EP0906613B1 - Procede et dispositif de codage d'un signal audiofrequence par analyse lpc "avant" et "arriere" - Google Patents

Procede et dispositif de codage d'un signal audiofrequence par analyse lpc "avant" et "arriere" Download PDF

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EP0906613B1
EP0906613B1 EP98920601A EP98920601A EP0906613B1 EP 0906613 B1 EP0906613 B1 EP 0906613B1 EP 98920601 A EP98920601 A EP 98920601A EP 98920601 A EP98920601 A EP 98920601A EP 0906613 B1 EP0906613 B1 EP 0906613B1
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value
lpc
analysis
stationarity
signal
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EP0906613A1 (fr
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Stéphane PROUST
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Orange SA
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France Telecom SA
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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
    • G10L19/04—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 predictive techniques
    • G10L19/16—Vocoder architecture
    • G10L19/18—Vocoders using multiple modes
    • 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
    • G10L19/04—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 predictive techniques
    • G10L19/06—Determination or coding of the spectral characteristics, e.g. of the short-term prediction coefficients

Definitions

  • the invention relates to a method and a device for coding an audio signal, such as a speech signal, by " forward " and “ backward “ LPC analysis.
  • coding techniques for audio signals including speech signals
  • LPC analysis for "Linear Predictive Coding" in Anglo-Saxon language, consists in performing a linear prediction of the audio frequency signal to be coded, the coding being carried out temporally by means of prediction filtering. linear applied to successive blocks of this signal.
  • the above coding techniques are said to be analysis by synthesis ".
  • they made it possible, for audio signals belonging to the band telephone frequency, reduce transmission rate of these signals from 64 kbps (MIC coding) to 16 kbps using the CELP coding technique, and even up to 8 kb / s in the case of coders implementing the most advanced recent results of this coding technique, without degradation noticeable from the quality of the speech restored after transmission and decoding.
  • a particularly important field of application of these coding techniques is, in particular, that of mobile telephony.
  • the necessary limitation of the frequency band granted to every mobile operator and the increase very rapid number of user subscribers make it necessary the corresponding decrease in coding rate, while user requirements for speech quality do not cease to grow.
  • Other areas of application of these coding techniques relate, for example, to storage digital data representative of these signals on storage media, high quality telephony for video or audio conference applications, multimedia, or digital satellite transmissions.
  • the linear prediction filters used in the aforementioned techniques are obtained using an analysis module called "LPC analysis" operating on successive blocks of the digital signal.
  • LPC analysis an analysis module
  • These filters are capable, according to the order of analysis, that is to say according to the number of coefficients of the filter, of modeling more or less faithfully the contours of the frequency spectrum of the signal to be coded. In the case of a speech signal, these contours are called formants.
  • the filter thus defined is not sufficient to perfectly model the signal. It is then essential to proceed to the coding of the linear prediction residue.
  • Such an operating mode relating to the linear prediction residue is notably implemented by the LD-CELP coding technique, for Low Delay CELP in Anglo-Saxon language, previously mentioned in the description.
  • the residual signal is in this case modeled by a waveform extracted from a stochastic dictionary and multiplied by a gain value.
  • the MP-LPC coding technique models this residue using variable position pulses assigned respective gain values, while the VSELP coding technique performs this modeling by a linear combination of pulse vectors extracts from appropriate directories.
  • the general envelope of the frequency spectrum is modeled by means of a short-term synthesis filter, constituting the LPC filter, the coefficients of which are evaluated by means of a linear prediction of the speech signal to be coded.
  • This LPC filter autoregressive filter, has a shape transfer function, relation (1): where p denotes the number of coefficients a i of the filter and the order of the linear prediction implemented, z denoting the variable of the transform in z of the frequency space.
  • a method for evaluating the coefficients a i consists in applying a criterion for minimizing the energy of the prediction error signal of the speech signal over the analysis length of the latter.
  • the analysis length for a digital speech signal formed by successive samples is in practice a number N of these samples, constituting a coding frame.
  • the energy of the prediction error signal then checks the relation (2): where s (n) denotes the sample of rank n in the frame of N samples.
  • the frame of coding can advantageously be divided into several subframes or adjacent LPC blocks.
  • Analysis length N then exceeds the length of each block to allow the taking into account a certain number of past samples and, where applicable, future, by means and at the cost of delays appropriate coding.
  • the analysis is called LPC "before” when the LPC analysis process is carried out on the block of the current frame of the speech signal to be coded, the coding at the level of the coder intervening "in real time", that is to say say during the block of the current frame to the sole processing delay introduced by calculating the coefficients of the filter.
