CA2191972A1 - Method and apparatus for low rate coding and decoding - Google Patents
Method and apparatus for low rate coding and decodingInfo
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- CA2191972A1 CA2191972A1 CA002191972A CA2191972A CA2191972A1 CA 2191972 A1 CA2191972 A1 CA 2191972A1 CA 002191972 A CA002191972 A CA 002191972A CA 2191972 A CA2191972 A CA 2191972A CA 2191972 A1 CA2191972 A1 CA 2191972A1
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- 239000013598 vector Substances 0.000 claims description 58
- 238000004891 communication Methods 0.000 claims description 8
- 230000005540 biological transmission Effects 0.000 claims description 5
- 238000012545 processing Methods 0.000 claims description 3
- 239000002131 composite material Substances 0.000 claims 2
- 230000000875 corresponding effect Effects 0.000 description 12
- 230000003595 spectral effect Effects 0.000 description 8
- 238000013459 approach Methods 0.000 description 4
- 230000008569 process Effects 0.000 description 4
- 230000004075 alteration Effects 0.000 description 2
- 230000015556 catabolic process Effects 0.000 description 2
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- G—PHYSICS
- G10—MUSICAL INSTRUMENTS; ACOUSTICS
- G10L—SPEECH ANALYSIS TECHNIQUES OR SPEECH SYNTHESIS; SPEECH RECOGNITION; SPEECH OR VOICE PROCESSING TECHNIQUES; SPEECH OR AUDIO CODING OR DECODING
- G10L19/00—Speech or audio signals analysis-synthesis techniques for redundancy reduction, e.g. in vocoders; Coding or decoding of speech or audio signals, using source filter models or psychoacoustic analysis
- G10L19/02—Speech or audio signals analysis-synthesis techniques for redundancy reduction, e.g. in vocoders; Coding or decoding of speech or audio signals, using source filter models or psychoacoustic analysis using spectral analysis, e.g. transform vocoders or subband vocoders
- G10L19/032—Quantisation or dequantisation of spectral components
- G10L19/038—Vector quantisation, e.g. TwinVQ audio
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Abstract
A system for low rate coding or decoding includes the use of a mode indicator in the coded signal to indicate whether prequantizer (i.e., coarse) or quantizer (i.e., fine) information is being sent. In a transmitter (300) embodiment both a prequantizer (212) and a restricted quantizer (214) search is performed of predefined prequantizer ("P") and quantizer ("V") codewords, respectively, to determine the codewords with the least residual energy. The index of the P or V codeword with the least residual is then determined (220), and this index is transmitted along with an appropriate P or V mode indicator.
In a receiver (330) embodiment, the mode indicator is used to set the decoder (e.g., a vocoder) (350) to form a signal replica from a P codeword (determined from the transmitted index), or from a V codeword (from the transmitted V
index) and previously determined P codeword (e.g., the centroid of the V
codeword).
In a receiver (330) embodiment, the mode indicator is used to set the decoder (e.g., a vocoder) (350) to form a signal replica from a P codeword (determined from the transmitted index), or from a V codeword (from the transmitted V
index) and previously determined P codeword (e.g., the centroid of the V
codeword).
Description
~ W 0 96/34383 21 9 1 972 P~r/US96/03235 METHOD AND APPARATUS FOR LOW RATE CODING AND DECODING
Field of the Invention The present invention relates, in general, to communication systems and, more particularly, to a method and apparatus for coding/decoding a signal for communication.
Background of the Invention Source coding is used extensively in modern commun.~a~ :ns in order to provide more efficient ways of communicating a signal. One common type of source coder is a vocoder, in which a voice signal is coded for trans",ission. One example of a vocoder is a linear pll:dk~ti~ coder tLPC). In an LPC vocoder each input speech vector is mapped to LPC
parameters, which in turn are mapped to a best-fit codeword of a finite cod~book. An example of such a codeword would be one which upon filtering/decoding results in the least residual energy when compared with the original input speech. Once the codeword is determined, the index for the oodeword is determined and ll~nslllill~d in lieu of the original speech signal. At a receiving unit, this index is used to look up the codeword from a codebook, and regenerate the signal via appropriate filtering.
While vocoders have been successful in permitting greater efficiencies in voice communication, this efficiency is gained by a certain degradation in voice quality in the replica 3 0 signal. This degradation is particularly noticeable in low-rate vocoders, such as the 1/8th rate proposed in Interim Standard (IS)-95 based CDMA (code division multiple access) cellular systems. When operating in full and half rate modes, an IS-95 system allocates 171 and 80 bits, respectively, for each 20 millisecond (ms) speech frame. In contrast, only 16 bits are . , _ ... , . . ,, .. , . .. , . _, . _ , .. ... ,, . , _ ... .. . .
W096/34383 2 1 9 1 q 2 PCTIUS96/03235 ~
allocated per speech frame for 1/8th rate transmissions. As a consequence, a typical embedded scheme such as the IS-95 system suffers from high quantization distorlion, since only a small fraction of the original bits are available during 1/8th 5 rate frame transmission.
No solutions to this problem have been satisfactorily proposed to date. For example, in proposed IS-96 speech coding, a differential encoding scheme which allocates one bit 10 for each 10 LPC parameters is suggest~ While providing better resolution, this method suffers the disadvantage that frame erasures can cause excessive quanli,ation error build-up, which can result in spectral drift and/or unstable synthesis filter co-efficients. A gross reduction in frame 15 energy may be used to mask the potential spectral drift effect during 1/8th rate, but this has resulted in the added problem of producing a "pumping" artifact when subjected to conditions of high-background noise levels. Another solution has been to inject random code vectors through the spectral filter 20 followed by a scaling of the resulting signal energy, when the 1/8th rate is used primarily for coding of bac'cground or "comfort" noise. However, without better resolution the synthesized background noise can still sound choppy under this approach, which will tend to be distracting to the user. There 25 thus remains a need for an improved method of coding signals at low rates, while maintaining low complexity and robustness to channel errors.
3 0 Brief Description of the Drawings FIG. 1 is a diagram illustrating the logical structure of a codebook for use in accordance with an embodiment of the invention;
Field of the Invention The present invention relates, in general, to communication systems and, more particularly, to a method and apparatus for coding/decoding a signal for communication.
