adaptive codebook gain unit
14
, a second fixed codebook gain unit
15
, and a synthesis filter
16
. The second vocoder
11
receives the LPC bits, ACBG bits, ACB bits, FCB bits, and FCBG bits. These bits are used by the second vocoder
11
to reconstruct an estimate of the original signal frame, in a manner well known in the art.
The total number of bit positions within the bit fame allocated to the various parameters, as given above, relate to the vocoder
1
(IS127 EVRC CDMA coder) operating at a full rate of 8 kbps. To summarize, the bit frame would include: 28 LPC bits; 7 ADC bits; 3+3+3=9 ACBG bits; 35+35+35=105 FCB bits; and 5+5+5=15 FCBG bits. Therefore, the total number of bits in the bit frame would be 164 bits.
As mentioned above, the vocoder
1
is a multi-rate vocoder, and the half rate of the vocoder
1
is 4 kbps. When the vocoder
1
operates at the half rate, it is no longer possible to transmit bit frames having a size of one hundred and sixty-four bit positions, while still keeping up with an incoming digital speech input, in real time. Instead, the bit frame size must be reduced to approximately eighty bit positions.
When the vocoder
1
(IS127 EVRC CDMA coder) operates at its half rate (4 kbps), the bit position are rationed in the following order: 22 LPC bits; 7 ACB bits; 3+3+3=9 ACBG bits; 10+10+10=30 FCB bits; and 4+4+4=12 FCBG bits. Therefore, the total number of bits in the bit frame would be 80 bits. It can be seen that the FCB bits suffer the predominate share of the bit frame's reduction in size.
Since the present invention concerns the fixed codebook, a brief summary of the operation of the fixed codebook computation in the vocoder
1
is in order. In the full rate (8 kbps), the one hundred and five bit positions allocated toward representing the fixed codebook response for the frame have the ability of placing eight estimation pulses in each of the three sub-frames. Graphically this is represented in FIG.
3
.
In
FIG. 3
, a first signal line
17
is illustrative of a second residual signal presented to the fixed codebook
7
for estimation. The first sub-frame
18
is divided into fifty-three sample points, the second sub-frame
19
is also divided into fifty-three sample points, and the third sub-frame
20
is divided into fifty-four sample points.
In order to best estimate the characteristics of the second residual signal on signal line
17
, positive and/or negative pulses
21
are located at select ones of the sample points. For example, second signal line
22
illustrates the polarities and placements of the pulses
21
, in estimating the second residual signal of first signal line
17
. The placements and polarities are the data characterized by the FCB bits for each of the sub-frames
18
,
19
,
20
. In other words, for each sub-frame, the fixed codebook
7
estimates the best placement of eight to ten pulses
21
to represent the second residual signal of the first signal line
17
, and the FCB bits for that sub-frame identify the placements and polarities of the pulses
21
.
When the second vocoder
11
receives the FCB b

Affiliated with

Also associated with

Rating

Shaped fixed codebook search for celp speech coding does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Shaped fixed codebook search for celp speech coding, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Shaped fixed codebook search for celp speech coding will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2873278