  • This analysis involves the transmission of the calculated values of the coefficients of the filters to the decoder.
  • the "rear" LPC analysis implemented in the LD-CELP coder at 16 kb / s is the subject of standard ITU-T G728.
  • This analysis technique consists in performing the LPC analysis, not on the block or the block of the current frame of the speech signal to be coded, but on the synthesis signal. It will then be understood that this LPC analysis is in fact carried out on the synthesis signal of the block preceding the current block, since this signal is available simultaneously at the level of the coder and the decoder. This simultaneous operation with the coder and the decoder thus makes it possible to avoid the transmission from the coder to the decoder of the value, obtained at the coder, of the coefficients of the LPC filter.
  • the “ backward ” LPC analysis makes it possible to free up the transmission rate, the rate thus freed up being able to be used for example in order to enrich the excitation dictionaries in the case of CELP coding.
  • the “ backward ” LPC analysis also allows an increase in the order of analysis, the number of coefficients of the LPC filter being able to reach 50 in the case of an LD-CELP coder against 10 coefficients for most coders implementing performs a " before " LPC analysis.
  • LPC frames coded by " forward " LPC analysis allows the coder and the decoder to converge again to the same synthesis signal in the event of transmission error. and therefore offers a robustness to these errors much superior to a coding by pure "back" LPC analysis.
  • the object of the present invention is to remedy the aforementioned drawbacks by implementing a method and a device for coding a digital audio frequency signal by specific " front " and “rear” LPC analysis.
  • Another object of the present invention is also the implementation of a process of dynamic adaptation of the function of choice between the "front" LPC analysis and the "rear” LPC analysis according to the degree of stationarity of the signal. to code.
  • Another object of the present invention is also the implementation of a process of dynamic adaptation of the aforementioned choice function on the basis of a discrimination between strongly stationary signals, such as music or background noise, and other signals. , such as speech, to allow the most appropriate coding processing by "back" and "front” LPC analysis respectively.
  • Another object of the present invention is also, the choice of the aforementioned most appropriate coding having been carried out, for a signal to be coded of a given type or characteristics, to avoid any untimely switchover in the non-LPC analysis mode. retained, and, thus, to avoid the appearance of transitions of LPC filters " front " - “ rear “ or vice versa likely to degrade the quality of the reproduced synthesis signal.
  • Another object of the present invention is finally the implemented a process of dynamic adaptation of the above-mentioned choice function for which the mode change LPC analysis is a true reflection of a change stationarity of the signal to be coded and risks, therefore, to be much less linked to a simple crossing effect point of the first and second threshold values.
  • the method and the device, objects of the present invention find application not only in the field of mobile telephony but also to the industry creation and reproduction of phonograms, at the high quality satellite and telephone transmission for video or audio conference applications, multimedia.
  • the transmitted coded signal noted s_c n (t)
  • s_c n (t) consists partly of LPC filtering parameters accompanied by LPC analysis decision information.
  • a non-transmitted coding residue signal res n (t) is available by the implementation of the coding method.
  • the digital audio frequency signal is subdivided into LPC frames, a succession of LPC blocks, each block, for the convenience of the description, being denoted B n and provided with a determined number N of samples.
  • the coding method which is the subject of the present invention, it consists in carrying out the aforementioned coding on the digital audio frequency signal as defined above from " forward " LPC filtering for the non-stationary areas, respectively on a synthesis signal obtained from the residual coding signal from a "rear” LPC filtering for stationary areas.
  • each current block denoted B n , being available in a starting step 10
  • STAT (n ) the degree of stationarity of the digital audio signal according to a stationarity parameter, denoted STAT (n ).
  • This stationarity parameter has a numerical value between a maximum stationarity value, denoted STAT M , and a minimum stationarity value, denoted STAT m .
  • the stationarity parameter has the maximum value STAT M for a very strongly stationary signal, while this stationarity parameter has the minimum value STAT m for a very strongly non-stationary signal.
  • the coding method which is the subject of the present invention consists in establishing, in a step 12, from the stationarity parameter STAT (n), an analysis choice value LPC, this choice value corresponding analysis of course, either to the choice of LPC analysis "before” , or on the contrary to the choice of LPC analysis "back".
  • the choice of analysis value is denoted d n (n) and is obtained from a specific decision function, denoted D n .