Background of the Invention Source coding is used extensively in modern commun.~a~ :ns in order to provide more efficient ways of communicating a signal. One common type of source coder is a vocoder, in which a voice signal is coded for trans",ission. One example of a vocoder is a linear pll:dk~ti~ coder tLPC). In an LPC vocoder each input speech vector is mapped to LPC
parameters, which in turn are mapped to a best-fit codeword of a finite cod~book. An example of such a codeword would be one which upon filtering/decoding results in the least residual energy when compared with the original input speech. Once the codeword is determined, the index for the oodeword is determined and ll~nslllill~d in lieu of the original speech signal. At a receiving unit, this index is used to look up the codeword from a codebook, and regenerate the signal via appropriate filtering.
While vocoders have been successful in permitting greater efficiencies in voice communication, this efficiency is gained by a certain degradation in voice quality in the replica 3 0 signal. This degradation is particularly noticeable in low-rate vocoders, such as the 1/8th rate proposed in Interim Standard (IS)-95 based CDMA (code division multiple access) cellular systems. When operating in full and half rate modes, an IS-95 system allocates 171 and 80 bits, respectively, for each 20 millisecond (ms) speech frame. In contrast, only 16 bits are . , _ ... , . . ,, .. , . .. , . _, . _ , .. ... ,, . , _ ... .. . .
W096/34383 2 1 9 1 q 2 PCTIUS96/03235 ~
allocated per speech frame for 1/8th rate transmissions. As a consequence, a typical embedded scheme such as the IS-95 system suffers from high quantization distorlion, since only a small fraction of the original bits are available during 1/8th 5 rate frame transmission.
No solutions to this problem have been satisfactorily proposed to date. For example, in proposed IS-96 speech coding, a differential encoding scheme which allocates one bit 10 for each 10 LPC parameters is suggest~ While providing better resolution, this method suffers the disadvantage that frame erasures can cause excessive quanli,ation error build-up, which can result in spectral drift and/or unstable synthesis filter co-efficients. A gross reduction in frame 15 energy may be used to mask the potential spectral drift effect during 1/8th rate, but this has resulted in the added problem of producing a "pumping" artifact when subjected to conditions of high-background noise levels. Another solution has been to inject random code vectors through the spectral filter 20 followed by a scaling of the resulting signal energy, when the 1/8th rate is used primarily for coding of bac'cground or "comfort" noise. However, without better resolution the synthesized background noise can still sound choppy under this approach, which will tend to be distracting to the user. There 25 thus remains a need for an improved method of coding signals at low rates, while maintaining low complexity and robustness to channel errors.
3 0 Brief Description of the Drawings FIG. 1 is a diagram illustrating the logical structure of a codebook for use in accordance with an embodiment of the invention;
WO 96r34383 P~
FIG. 2 is a flow-chart illustrating a preferred method for low-rate encoding in accordance with the invention;
FIG. 3 is a block diagram illustrating a first embodiment apparatus according to the invention.
Detailed Description of the Drawings These problems and others are solved by an improved method and apparatus according to the invention. A presently 10 preferred embodiment of the invention is a method in which a prequantizer search is performed to determine a best prequantizer (~P") codeword (i.e., having a least prequantizer residual co",ponent), and a restricted quantizer search is pe,r~,r,-,ed to deIel",' ,e the best quantizer (~QD) code~.Jrd (i.e., 15 having the least quantizer residual component). The residual components are then compared to determine which is least, and the co"e:spondi"g index of the codeword yielding the least residual is formatted for I,dns",i:jsion, along with a mode indicator indicating which type of index (quantizer or 20 prequantizer) is being sent. At a receiving unit, the coded signal is fed to a vocoder which first determines which mode indicator was sent. If a quantizer ("V") mode indicator (e.g., a O bit) is received, then the vocoder uses the ~V" cod~,..uld co"~:,pondi"g to the received V index along with a previously 2 5 determined P codeword to form a replica of the original signal.
If the prequantizer (~P") mode indicator is received, the vocoder uses the P codeword corresponding to the received P
index to form the signal replica. As a consequence of this approach, only a reduced number of bits (e.g., 12 bits in the 3 0 embodiment described below) are required to effectively code the signal information at lower rates.
Turning now to FIG. 1, a codebook structure such as may be used with the present invention is illustrated. As can be 3 5 seen, vector quantizer 100 is a two-level, tree-structured WO 96/34383 2 1 9 1 9 7 2 PCT/US96/03235 ~
vector quantizer. This structure illustrates the prequantizer and quantizer hierarchy used in a typical predetermined vector quantizer. The quantizer stage 110 include& 1 through N
prequantizer codewords, e.g., codeword P1 111, each P
5 codeword corresponding to one of plural prequantizer (~P") indices of the predetermined vector quantizer. The lower quantizer stage 120 includes plural sets of plural quantizer ("V") codewords 121-123, 131-133, each quantizer ("V~) code~JId of a set cor,t,spond;"g to single one of plural 10 quantizer ("V") indices. Each set of V codewords is ~soc;~ied with a P codeword. For example, in FIG. 1 the first set of V
cod_words 121-123 is associated with P codeword 111, in which P codeword 111 forms a centroid (typically of lower resolution) for the (typically higher resolution) V codeword 15 vectors 121-123.
As one skilled in the art will appreciate, it is common in LPC vocoders to use codebooks having multiple tree structures.
in such vocoders, the tree structure of FIG. 1 would be 20 .~preser,l~live of each of the vector sey",d"l~i that are used in determining a -sflection coefficient vector estimate. For example, in a three-vector-segment vector quantizer, in which a ten-element reflection co-efficient vector estimate is used, this reflection co-efficient vector estimate might typically be 2 5 segmented as follows:
r 1 - 1 o = r 1 - 3 + rA4- 6 + rA7 1 O
Equation 1 3 0 In such a vector quantizer, the following bid allocation is representative of what one would expect in a full or half-rate sub-frame in each of the respective three vector segments:
Table 1. Full/Half Rate LPC Bit Allocation WO 96/34383 2 1 9 1 ~ 7 2 PCTIUS96/03235 Segment P bits V bits Total Total 15 13 28 Thus, in the tree for segment 1 there will be 26 = 64 possible P cod~..o,-l~, and 25 = 32 possible V codewords per associated 10 set under each P cod_wur.l. Again, the P code~,rds l~:,urtlser,l the prequantizer cells of the codebook and are used to code the coarse spectral information, while the V codewords are used to code the fine or detailed spectral information.
