Data processing: speech signal processing – linguistics – language – Speech signal processing – For storage or transmission
Reexamination Certificate
1999-10-07
2002-05-07
{overscore (S)}mits, T{overscore (a)}livaldis Ivars (Department: 2641)
Data processing: speech signal processing, linguistics, language
Speech signal processing
For storage or transmission
C704S219000, C704S228000, C381S094100
Reexamination Certificate
active
06385578
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for eliminating noises in code division multiple access (CDMA) systems and, more particularly to a method for eliminating annoying noises of an Enhanced Variable Rate Codec (EVRC) of a CDMA system in a weak electromagnetic field.
BACKGROUND OF THE INVENTION
Vocoders are used in communication devices, such as cellular phones or personal communication services (PCS), to provide digital signal compression of an analog audio signal that is converted into a digital form for transmission purpose. A conventional variable rate vocoder is disclosed in U.S. Pat. No. 5,414,796 issued to Jacobs et al. In this particular implementation of a variable rate vocoder, an input speech is encoded by using Code Excited Linear Predictive (CELP) coding technique at one of several rates as determined by a level of speech activity.
The CDMA Development Group (CDG) and Telecommunication Industry Association (TIA) finalized the specification of a new vocoder called Enhanced Variable Rate Codec (EVRC) for providing better quality than that of an existing vocoder. An EVRC algorithm is specified in TIA/EIA/IS-127, and an EVRC method is proposed in TIA/EIA/IS-718 with a fixed point C code.
The core of the standard EVRC algorithm is a Relaxed Code-Excited Linear Predictive (RCELP) coding. The RCELP coding is a generalization of the CELP speech coding algorithm and is particularly well suited for variable rate operation and robustness in CDMA environment.
FIG. 1
is a block diagram illustrating a conventional mobile station having an EVRC. Referring to
FIG. 1
, the mobile station comprises a Radio Frequency (RF) transmitter/receiver
10
, a baseband analog processor
20
, a Mobile Station Modem (MSM)
70
, a Digital-to-Analog Converter
80
, and an Analog-to-Digital Converter
90
. The MSM
70
includes a baseband digital modem
30
, a variable vocoder
40
having an encoder
50
and a decoder
60
, and a Central Processing Unit (CPU)
70
.
FIG. 2
is a block diagram illustrating the encoder of the EVRC shown in FIG.
1
. Referring to
FIG. 2
, the encoder
50
eliminates DC components and noise contained in the input speech signal IN_SPEECH from the A/D converter
90
by using high-pass and adaptive noise suppression filtering. And the encoder
50
determines Linear Prediction Coefficients (LPCs) and a rate. An LPC analysis filter
55
generates an excitation signal EXCITATION in response to the LPC coefficients and the filtered input speech signal IN_SPEECH′. The LPCs are converted to Line Spectral Pairs (LSPs). An adaptive codebook search process and a fixed codebook search process are respectively executed, thereby an adaptive codebook memory
58
is updated. Through above described processes, the input speech signal IN_SPEECH is converted to a digital speech packet that is supplied to the baseband digital modem
30
.
FIG. 3
is a block diagram illustrating the decoder of the EVRC shown in FIG.
1
. Referring to
FIG. 3
, the decoder
60
includes an adaptive codebook decoder
61
, a fixed codebook decoder
62
, an adaptive codebook memory
63
, an LPC synthesis filter
64
, and an adaptive post filter
67
including a pitch post filter
65
and an LPC synthesis post filter
66
. The adaptive codebook decoder
61
decodes adaptive codebook factors acb_gain and acb_delay from the encoder
50
and generates an adaptive codebook excitation signal. Similarly, the fixed codebook decoder
62
decodes fixed codebook factors fcb_gain and fcb_index from the encoder
50
and generates a fixed codebook excitation signal. The adaptive codebook excitation signal and the fixed codebook excitation signal are added for generating a total excitation signal EXCITATION. The excitation signal EXCITATION is used for updating the adaptive codebook memory
63
. The LPC synthesis filter
64
generates a synthesized signal OUT_SPEECH′ in response to the excitation signal EXCITATION and the LPC coefficients from the encoder
50
which are interpolated. The synthesized signal OUT_SPEECH′ is post-filtered through the pitch post filter
65
and the LPC post filter
66
so as to generate an output speech signal OUT_SPEECH. The adaptive post filter
67
improves the perceived speech quality of the decoder output signal OUT_SPEECH.