  • step 12 is then followed by a test step 13 allowing the application of the analysis choice value d n (n), symbolized by C, to the LPC filtering to effect the coding of the digital audio-frequency signal by filtering "Front" LPC for non-stationary areas on the digital audio signal, respectively by "rear” LPC filtering for stationary areas on the synthesis signal.
  • step 12 the decision function implemented in step 12, this decision function being denoted D n , is an adaptive function updated for each current block B n , from the stationarity parameter.
  • the updating of the adaptive function makes it possible to privilege the maintenance in one of the LPC filtering modes "front”, respectively “rear” , according to the degree of stationarity of the digital audio-frequency signal and thus limit the number of toggles of l 'to one another filtering modes, and vice versa.
  • the analysis choice value d n (n) established from the above-mentioned decision function D n corresponds to a filtering mode priority value LPC " front " or “rear” as well as to another priority value representing in fact a value of absence of priority to return to the LPC filtering mode " rear " or " front ".
  • the analysis choice value d n (n) can for example correspond to a logical value, the true value of this logical value, value 1 for example, corresponding to a LPC filtering choice "backward” while the value complemented by this true value, the value zero, corresponds to a LPC filtering choice "forward". It is thus understood that the test function in step 13 can be summed up as a test value on the logical value of the above-mentioned analysis choice value to ensure in step 14 the "back" LPC filtering for the zones.
  • the analysis choice value d n (n) is represented by a logical value, it is understood that this logical value can be associated with a priority and probability value of filtering mode established by the decision function D n specifically. It is understood in particular that this probability value can correspond, for each current block B n , to the true logic value for a range of probability values between zero and 1 for "backward" LPC filtering whereas the complemented logic value, value logic zero, for example, may correspond to the complement of the above range of probability values between zero and 1 of the first aforementioned range. This probability is linked to the number of successive filtering decisions in the same filtering mode.
  • the operating mode of the decision function D n making it possible in fact to associate with the logic variable d n (n) the priority of filtering mode, is adaptive over time, for each current block B n .
  • the adaptation of the decision function D n aims to progressively favor the "back" LPC filtering mode or on the contrary the "front” LPC filtering mode which works best, counts given the overall stationarity of the signal to be coded, in order to avoid as much as possible any unnecessary switching from one of the filtering modes to the other.
  • step 11 consisting in determining the degree of stationarity of each current block B n of the digital audio frequency signal consists, from an arbitrary starting value of the stationarity parameter, as represented in l step 110 of FIG.
  • step 111 to be calculated in a step 111 for this current block B n an intermediate stationarity parameter value, denoted STAT * (n), function of a number determined from successive analysis choice values, these LPC analysis choice values, denoted d n-1 (n-1), ..., to d np (np), being obtained for different successive blocks prior to the block current B n of the succession of blocks LPC, and of the value of the stationarity parameter of the block preceding the current block, this stationarity value being denoted STAT (n-1).
  • step 111 represented in FIG.
  • step 112 which consists in refining the value of the intermediate stationarity parameter as a function of the value of the prediction gains of the filters or LPC analysis mode "front" and “rear” of the frame preceding the current frame.
  • step 112 of FIG. 2a it is indicated that the above-mentioned function is denoted g (STAT * (n), Gpf, Gpb) where Gpf denotes the prediction gain of the LPC filter " before " and Gpb denotes the prediction gain the "back" LPC filter for the frame preceding the current frame.
  • step 111 consists, from an initialization step 1110 in which the value of the stationarity parameter STAT (n-1) and the value of analysis choice d n-1 (n -1) relating to the block LPC B n-1 prior to the current block B n is available, to be carried out, in a step 1111, a step consisting in discriminating the analysis mode LPC " front " or LPC "rear” of block B n-1 preceding the current block B n .
  • This discrimination step 1111 can, as shown in FIG. 2b, consist of a test step on the value of choice of analysis d n-1 (n-1) with respect to the symbolic value "fwd” or to the value logic zero corresponding to the value complemented by the true logic value.
  • the step of calculating the parameter value of intermediate stationarity consists, in a step 1113, in determining the number of anterior frames analyzed consecutively in LPC analysis mode " rear ", number noted N_BWD, then, in a step 1114, in comparing on criterion of comparison of superiority the number of frames prior to a first arbitrary value, denoted Na, representative of a number of successive frames analyzed in "backward” LPC mode .