However, not all 28 bits of i"fcr",~(ion are necessary to maintain a high-quality signal. For example, if the signal that is being encoded is relatively stationary, then the prequantizer index (P index) for each respective segment will typically be highly correlated to the previously detetmined prequantizer 20 index. One need only then transmit the detailed spectral information (i.e., the V index) within a low, for example 1/8th, rate frame, and concatenate this information to the previously determined P codeword (i.e., the codeword corresponding to the previously used P index, per segment) at the synthesizer or 25 decoder of the receiver. On the other hand, if the signal being coded has changing spectral characteristics, one need only transmit the P indices within the 1/8th rate frame and update the prequantizer state at the decoder. The P codewords corresponding to the lldlls,,,illed P indices would then be used 30 in the decoding filter for the current frame, with fine spectral ~ information (V codewords) concatenated on successive frames.
This allows for an adequate representation of the signal by a reduced number of bits, such as is illustrated by the following table:
W 0 96134383 2 1 9 1 972 P~r~US96103235 Table 2: 1/8 Rate LPC Bit Allocation Segment P bits -V
bits (mode=1 ) (mode=0) Total 12 12 While the same number of bits could be used for P
codewords and V codewords per segment for all of the variable rates used by vocoder, Table 2 illustrates one approach for further reducing the number of bits required for l,~ns".ission 15 at lower rates. In this case, assuming the same codebook is used for all rates, the least sig"i~icant bits for certain of the P and V cod~,..urds are dropped prior to ~,~ns",;sa;on. Thus, for example, the 1/8th rate P code..old for segment 1 would only include the five most :,iyl ,iricant bits of the stored P
20 codeword, which as shown in Table 1 is 6 bits in length. At the same time, certain other codewords would be sent in their entirety (e.g., the V codeword of segment 1). One skilled in the art will app,e~ le that there are numerous ways in which the P and V codewords within the codebook can be defined, as well 2 5 as the bit structure that is transmitted at the different rates.
What should be appreciated is that by this altarnate approach further reduction in the size of the codeword index (or indices) for transmission is achieved. Thus, for example, where only a 16 bit structure is permitted for 1/8th rate framing, only 12 30 bits of vector quantizer information are needed, leaving one bit for a mode indicator and three bits for the coding of difference energy. The mode indicator bit would either be used as a quantizer mode indicator (V mode indicator) when only the V index is to be transmitted, and a prequantizer mode indicator WO 96/34383 2 1 9 1 ~ 7 2 PCTIUS96/03235 (P mode indicator, e.g., a 1 bit) when the P index is being transmitted.
Turning now to FIG. 2, a flow chart illustrating a 5 preferred emhoo' "er,l of a coding process according to the invention as shown. This process cor"",ences with an initial determination that the vocoder is in 1/8th rat~ mode (Step 210). A prequantizer search (step 212) is then performed on an input signal. This search is performed by comparing each P
10 codeword of the codebook with the signal and delen"i~ ~9 the P codeword having the least prequantizer residual component (e.g., energy). In the case where the vector quantizer is a plural-vector-segment vector quantizer (e.g., three segments), a determination of the best P codeword for each vector 15 segment is made based on the P codewords having the least prequantizer residual energy, then each of the best P
codewords are vector added to form a prequantizer vector sum, and the least prequantizer residual component is determined using this vector sum. Those skilled in the art will app,t:oiaIe 20 that other methods for deler",i" ,9 the least prequantizer residual component may be used. Further, the prequantizer search may be restricted to only those P codewords corresponding to the P indices that can be transmitted, where only a restricted number of bits can be used in transmission 25 (such as shown in Table 2). Thus, in the example of Table 2, only those P codewords having the same least significant bit, for example a 0 bit, would be searched in the encoding process;
at the same time, the receiver units would be configured to add a 0 bit as the least significant bit to the corresponding 30 indices received prior to looking up the codewords in the codebook.
In step 214, a restricted quantizer search is performed. The input signal is again compared, but this time 35 against each quantizer codeword (or V codewords, each _ _ _ _ _ . . . .. . . .. _ .... .. .. . . . . . . _ W096134383 2 1 9 1 972 PCTIUS96/03U5 ~
corresponding to one of plural quantizer indices of the codebook) of one set, or one set per segment if multiple vector seg",e"l:, are used. This set is d~le~",- ,ed by a pre; ousiy determined P codeword, which forms the prequantizer (or 5 centroid) of the set of V codewords. In the case where the determination is made for an initial 118th rate frame, the previously determined P code~on~ could be a predetermined initialization P codeword (which for convenience could simply be the P codeword having index 000000 for a Table 2 segment 10 1 set). Otherwise, the last P codeword l,dns",illed is used (both transmitting and receiving vocoders thereby possessing the P cocle..Jrd). The V cod~,~.Jrd of the set having the least quantizer residual component is then determined, and its corresponding V index stored in memory. As in the case of the 15 prequantizer search, the best V cod~ "d of the predetermined set of V code~nls is determined for each vector segment if there are plural segments, with a quantizer vector sum being formed by vector adding each of the best V cod~..or.ls, and a least quantizer residual component being determined 20 therefrom. Alternately, a least quantizer residual component could be determined for each V codeword per s~gme:ll, or more simply just that of the first segment; these value(s) would in turn then be used in step 220. One skilled in the art will appreciate that other alternatives are also available in 25 forming a residual measure for use in comparing and determining which index to use.
Next, in step 220 a comparison is made between the least quantizer residual component and the least 3 0 prequantizer residual component to determine which one is less. Alternatively, in the case of a segmented codebook, multiple residuals, or the residual relating to the lowest order coefficients (i.e., segment 1) could be used. When the least prequantizer residual component is the lesser of the two, a 3 5 prequantizer mode indicator (P mode indicator) is output (step ... .... _ . . ... .... . ... . .. . . . .. ... . . _ _ _ .
~ W 0 96/34383 2 1 9 1 q72 p(~rruS96/0323~
_ g _ 222) along with the P index corresponding to the P code~,rd generating the least prequantizer residual co",ponent. Orl the other hand, if the least quantizer residual component is the lesser of the two, a quantizer mode indicator (V mode 5 indicator) is output along with the V index corresponding to the V codeword generating it (step 224).