Additionally, the decoder
60
of the EVRC
40
is used for processing error packets as well as the above described operations. When the decoder
60
receives an error packet, the decoder
60
detects a last valid rate of the error packet inputted and decays a decoded signal by the detected rate. For example, the detected rate is either a full rate or a half rate.
The decoder
60
based on the specifications TIA/EIA/IS-127 and TIA/EIA/IS-718 stores an average adaptive codebook gain avg_acb_gain and an average fixed codebook gain avg_fcb_gain, so as to determine a codebook gain of the inputted error packet. If the stored average adaptive codebook gain avg_acb_gain is greater than a predetermined reference value (for example, 0.2), the excitation signal EXCITATION is post-filtered by the pitch post filter
65
of the adaptive post filter
67
, wherein the excitation signal EXCITATION includes the adaptive codebook factors.
If the stored average adaptive codebook gain avg_acb_gain is less than the reference value (for example, 0.2), a seed value for generating background noise is set by a certain LSP coefficient and the background noise is generated to the excitation signal EXCITATION. In that case, the excitation signal EXCITATION includes the sum of the adaptive codebook factors and the fixed codebook factors. The excitation signal EXCITATION is filtered by the adaptive post filter
67
with the pitch post filter
65
. More detailed error packet processing routine of the decoder
60
will be described below with reference to
FIG. 4
, which is a flowchart illustrating a conventional method for processing an error packet of the decoder included in the EVRC and illustrates an algorithm based on the specifications TIA/EIA/IS-127 and TIA/EIA/IS-718 for the EVRC
40
.
Referring to
FIG. 4
, at step S
110
, an error packet is inputted to the decoder
60
of the EVRC
40
. A last valid rate of the input packet is determined at step S
112
. If the determined valid rate is either ‘full’ or ‘half’, it is asked at step S
114
whether the error packet is occurring continually. If so, an average adaptive codebook gain avg_acb_gain is reduced to 75% at step S
116
, and then an excitation signal EXCITATION is generated from the calculated average adaptive codebook gain avg_acb_gain at step S
118
. The magnitude of the excitation signal EXCITATION is reduced to 75% because of the reduced average adaptive codebook gain avg_acb_gain.
If the error packet is determined not to occur continually at step
114
, the flow continues to step S
118
wherein the excitation signal EXCITATION is generated without the reduction. At step S
120
, it is determined whether the average adaptive codebook gain avg_acb_gain is less than a predetermined reference value of 0.2. If so, at step S
122
, the seed value is set and the background noise is generated to the excitation signal EXCITATION. If not, the background noise is not generated. At step S
124
, the excitation signal EXCITATION is post-filtered through the pitch post filter
65
and the LPC post filter
66
, so as to improve the perceived speech quality of the output speech signal. At step S
126
, the final output speech signal is doubled to be outputted from the decoder
60
and the error packet processing routine of the decoder
60
of the EVRC
40
is ended.
At step S
112
, if the error packet has the last valid rate of an eighth, the control flow proceeds to step S
128
wherein the background noise is produced in response to a seed value, and an excitation signal EXCITATION is generated by an average eighth gain and the generated background noise, wherein the seed value in the eighth rate is set at the beginning of the decoder. At step S
130
, the excitation signal EXCITATION is filtered through the LP
Kim Young-Jin
Lee Sang-Min
F. Chau & Associates LLP
{overscore (S)}mits T{overscore (a)}livaldis Ivars
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