  • the calculation step then consists in assigning, in a step 1114b, to the value of the intermediate stationarity parameter STAT * (n), the value of the stationarity parameter of the preceding block the current block, STAT (n-1), increased by a determined value as a function of the first arbitrary value representative of a number of successive frames analyzed, that is to say in fact of the number of previous frames N_BWD analyzed consecutively in LPC analysis mode " back ".
  • the determined value as a function of the first arbitrary value is denoted f a (N_BWD).
  • the value of the intermediate stationarity parameter STAT * (n) is assigned, in a step 1114a, the value of the stationarity parameter STAT (n-1) of the preceding block the current block B n .
  • test 1112 indicates the existence of such a transition from the "backward" analysis mode for the LPC block B n-2 preceding the block preceding the current block B n-1 , while a negative response to the test 1112 cited above indicates the absence of such a transition.
  • the calculation step 111 then consists in comparing, on the basis of an inferiority comparison criterion, the number of anterior frames N_BWD mentioned above with a second arbitrary value N b representative of a number of successive frames analyzed in "backward" LPC mode preceding block B n-1 preceding the current block.
  • step 1118a consisting in assigning to the value of the intermediate stationarity parameter STAT * (n) the value of the stationarity parameter of the block preceding the current block, STAT (n-1) reduced by a determined value, function of the second arbitrary value N b , this determined value being noted f b (N_BWD). It is thus understood that during the allocation step 1118a, the value of the intermediate stationarity parameter is thus reduced accordingly.
  • step 111 consists then to assign, in a step 1118b, to the value of intermediate stationary parameter STAT * (n) the value of stationarity parameter of the block preceding the block current, i.e. STAT (n-1).
  • step 111 there is the value of the intermediate stationarity parameter STAT * (n) for the current block B n .
  • step 112 consisting in refining the value of the aforementioned intermediate stationarity parameter
  • this can advantageously consist, in a step 1120, in discriminating the prediction gains from the LPC filtering "backward” LPC filtering and "advan t", the gain values being denoted Gpb and Gpf respectively.
  • the aforementioned discrimination step simply consists in storing and reading the gain values calculated for the aforementioned "front” and “back” LPC filtering.
  • step 1120 can consist in calculating the relative value of the prediction gains, denoted DGfb, such as the difference or the ratio between the aforementioned "front” and “rear” prediction gains.
  • step 112 of FIG. 2a comprises, after the above-mentioned step 1120, a step 1121 consisting in modifying the value of the intermediate stationarity parameter STAT * (n) a refinement value ⁇ S, this refinement value in accordance with a particularly remarkable characteristic of the method which is the subject of the present invention being a function of the relative value of the LPC filtering prediction gains "front" and "rear".
  • the function f r (GPf, Gpb) making it possible to establish the refinement value ⁇ S is an increasing respectively decreasing function of this relative value, according to the direction in which this relative value is considered.
  • the relative value designates the value of the "back" LPC filtering gain compared to the "front" LPC filtering gain , this choice can be arbitrarily retained without in any way detracting from the generality of the process, object of the invention, relative value DGfb above, the function f r is then increasing. Otherwise, it decreases.
  • the modification, by increase or by decrease, of the value of the intermediate stationarity parameter of the ripening value ⁇ S is proportional to this relative value of the gains.
  • step 1121 we thus have the value of the stationarity parameter STAT (n) in step 1122.
  • step 1121 of FIG. 2b A more detailed description of step 1121 of FIG. 2b will now be given in conjunction with FIG. 2c in a preferred embodiment in which a plurality of test criteria are applied both to the refining value and to the values of "forward" and "backward" LPC prediction gain in order to optimize the stationarity parameter calculation process.
  • step 1121 can consist of a first step 1121a making it possible to calculate the refinement value ⁇ S from the function f r (Gpf, Gpb) previously cited.
  • f r the refinement value ⁇ S from the function f r (Gpf, Gpb) previously cited.
  • Different examples of usable functions will be given later in the description.
  • the refinement value ⁇ S is subject to a comparison test of superiority to the value 0, in a step 1121b, this comparison test in fact allowing to determine the increase in this refinement value ⁇ S.
  • the step of increasing the value of intermediate stationarity parameter of the refinement value ⁇ S is further subject to a condition of superiority of the “ backward ” filtering gain value LPC, compared with a first determined positive value, in a step of comparing the superiority of the value of the "back" LPC filtering gain Gpb with respect to this first determined positive value, denoted S i .
  • the increase in the value of the intermediate stationarity parameter of the ripening value ⁇ S is furthermore subject to a condition of inferiority of the value of the intermediate stationarity parameter STAT * (n) by compared to a second determined positive value STAT i of course representing a stationarity value.