- In a preferred embodiment, prior to outputting the determined mode indicator and index a further determination 10 is made as to whether a higher rate of coding is needed, for better resolution. Thus, in step 230 both residual cGI"poner~
are cor"pared against a predetermined threshold to determine if too much residual energy is present. This is an i, '- "or, that an unacce,l i b!e signal quality will be present when 15 decoded. One skilled in the art will know how to design an apploprid~ threshold, based upon such factors as the ap~' ~n involved, user preferences and the like. If the threshold is exceederl, the vocoder is forced to a higher rate (step 232). If the threshold is not ~ceeded the detel" ~ ,ed 20 index and mode indicator, along with other desired signal parameters (e.g., the quantizer residual energy for certai vocoders) are then formed into the coded signal and transmitted (step 234). The process is then repeated for the next signal (step 236).
Finally, FIG. 3 illustrates one embodiment of a communication system in which low-rate coding according to the invention is used. When an input signal 301 such as speech is received by variable rate vocoder 310, the signal is fed to 30 both prequantizer searcher 312 and quantizer searcher 314.
Both searchers perform a comparison in accordance with steps 212 and 214, respectively, of FIG. 2 (using the P and V indices to retrieve the P and V codewords, respectively, from codebook 311). The determined P index and residual component (signal 313~ and V index and residual component (signal 315) are fed , _ _ ... .. ..... .. ... .... ... . . . . .
to a coder 316 for determination of which low-rate mode (if either) is to be used. A coded version of the signal is then formed using the apprcpriate index and moda indicator, along with other applopridle (conventional) parameters. This coded 5 signal (signal 317) is then modulated in modulator 3ZO and amplified and l,dnsr"illed via transmitter 325 of unit 300.
A receiver cornmunication unit 330 receives and demodulates the l,dns",illed signal via receiver 335 and 1 0 demodulator 340, outputting a coded version of the original signal. A decoder (e.g., vocoder 350) inputs the coded version of the signal 341 to mode controller 352 and signal generator 354. Controller 352 d~l~r",i"es if the coded signal includes a V or P mode indicator, and if not which of the higher decoding 1 5 rates is ,, ' ~~'-'e. When a P mode indicator is received, controller 352 controls signal generator 354 to retrieve the P
cod~ ,rd corresponding to the received P index from codebook 351, and generate a replica of the original signal using the P
codeword, along with other received signal parameters (e.g., 2û the residual energy). When a V mode indicator is received, the signal generatcr 354 is controlled to use bo"l the previously received P codeword (i.e., the P codeword used in the prior frame) and the V codeword corresponding to the received V
index, along with other signal parameters, to generate the 2 5 signal replica. Finally, the signal replica is outputted as output 356 for further processing, such as transduction into an audible signal.
Thus, it will be apparent to one skilled in the art that 30 there has been provided in accordance with the invention, a method and apparatus for low rate coding/decoding of a signal that fully satisfies the objectives, aims, and advantages set forth above.
~ W 0 96134383 2 1 9 l q72 p(~rruS96/03235 While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alterations, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description.
5 For example, while the low rate vocoding apparatus of FIG. 3 has been described in terms of specific logical/functional circuitry relationshil-s, one skilled in the art will appreciate that such may be embodied in a variety of ways, such as appropriately configured and pruy,d"""ed processors, ASlCs 10 (application specific integrated circuits), and DSPs (digital signal prucessor~). Further, the invention is not limited in application to just vocoders in cellular communication systems, but also applies to other source coders for other types of communication systems. Accordi"yly, the invention 15 is intended to embrace all such alterations, modifications, and variations within the spirit and scope of the appended claims.
We claim:
- !, ! A ;,~
FIG. 3 is a block diagram illustrating a first embodiment apparatus according to the invention.
Detailed Description of the Drawings These problems and others are solved by an improved method and apparatus according to the invention. A presently 10 preferred embodiment of the invention is a method in which a prequantizer search is performed to determine a best prequantizer (~P") codeword (i.e., having a least prequantizer residual co",ponent), and a restricted quantizer search is pe,r~,r,-,ed to deIel",' ,e the best quantizer (~QD) code~.Jrd (i.e., 15 having the least quantizer residual component). The residual components are then compared to determine which is least, and the co"e:spondi"g index of the codeword yielding the least residual is formatted for I,dns",i:jsion, along with a mode indicator indicating which type of index (quantizer or 20 prequantizer) is being sent. At a receiving unit, the coded signal is fed to a vocoder which first determines which mode indicator was sent. If a quantizer ("V") mode indicator (e.g., a O bit) is received, then the vocoder uses the ~V" cod~,..uld co"~:,pondi"g to the received V index along with a previously 2 5 determined P codeword to form a replica of the original signal.
If the prequantizer (~P") mode indicator is received, the vocoder uses the P codeword corresponding to the received P
index to form the signal replica. As a consequence of this approach, only a reduced number of bits (e.g., 12 bits in the 3 0 embodiment described below) are required to effectively code the signal information at lower rates.
Turning now to FIG. 1, a codebook structure such as may be used with the present invention is illustrated. As can be 3 5 seen, vector quantizer 100 is a two-level, tree-structured WO 96/34383 2 1 9 1 9 7 2 PCT/US96/03235 ~
vector quantizer. This structure illustrates the prequantizer and quantizer hierarchy used in a typical predetermined vector quantizer. The quantizer stage 110 include& 1 through N
prequantizer codewords, e.g., codeword P1 111, each P
5 codeword corresponding to one of plural prequantizer (~P") indices of the predetermined vector quantizer. The lower quantizer stage 120 includes plural sets of plural quantizer ("V") codewords 121-123, 131-133, each quantizer ("V~) code~JId of a set cor,t,spond;"g to single one of plural 10 quantizer ("V") indices. Each set of V codewords is ~soc;~ied with a P codeword. For example, in FIG. 1 the first set of V
cod_words 121-123 is associated with P codeword 111, in which P codeword 111 forms a centroid (typically of lower resolution) for the (typically higher resolution) V codeword 15 vectors 121-123.