  • This inferiority condition test is carried out in step 1121e.
  • the step of decreasing the value of the parameter of intermediate stationarity of the ripening value ⁇ S is further subject to a value superiority condition of the intermediate stationarity parameter STAT * (n) by compared to a fourth determined positive value, noted STATd in a comparison test noted 1121f.
  • the fourth determined positive value is representative of a selected stationarity parameter value.
  • step 1122 of FIG. 2b of the parameter of STAT (n) stationarity we have thus in step 1122 of FIG. 2b of the parameter of STAT (n) stationarity.
  • FIG. 2d A first example of a non-linear function f r (Gpf, Gpb) is shown in Figure 2d.
  • step 1111 of step 111 shown in FIG. 2b can be preceded by a step 1111a consisting, for each successive current block, in determining the average energy of the digital audio frequency signal and in comparing in this same step ,. on an inferiority comparison criterion, this average energy at a determined threshold value representative of a frame of silence.
  • this threshold value is noted ENER_SIL.
  • the value of the stationarity parameter of the current block STAT (n) is assigned the value of the stationarity parameter of the previous block STAT (n-1) in the allocation step 1111b represented in the figure. 2b.
  • the steps 1111a and 1111b are, in the above-mentioned figure, shown in dotted lines, since they are reserved for example for coding a speech signal.
  • a distance denoted d LPC , is first calculated between the filter LPC of the current block and that of the previous block B n-1 . This distance calculation is carried out for example using the LSP frequency parameters as mentioned previously in the description relative to the method described in the aforementioned article.
  • the " forward " LPC filtering mode can be advantageously chosen as soon as the energy of the signal to be encoded E n , that is to say the energy of the corresponding block B n , becomes lower than the value of the energy of a frame of silence ENER_SIL, this value of energy corresponding to the minimum audible level.
  • the set of conditions allowing the establishment of the decision function D n and the obtaining of the corresponding analysis choice values d n (n), is illustrated in FIG. 2f with temporal adaptation of the decision function D n .
  • the value of the stationary parameter STAT (n) can for example be located on a scale of 0, corresponding to the value STAT m very little stationary, to 100, corresponding to the value STAT M very stationary.
  • the decision function D n is modified by adapting the value of the thresholds.
  • the thresholds S_PRED, S_LSP_L and S_LSP_H are increased.
  • the threshold values S_TRANS, S_STAT and G 1 keep a fixed value, these values being able for example to be equal to -1 dB, 5 dB and 0 dB respectively.
  • step 120 carrying out a step of test 121 relating to the energy of the current LPC block B n , by a comparison of inferiority to the value of energy of silence ENER_SIL or of the value of the stationarity parameter STAT (n), compared by a comparison of inferiority to the value S FWD previously cited in the description.
  • step 121 the value of choice of analysis d n (n) is taken equal to 0, that is to say symbolic value "fwd" in step 122.
  • a new test is performed on the aforementioned filtering distance LPC d LPC , in a step 124, relative to the threshold value S_LSP_H (n) by comparison of superiority to this threshold value.
  • a new test 126a is carried out, consisting in comparing the prediction gain of the LPC filtering " before ", Gpf, to the prediction gain of the LPC filtering "back", Gpb, reduced by the threshold value S_TRANS.
  • the analysis value d n (n) is assigned the logical value 0, symbolic value " fwd ", and on a negative response to the above test 126a, is assigned to the same value choice of analysis the logical value 1, symbolic value " bwd ".
  • the corresponding steps are noted 128 and 129.
  • Test 125 consists in making a comparison of the filtering distance LPC, d LPC , by comparison of inferiority to the threshold value S_LSP_L (n).
  • a new test 126b is carried out by comparing the superiority of the LPC filtering prediction gain "back" to the LPC filtering prediction gain "before” reduced by the value S_STAT previously mentioned.
  • step 129 the value of analysis choice d n (n) is assigned in step 129 the logical value 1, that is to say the symbolic value " bwd ".
  • a new test is carried out, in a step 127, this test consisting in verifying the conditions of comparison of the "back" LPC filtering gain Gpb with the LPC filtering prediction gain "before” decreased of the threshold value S_PRED (n), of comparing the superiority of the intermediate LPC filtering prediction gain Gpi to the value of the LPC filtering prediction gain " before " minus the aforementioned threshold value S_PRED (n) and of comparison of superiority of the “backward” filtering prediction gain Gpb to the threshold value G 1 , as well as of comparing the value of the intermediate filtering prediction gain Gpi to the threshold value G 1 .