As one skilled in the art will appreciate, it is common in LPC vocoders to use codebooks having multiple tree structures.
in such vocoders, the tree structure of FIG. 1 would be 20 .~preser,l~live of each of the vector sey",d"l~i that are used in determining a -sflection coefficient vector estimate. For example, in a three-vector-segment vector quantizer, in which a ten-element reflection co-efficient vector estimate is used, this reflection co-efficient vector estimate might typically be 2 5 segmented as follows:
r 1 - 1 o = r 1 - 3 + rA4- 6 + rA7 1 O
Equation 1 3 0 In such a vector quantizer, the following bid allocation is representative of what one would expect in a full or half-rate sub-frame in each of the respective three vector segments:
Table 1. Full/Half Rate LPC Bit Allocation WO 96/34383 2 1 9 1 ~ 7 2 PCTIUS96/03235 Segment P bits V bits Total Total 15 13 28 Thus, in the tree for segment 1 there will be 26 = 64 possible P cod~..o,-l~, and 25 = 32 possible V codewords per associated 10 set under each P cod_wur.l. Again, the P code~,rds l~:,urtlser,l the prequantizer cells of the codebook and are used to code the coarse spectral information, while the V codewords are used to code the fine or detailed spectral information.
However, not all 28 bits of i"fcr",~(ion are necessary to maintain a high-quality signal. For example, if the signal that is being encoded is relatively stationary, then the prequantizer index (P index) for each respective segment will typically be highly correlated to the previously detetmined prequantizer 20 index. One need only then transmit the detailed spectral information (i.e., the V index) within a low, for example 1/8th, rate frame, and concatenate this information to the previously determined P codeword (i.e., the codeword corresponding to the previously used P index, per segment) at the synthesizer or 25 decoder of the receiver. On the other hand, if the signal being coded has changing spectral characteristics, one need only transmit the P indices within the 1/8th rate frame and update the prequantizer state at the decoder. The P codewords corresponding to the lldlls,,,illed P indices would then be used 30 in the decoding filter for the current frame, with fine spectral ~ information (V codewords) concatenated on successive frames.
This allows for an adequate representation of the signal by a reduced number of bits, such as is illustrated by the following table:
W 0 96134383 2 1 9 1 972 P~r~US96103235 Table 2: 1/8 Rate LPC Bit Allocation Segment P bits -V
bits (mode=1 ) (mode=0) Total 12 12 While the same number of bits could be used for P
codewords and V codewords per segment for all of the variable rates used by vocoder, Table 2 illustrates one approach for further reducing the number of bits required for l,~ns".ission 15 at lower rates. In this case, assuming the same codebook is used for all rates, the least sig"i~icant bits for certain of the P and V cod~,..urds are dropped prior to ~,~ns",;sa;on. Thus, for example, the 1/8th rate P code..old for segment 1 would only include the five most :,iyl ,iricant bits of the stored P
20 codeword, which as shown in Table 1 is 6 bits in length. At the same time, certain other codewords would be sent in their entirety (e.g., the V codeword of segment 1). One skilled in the art will app,e~ le that there are numerous ways in which the P and V codewords within the codebook can be defined, as well 2 5 as the bit structure that is transmitted at the different rates.
What should be appreciated is that by this altarnate approach further reduction in the size of the codeword index (or indices) for transmission is achieved. Thus, for example, where only a 16 bit structure is permitted for 1/8th rate framing, only 12 30 bits of vector quantizer information are needed, leaving one bit for a mode indicator and three bits for the coding of difference energy. The mode indicator bit would either be used as a quantizer mode indicator (V mode indicator) when only the V index is to be transmitted, and a prequantizer mode indicator WO 96/34383 2 1 9 1 ~ 7 2 PCTIUS96/03235 (P mode indicator, e.g., a 1 bit) when the P index is being transmitted.
Turning now to FIG. 2, a flow chart illustrating a 5 preferred emhoo' "er,l of a coding process according to the invention as shown. This process cor"",ences with an initial determination that the vocoder is in 1/8th rat~ mode (Step 210). A prequantizer search (step 212) is then performed on an input signal. This search is performed by comparing each P
10 codeword of the codebook with the signal and delen"i~ ~9 the P codeword having the least prequantizer residual component (e.g., energy). In the case where the vector quantizer is a plural-vector-segment vector quantizer (e.g., three segments), a determination of the best P codeword for each vector 15 segment is made based on the P codewords having the least prequantizer residual energy, then each of the best P
codewords are vector added to form a prequantizer vector sum, and the least prequantizer residual component is determined using this vector sum. Those skilled in the art will app,t:oiaIe 20 that other methods for deler",i" ,9 the least prequantizer residual component may be used. Further, the prequantizer search may be restricted to only those P codewords corresponding to the P indices that can be transmitted, where only a restricted number of bits can be used in transmission 25 (such as shown in Table 2). Thus, in the example of Table 2, only those P codewords having the same least significant bit, for example a 0 bit, would be searched in the encoding process;
at the same time, the receiver units would be configured to add a 0 bit as the least significant bit to the corresponding 30 indices received prior to looking up the codewords in the codebook.
In step 214, a restricted quantizer search is performed. The input signal is again compared, but this time 35 against each quantizer codeword (or V codewords, each _ _ _ _ _ . . . .. . . .. _ .... .. .. . . . . . . _ W096134383 2 1 9 1 972 PCTIUS96/03U5 ~
corresponding to one of plural quantizer indices of the codebook) of one set, or one set per segment if multiple vector seg",e"l:, are used. This set is d~le~",- ,ed by a pre; ousiy determined P codeword, which forms the prequantizer (or 5 centroid) of the set of V codewords. In the case where the determination is made for an initial 118th rate frame, the previously determined P code~on~ could be a predetermined initialization P codeword (which for convenience could simply be the P codeword having index 000000 for a Table 2 segment 10 1 set). Otherwise, the last P codeword l,dns",illed is used (both transmitting and receiving vocoders thereby possessing the P cocle..Jrd). The V cod~,~.Jrd of the set having the least quantizer residual component is then determined, and its corresponding V index stored in memory. As in the case of the 15 prequantizer search, the best V cod~ "d of the predetermined set of V code~nls is determined for each vector segment if there are plural segments, with a quantizer vector sum being formed by vector adding each of the best V cod~..or.ls, and a least quantizer residual component being determined 20 therefrom. Alternately, a least quantizer residual component could be determined for each V codeword per s~gme:ll, or more simply just that of the first segment; these value(s) would in turn then be used in step 220. One skilled in the art will appreciate that other alternatives are also available in 25 forming a residual measure for use in comparing and determining which index to use.
Next, in step 220 a comparison is made between the least quantizer residual component and the least 3 0 prequantizer residual component to determine which one is less. Alternatively, in the case of a segmented codebook, multiple residuals, or the residual relating to the lowest order coefficients (i.e., segment 1) could be used. When the least prequantizer residual component is the lesser of the two, a 3 5 prequantizer mode indicator (P mode indicator) is output (step ... .... _ . . ... .... . ... . .. . . . .. ... . . _ _ _ .
~ W 0 96/34383 2 1 9 1 q72 p(~rruS96/0323~
_ g _ 222) along with the P index corresponding to the P code~,rd generating the least prequantizer residual co",ponent. Orl the other hand, if the least quantizer residual component is the lesser of the two, a quantizer mode indicator (V mode 5 indicator) is output along with the V index corresponding to the V codeword generating it (step 224).
- In a preferred embodiment, prior to outputting the determined mode indicator and index a further determination 10 is made as to whether a higher rate of coding is needed, for better resolution. Thus, in step 230 both residual cGI"poner~
are cor"pared against a predetermined threshold to determine if too much residual energy is present. This is an i, '- "or, that an unacce,l i b!e signal quality will be present when 15 decoded. One skilled in the art will know how to design an apploprid~ threshold, based upon such factors as the ap~' ~n involved, user preferences and the like. If the threshold is exceederl, the vocoder is forced to a higher rate (step 232). If the threshold is not ~ceeded the detel" ~ ,ed 20 index and mode indicator, along with other desired signal parameters (e.g., the quantizer residual energy for certai vocoders) are then formed into the coded signal and transmitted (step 234). The process is then repeated for the next signal (step 236).
Finally, FIG. 3 illustrates one embodiment of a communication system in which low-rate coding according to the invention is used. When an input signal 301 such as speech is received by variable rate vocoder 310, the signal is fed to 30 both prequantizer searcher 312 and quantizer searcher 314.
Both searchers perform a comparison in accordance with steps 212 and 214, respectively, of FIG. 2 (using the P and V indices to retrieve the P and V codewords, respectively, from codebook 311). The determined P index and residual component (signal 313~ and V index and residual component (signal 315) are fed , _ _ ... .. ..... .. ... .... ... . . . . .
to a coder 316 for determination of which low-rate mode (if either) is to be used. A coded version of the signal is then formed using the apprcpriate index and moda indicator, along with other applopridle (conventional) parameters. This coded 5 signal (signal 317) is then modulated in modulator 3ZO and amplified and l,dnsr"illed via transmitter 325 of unit 300.
A receiver cornmunication unit 330 receives and demodulates the l,dns",illed signal via receiver 335 and 1 0 demodulator 340, outputting a coded version of the original signal. A decoder (e.g., vocoder 350) inputs the coded version of the signal 341 to mode controller 352 and signal generator 354. Controller 352 d~l~r",i"es if the coded signal includes a V or P mode indicator, and if not which of the higher decoding 1 5 rates is ,, ' ~~'-'e. When a P mode indicator is received, controller 352 controls signal generator 354 to retrieve the P
cod~ ,rd corresponding to the received P index from codebook 351, and generate a replica of the original signal using the P
codeword, along with other received signal parameters (e.g., 2û the residual energy). When a V mode indicator is received, the signal generatcr 354 is controlled to use bo"l the previously received P codeword (i.e., the P codeword used in the prior frame) and the V codeword corresponding to the received V
index, along with other signal parameters, to generate the 2 5 signal replica. Finally, the signal replica is outputted as output 356 for further processing, such as transduction into an audible signal.
Thus, it will be apparent to one skilled in the art that 30 there has been provided in accordance with the invention, a method and apparatus for low rate coding/decoding of a signal that fully satisfies the objectives, aims, and advantages set forth above.
~ W 0 96134383 2 1 9 l q72 p(~rruS96/03235 While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alterations, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description.
5 For example, while the low rate vocoding apparatus of FIG. 3 has been described in terms of specific logical/functional circuitry relationshil-s, one skilled in the art will appreciate that such may be embodied in a variety of ways, such as appropriately configured and pruy,d"""ed processors, ASlCs 10 (application specific integrated circuits), and DSPs (digital signal prucessor~). Further, the invention is not limited in application to just vocoders in cellular communication systems, but also applies to other source coders for other types of communication systems. Accordi"yly, the invention 15 is intended to embrace all such alterations, modifications, and variations within the spirit and scope of the appended claims.
We claim:
- !, ! A ;,~
Claims (10)
1. A method for low rate vocoding and communication of a first signal comprising:
(a) comparing each prequantizer ("P") codeword of a predetermined vector quantizer, each P codeword corresponding to one of plural prequantizer ("P") indices, with the first signal and determining a first P codeword, with a corresponding first P
index, having a least prequantizer residual component;
(b) comparing each quantizer codeword ("V codeword") of a first set of the predetermined vector quantizer, each V codeword corresponding to one of plural quantizer indices ("V indices") and the first set being associated with a previously determined P
codeword, with the first signal and determining a first V
codeword, with a corresponding first V index but not the first P
index, having a least quantizer residual component; and (c) transmitting a coded signal comprising the first V index and a quantizer mode indicator ("V mode indicator") when the least prequantizer residual component is greater than the least quantizer residual component.
(a) comparing each prequantizer ("P") codeword of a predetermined vector quantizer, each P codeword corresponding to one of plural prequantizer ("P") indices, with the first signal and determining a first P codeword, with a corresponding first P
index, having a least prequantizer residual component;
(b) comparing each quantizer codeword ("V codeword") of a first set of the predetermined vector quantizer, each V codeword corresponding to one of plural quantizer indices ("V indices") and the first set being associated with a previously determined P
codeword, with the first signal and determining a first V
codeword, with a corresponding first V index but not the first P
index, having a least quantizer residual component; and (c) transmitting a coded signal comprising the first V index and a quantizer mode indicator ("V mode indicator") when the least prequantizer residual component is greater than the least quantizer residual component.