  • the value of analysis choice d n (n) is assigned the logical value 1, that is to say the symbolic value "bwd” in step 129, while qu 'to the negative response to the above test 127, to the analysis choice value d n (n), on the contrary, is assigned the logical value 0, that is to say the symbolic value "fwd” in step 128 .
  • the signal digital to be coded is subdivided into frames consisting of successive blocks of samples, each block comprising a given number N of samples for example.
  • the coding device which is the subject of the invention comprises a " front " LPC analysis filter, bearing the reference 1A, and a “rear” LPC analysis filter , bearing the reference 1B, in order to enable a transmitted coded signal consisting of LPC filtering parameters accompanied by an analysis decision indication to be delivered, as well as parameters Pr n relating to the harmonic analysis and to the CELP excitation signal.
  • the analysis decision indication corresponds to the analysis choice value d n (n) as mentioned previously in the description.
  • the LPC filtering parameters it is indicated that these correspond to specific parameters, in accordance with the mode of implementation of the coding method which is the subject of the present invention, as will be described below in the description.
  • FIG. 3 the existence of an adaptive filter as a function of the value of the stationarity parameter has also been shown in the coding device according to the invention, this adaptive filter bearing the reference 1E.
  • This adaptive filter 1E naturally receives the original digital signal, noted s n (t) , that is to say the current block B n .
  • the 1E filter uses the LPC filtering parameters to calculate the residual signal which will then be coded by the 1F module. These LPC parameters, as well as the filter decision indication constitute a part of the coded signal which is transmitted to the decoder.
  • the coding device which is the subject of the present invention comprises a coding means, bearing the reference 1F, of a residual coding signal not transmitted, the residual coding signal, designated by res n (t) is directly available at the output of the adaptive filter 1E, this signal thus being delivered at the input with the digital audio frequency signal to the coding module of the non-transmitted coding residue signal, to generate a synthesis residue signal, res_syn n (t).
  • a reverse filtering module bearing the reference 1G, receives the synthesis residue signal and makes it possible to deliver a synthesis signal referenced s_syn n (t) .
  • a storage module 1H receives the aforementioned synthesis signal s_syn n (t) to deliver the aforementioned synthesis signal for the block prior to the current block B n , the synthesis signal thus obtained being designated by s_syn n-1 (t). This synthesis signal is delivered to the "rear" LPC analysis filter bearing the reference 1B in FIG. 3 above.
  • the coding device, object of the present invention makes it possible to carry out coding of the digital audio frequency signal on the aforementioned digital audio signal using the " front " LPC filter for the non-stationary zones and on the aforementioned synthesis signal s_syn n-1 (t) from the “rear” LPC filter 1B for the stationary zones, as will be described below.
  • the device which is the subject of the invention comprises for this purpose, for each current LPC block B n , a module 1C for calculating the degree of stationarity of the digital audio signal according to a parameter of stationarity whose value is between a maximum stationarity value and a minimum stationarity value.
  • the stationarity parameter is the STAT (n) parameter previously described in the description in accordance with the coding method which is the subject of the present invention.
  • the maximum and minimum stationarity values are also defined above.
  • the coding device which is the subject of the invention comprises a module, denoted 1D 1 , for establishing from the above-mentioned stationarity parameter STAT (n) a function of decision and an LPC analysis choice value, the decision function being denoted D n as mentioned previously in the description, and the LPC analysis choice value being of course and corresponding to the choice value of LPC analysis noted d n (n) previously described in the description.
  • the value of choice of analysis d n (n) can take the values 0 or 1, logical values, which correspond to the symbolic value of choice of analysis "fwd" and "bwd” for LPC analysis " front " and” rear "respectively.
  • the coding device comprises an LPC filtering analysis discrimination module, denoted 1D 2 , this module receiving the analysis choice value d n (n) and allowing to deliver, for the current LPC block B n, the value of the LPC filtering parameters "rear” respectively "front” according to the above-mentioned analysis choice value.
  • LPC filtering analysis discrimination module denoted 1D 2
  • this module receiving the analysis choice value d n (n) and allowing to deliver, for the current LPC block B n, the value of the LPC filtering parameters "rear” respectively "front” according to the above-mentioned analysis choice value.
  • the " back " LPC filtering analysis parameters as well as the " front " LPC analysis filtering parameters are of course available in digital form at the filters bearing the reference 1B and 1A respectively in the figure.
  • the discrimination module 1D 2 can for example, in a non-limiting embodiment, consist of two distinct memory zones allowing the memorization of the filtering parameters Af n (z) and Ab n (z) respectively, the analysis choice value d n (n) as a function of its logical current value, 0 or 1, allowing the addressing in reading of the values of filtering parameters stored by the module 1D 2 for example and the transmission of these filtering parameters by the latter.
  • the coding device in accordance with the object of the present invention for producing the adaptive filter as a function of the stationarity value carrying the reference 1E, can be produced by a filter element whose transfer function, denoted A (z), is established from the values of filter parameters delivered by the discrimination module 1D 2 previously mentioned.
  • the adaptive filtering module 1E can be produced by a filter with adjustable coefficients, to the value of the coefficients of the latter being assigned the values of filtering parameters delivered by the discrimination module 1D 2 previously mentioned.
  • the filtering performed by the module 1E is thus of the adaptive type as a function of the degree of stationarity of the digital audio frequency signal to be coded.
  • the module 1E thus delivers, from the original digital audio signal s n (t) , the residual filtering signal LPC designated by res n (t) to the coding module for the residue 1F, which then makes it possible to deliver the residual signal LPC synthesis designated by res_syn n (t).
  • the module 1G is a filtering module whose transfer function is the inverse of the transfer function of the module 1E obtained from the memorized parameters of the latter. It receives the LPC synthesis residue signal res_syn n (t) delivered by the coding module from the coding residue delivered by the module 1F.
  • the coding of the digital audio signal s n (t) is carried out at the level of the module 1E by virtue of the LPC analysis "front”, respectively “rear” carried out by the LPC analysis filters “before” 1A and d analysis LPC “back” 1B, the coded signal s_c n (t) consisting of the transmission of filtering parameters LPC "before” when the value of analysis choice d n (n) has the symbolic value "fwd" as well as the indication of the choice of analysis, that is to say of the value of the choice of analysis previously cited.
  • This operating mode makes it possible to carry out the coding of the digital audio-frequency signal and to favor the maintenance in one of the LPC filtering modes "front”, respectively “rear”, according to the degree of stationarity of the digital signal and to further limit the number of switches from one to the other of the filtering modes considered.
  • a device for decoding a digital audio signal coded in double analysis on the criterion of choice of LPC analysis "front”, respectively “rear”, into a coded signal transmitted in accordance with the coding method object of the present invention and thanks to the implementation of a coding device as shown in FIG. 3 for example, will now be described in conjunction with FIG. 4.
  • the transmitted coded signal s_c n (t) consists for each analysis block LPC in the value of choice of analysis mentioned above and, in the case where the value of choice of analysis corresponds for the LPC analysis block considered in a " before " LPC analysis, in the "before" LPC filtering parameters as well as the LPC filtering residue coding parameters, parameters Pr n , that is to say of the res signal n (t) into a synthesis residue signal res_syn (t) by the coding module of the residue 1F.
  • the decoding device comprises at least one synthesis module, referenced 2A, of the filtering residue signal receiving the coding parameters of the LPC residue delivered by the module 1F.
  • the module 2A decodes the coding parameters supplied by the module 1F and consequently delivers a synthesis residue signal, which is referenced in FIG. 4 res_syn n (t).
  • the decoding device as shown in FIG. 4 also includes a module, bearing the reference 2B, of adaptive reverse filtering as a function of the degree of stationarity, receiving the above-mentioned synthesis residue signal, delivered by the module 2A, and allowing d 'generating a synthesis signal s_syn n (t) representative of the digital audio frequency signal, this signal constituting in fact the decoded signal.
  • the reverse filtering module 2B implements the filtering parameters received by the decoder due to the transmission, ie the LPC analysis parameters " before " when these are transmitted and the decision to analysis corresponds to a “ forward ” LPC analysis or, on the contrary, the “rear” filtering analysis parameters as will be described below.
  • the decoding device which is the subject of the present invention of course comprises a "rear" LPC filter module , carrying the 2D reference, receiving the synthesis signal, that is to say the signal referenced s_syn n (t ) for the LPC block prior to the current LPC block, this synthesis signal thus being referenced s_syn n-1 (t) in FIG. 4.
  • the synthesis signal relating to the current block B n and referenced s_syn n (t) can then be delivered to the 2D "rear" LPC filtering module by means of a storage module, bearing the reference 2E, making it possible in fact, by addressing in read-appropriate mode, to offset the reading of the signal from synthesis to that corresponding to the block preceding the current block B n .