2. The method of claim 1, wherein in step (c) the coded signal comprises the first P index and a prequantizer ("P") mode indicator when the least prequantizer residual component is less than the least quantizer residual component.
3. The method of claim 2, wherein the predetermined vector quantizer is a plural vector segment vector quantizer, and:
step (a) further comprises determining a best P codeword, for each vector segment of the predetermined vector quantizer, having a least prequantizer residual component, vector adding each best P codeword to form a prequantizer vector sum, and determining the least prequantizer residual component using the prequantizer vector sum;
step (b) further comprises determining a best V codeword of a predetermined set of V codewords for each vector segment, wherein for each vector segment one predetermined set of V
codewords is determined by an associated predetermined P
codeword of said each vector segment, vector adding each best V
codeword to form a quantizer vector sum, and determining the least quantizer residual component using the quantizer vector sum; and step (c) further comprises transmitting each one of the plural P indices corresponding to each best P codeword and the prequantizer mode indicator when the least prequantizer residual component is less than the least quantizer residual component, and transmitting each one of the plural V indices corresponding to each best V codeword and a quantizer mode indicator when the least prequantizer residual component is greater than the least quantizer residual component.
step (a) further comprises determining a best P codeword, for each vector segment of the predetermined vector quantizer, having a least prequantizer residual component, vector adding each best P codeword to form a prequantizer vector sum, and determining the least prequantizer residual component using the prequantizer vector sum;
step (b) further comprises determining a best V codeword of a predetermined set of V codewords for each vector segment, wherein for each vector segment one predetermined set of V
codewords is determined by an associated predetermined P
codeword of said each vector segment, vector adding each best V
codeword to form a quantizer vector sum, and determining the least quantizer residual component using the quantizer vector sum; and step (c) further comprises transmitting each one of the plural P indices corresponding to each best P codeword and the prequantizer mode indicator when the least prequantizer residual component is less than the least quantizer residual component, and transmitting each one of the plural V indices corresponding to each best V codeword and a quantizer mode indicator when the least prequantizer residual component is greater than the least quantizer residual component.
4. A system for low rate coding and communication of a first signal comprising:
(a) prequantizer search means for comparing each prequantizer ("P") codeword of a predetermined vector quantizer, each P
codeword corresponding to one of plural prequantizer ("P") indices, with the first signal and determining a first P codeword, with a corresponding first P index, having a least prequantizer residual component;
(b) quantizer search means for comparing each quantizer codeword ("V codeword") of a first set of the predetermined vector quantizer, each V codeword corresponding to one of plural quantizer indices ("V indices") and the first set being associated with a previously determined P codeword, with the first signal and determining a first V codeword, with a corresponding first V
index, having a least quantizer residual component; and (c) coding means for transmitting a coded signal comprising the first V index but not the first P index and a quantizer mode indicator ("V mode indicator") when the least prequantizer residual component is greater than the least quantizer residual component.
(a) prequantizer search means for comparing each prequantizer ("P") codeword of a predetermined vector quantizer, each P
codeword corresponding to one of plural prequantizer ("P") indices, with the first signal and determining a first P codeword, with a corresponding first P index, having a least prequantizer residual component;
(b) quantizer search means for comparing each quantizer codeword ("V codeword") of a first set of the predetermined vector quantizer, each V codeword corresponding to one of plural quantizer indices ("V indices") and the first set being associated with a previously determined P codeword, with the first signal and determining a first V codeword, with a corresponding first V
index, having a least quantizer residual component; and (c) coding means for transmitting a coded signal comprising the first V index but not the first P index and a quantizer mode indicator ("V mode indicator") when the least prequantizer residual component is greater than the least quantizer residual component.
5. The system of claim 4, further comprising:
(d) receiving means for receiving the coded signal when the coded signal includes the V mode indicator, and (e) decoding means for decoding the coded signal in a first mode, in response to the V mode indicator, into a replica of the first signal based on the previously determined P codeword and the first V codeword corresponding to the transmitted first V
index.
(d) receiving means for receiving the coded signal when the coded signal includes the V mode indicator, and (e) decoding means for decoding the coded signal in a first mode, in response to the V mode indicator, into a replica of the first signal based on the previously determined P codeword and the first V codeword corresponding to the transmitted first V
index.
6. A variable rate vocoder for coding a first signal for transmission comprising:
a codebook comprising plural prequantizer ("P") codewords, each of the plural P codewords corresponding to one of plural prequantizer ("P") indices, and plural sets of plural quantizer codewords ("V codewords"), each of the plural V codewords corresponding to one of plural quantizer indices ("V indices") and each of the plural sets being associated with one of the plural P
codewords;
a prequantizer searcher operable for comparing each of the plural P codewords with the first signal and determining a first P
codeword, with a corresponding first P index, having a least prequantizer residual component;
a quantizer searcher operable for comparing each V
codeword of a first set, the first set being associated with a previously determined P codeword of the plural P codewords, with the first signal and determining a first V codeword, with a corresponding first V index, having a least quantizer residual component; and a coder operable for forming a coded version of the signal comprising the first V index and a quantizer mode indicator but not the first P index ("V mode indicator") when the least prequantizer residual component is greater than the least quantizer residual component.
a codebook comprising plural prequantizer ("P") codewords, each of the plural P codewords corresponding to one of plural prequantizer ("P") indices, and plural sets of plural quantizer codewords ("V codewords"), each of the plural V codewords corresponding to one of plural quantizer indices ("V indices") and each of the plural sets being associated with one of the plural P
codewords;
a prequantizer searcher operable for comparing each of the plural P codewords with the first signal and determining a first P
codeword, with a corresponding first P index, having a least prequantizer residual component;
a quantizer searcher operable for comparing each V
codeword of a first set, the first set being associated with a previously determined P codeword of the plural P codewords, with the first signal and determining a first V codeword, with a corresponding first V index, having a least quantizer residual component; and a coder operable for forming a coded version of the signal comprising the first V index and a quantizer mode indicator but not the first P index ("V mode indicator") when the least prequantizer residual component is greater than the least quantizer residual component.
7. A communication unit including a low rate vocoder for decoding a coded version of a signal comprising:
a receiver for receiving the coded version of the signal;
a decoder coupled to the receiver comprising:
a codebook comprising plural prequantizer ("P") codewords, each of the plural P codewords corresponding to one of plural prequantizer ("P") indicators, and plural sets of plural quantizer codewords ("V codewords"), each of the plural V codewords corresponding to one of plural quantizer indicators ("V indicators") and each of the plural sets being associated with one of the plural P codewords;
a mode controller operable for receiving the coded version of the signal from the receiver and determining if the coded version of the signal includes one of a quantizer mode indicator ("V mode indicator") and a prequantizer ("P") mode indicator, wherein when the coded version of the signal includes a V mode indicator the coded version of the signal further includes a first V indicator but not a P
indicator, and when the coded version of the signal includes a P mode indicator the coded version of the signal further includes a first P indicator but not a V indicator;
a signal generator coupled to the receiver and mode controller operable for decoding the coded version of the signal: (a) in a first mode, when the coded version of the signal includes the V mode indicator, into a replica of the signal based on a previously received P codeword and a first V codeword of the plural V codewords corresponding to the first V indicator, and (b) in a second mode, when the coded version of the signal includes the P mode indicator, into a replica of the first signal based on a first P codeword of the plural P codewords corresponding to the first P indicator.
a receiver for receiving the coded version of the signal;
a decoder coupled to the receiver comprising:
a codebook comprising plural prequantizer ("P") codewords, each of the plural P codewords corresponding to one of plural prequantizer ("P") indicators, and plural sets of plural quantizer codewords ("V codewords"), each of the plural V codewords corresponding to one of plural quantizer indicators ("V indicators") and each of the plural sets being associated with one of the plural P codewords;
a mode controller operable for receiving the coded version of the signal from the receiver and determining if the coded version of the signal includes one of a quantizer mode indicator ("V mode indicator") and a prequantizer ("P") mode indicator, wherein when the coded version of the signal includes a V mode indicator the coded version of the signal further includes a first V indicator but not a P
indicator, and when the coded version of the signal includes a P mode indicator the coded version of the signal further includes a first P indicator but not a V indicator;
a signal generator coupled to the receiver and mode controller operable for decoding the coded version of the signal: (a) in a first mode, when the coded version of the signal includes the V mode indicator, into a replica of the signal based on a previously received P codeword and a first V codeword of the plural V codewords corresponding to the first V indicator, and (b) in a second mode, when the coded version of the signal includes the P mode indicator, into a replica of the first signal based on a first P codeword of the plural P codewords corresponding to the first P indicator.
8. A method for processing low rate vocoded signals using a code vector index, the method comprising:
determining an input vector derived from an input signal;
selecting an initial prequantizer index (P1) from a plurality of prequantizer indices at a first time period;
at a second time period after the first time period:
selecting a vector index (V) from a plurality of vector indices corresponding to the initial prequantizer index (P1);
determining a first distortion indicator based on a comparison between the input vector and a codeword associated with the selected vector index (V);
determining a second distortion indicator based on a comparison between the input vector and a codeword associated with a second prequantizer index (P2) from the plurality of prequantizer indices;
comparing the first distortion indicator with the second distortion indicator;
selecting a mode indicator based on the comparsion; and transmitting the mode indicator and one of the vector index (V) and the prequantizer index P2.
determining an input vector derived from an input signal;
selecting an initial prequantizer index (P1) from a plurality of prequantizer indices at a first time period;
at a second time period after the first time period:
selecting a vector index (V) from a plurality of vector indices corresponding to the initial prequantizer index (P1);
determining a first distortion indicator based on a comparison between the input vector and a codeword associated with the selected vector index (V);
determining a second distortion indicator based on a comparison between the input vector and a codeword associated with a second prequantizer index (P2) from the plurality of prequantizer indices;
comparing the first distortion indicator with the second distortion indicator;
selecting a mode indicator based on the comparsion; and transmitting the mode indicator and one of the vector index (V) and the prequantizer index P2.
9. The method of claim 8, wherein:
second prequantizer index P2 is a portion of a composite quantized codeword index;
vector index V is a portion of a composite quantized codeword index;
first distortion indicator comprises a least quantizer residual component; and a codeword index includes P1 and V.
second prequantizer index P2 is a portion of a composite quantized codeword index;
vector index V is a portion of a composite quantized codeword index;
first distortion indicator comprises a least quantizer residual component; and a codeword index includes P1 and V.
10. A method for processing low rate vocoded signals using a code vector index, the method comprising:
determining an input vector derived from an input signal;
selecting a first index from a first plurality of indices at a first time period;
at a second time period:
selecting a first index from a second plurality of vector indices, the second plurality of indices corresponding to the first index;
determining a first distortion indicator based on a comparison between the input vector and a codeword associated with the first index from the second plurality of vector indices;
determining a second distortion indicator based on a comparison between the input vector and a codeword associated with a second index from the first plurality of indices;
comparing the first distortion indicator with the second distortion indicator;
selecting a mode indicator based on the comparsion; and transmitting the mode indicator and only one of the first and second indexes.
determining an input vector derived from an input signal;
selecting a first index from a first plurality of indices at a first time period;
at a second time period:
selecting a first index from a second plurality of vector indices, the second plurality of indices corresponding to the first index;
determining a first distortion indicator based on a comparison between the input vector and a codeword associated with the first index from the second plurality of vector indices;
determining a second distortion indicator based on a comparison between the input vector and a codeword associated with a second index from the first plurality of indices;
comparing the first distortion indicator with the second distortion indicator;
selecting a mode indicator based on the comparsion; and transmitting the mode indicator and only one of the first and second indexes.
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| CN104637483A (en) * | 2015-02-03 | 2015-05-20 | 中国电子科技集团公司第五十八研究所 | Multichannel-based low-speed voice coding/decoding system |
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| JPH02287399A (en) * | 1989-04-28 | 1990-11-27 | Fujitsu Ltd | Vector quantization control system |
| US5010574A (en) * | 1989-06-13 | 1991-04-23 | At&T Bell Laboratories | Vector quantizer search arrangement |
| US4974099A (en) * | 1989-06-21 | 1990-11-27 | International Mobile Machines Corporation | Communication signal compression system and method |
| US4963030A (en) * | 1989-11-29 | 1990-10-16 | California Institute Of Technology | Distributed-block vector quantization coder |
| FR2709366B1 (en) * | 1993-03-26 | 2001-09-14 | Motorola Inc | Method for storing reflection coefficient vectors. |
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