  • the decoding device which is the subject of the present invention, as shown in FIG. 4, finally comprises a discriminator module bearing the reference 2C, making it possible to discriminate the LPC analysis "before ", respectively " rear ".
  • the module 2C receives, on the one hand, for a discrimination command, the value of analysis choice received, that is to say the value d n (n), and, on the other hand, the filtering parameters " Front " LPC, ie the parameters Af n (z) transmitted, as well as the "rear” LPC filtering parameters Ab n (z) obtained by means of the 2D module.
  • the module 2C thus makes it possible to deliver, as a function of the value of choice of analysis, that is to say of the value d n (n), that is to say the filtering parameters LPC "before” Af n (z), or the "rear” LPC filtering parameters Ab n (z) to the adaptive reverse filtering module 2B as a function of the degree of stationarity.
  • modules 2C and 2B can simply consist of modules substantially identical to the modules 1D 2 and 1E or, more particularly, 1G of FIG. 3.
  • the actual coder consisted of a telephone band from 300 to 3400 Hz, at a speed of 12 kb / s CELP type.
  • the frames were formed over a period 10 ms for excitation provided by dictionary algebraic according to the technique called ACELP previously mentioned in the description.
  • the " forward " LPC analysis was a 10-order analysis and the "back” LPC analysis a 30-order analysis every 80 samples.
  • Each block B n contained 80 samples.
  • the above-mentioned stationarity parameter varies between two extreme values 0 and 100, the aforementioned values STAT m and STAT M.
  • test conditions referenced 1121d, 1121c and 1121f in Figure 2c were not used in this embodiment.
  • the threshold S_LSP_L is adapted using the following staircase function: The value of the threshold S_STAT used in case of stationarity of the LPC filters measured using the threshold S_LSP_L has been fixed at 4.0 dB. The threshold S_LSP_H was not used in this embodiment. The value of the G 1 threshold has been set at OdB. Regarding the energy value characterizing an ENER_SIL silence frame, this value was set at 40 dB measured on the 80 samples s (i) of the current block B n :
  • this value S FWD has was set at 40.6.
  • the above-mentioned stationarity parameter varies between the two extreme values 0 and 120, the aforementioned values STAT m and STAT M.
  • the values of the functions f a (N_BWD) and f c (N_BWD) are such that:
  • the threshold S_LSP_L is adapted using the following staircase function:
  • the threshold S_LSP_H is adapted using the following staircase function: The value of the threshold S_TRANS used in the event of transition of the LPC filters measured using the threshold S_LSP_H was fixed at 0 dB.
  • the value of the threshold S_STAT used in case of stationarity of the LPC filters measured using the threshold S_LSP_L was fixed at 2.5 dB.
  • the value of the threshold G 1 has been set at 0dB.
  • the energy value characterizing an ENER_SIL silence frame this value was fixed at 50 dB measured on the 120 samples s (i) of the current block B n :
  • this value S FWD has was set at 60.

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EP98920601A 1997-04-16 1998-04-09 Procede et dispositif de codage d'un signal audiofrequence par analyse lpc "avant" et "arriere" Expired - Lifetime EP0906613B1 (fr)

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FR9704684A FR2762464B1 (fr) 1997-04-16 1997-04-16 Procede et dispositif de codage d'un signal audiofrequence par analyse lpc "avant" et "arriere"
FR9704684 1997-04-16
PCT/FR1998/000723 WO1998047134A1 (fr) 1997-04-16 1998-04-09 Procede et dispositif de codage d'un signal audiofrequence par analyse lpc 'avant' et 'arriere'

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CA2258695C (en) 2003-02-11
BR9804852B1 (pt) 2011-04-19
DE69807806T2 (de) 2003-08-07
AU729584B2 (en) 2001-02-08
HK1020101A1 (en) 2000-03-10
ES2183358T3 (es) 2003-03-16
JP2000512777A (ja) 2000-09-26
JP3564144B2 (ja) 2004-09-08
AU7340498A (en) 1998-11-11
CN1122256C (zh) 2003-09-24
CA2258695A1 (en) 1998-10-22
BR9804852A (pt) 1999-08-24
FR2762464B1 (fr) 1999-06-25
FR2762464A1 (fr) 1998-10-23
CN1229501A (zh) 1999-09-22
WO1998047134A1 (fr) 1998-10-22
DE69807806D1 (de) 2002-10-17
US6327562B1 (en) 2001-12-04

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