Classifications

• • 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 frequency signal, such as a speech signal, by "forward" and "backward” LPC analysis.
• the coding techniques for audio frequency signals aim to allow the transmission of these signals in digital form, under conditions of reduction of the transmission rate, in order, in particular, to ensure adequate management of the networks for the transmission of these signals, taking into account the significant increase in transactions between users.
• the coding techniques used that designated by 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 aforementioned coding techniques are said to be "by analysis by synthesis". In particular, they have made it possible, for audio frequency signals belonging to the telephone frequency band, to reduce the transmission rate of these signals from 64 b / s (MIC coding) to 16 kb / s using the coding technique CELP, and even up to 8 kb / s in the case of coders implementing the most recent evolutions of this coding technique, without degradation noticeable quality of 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 each mobile operator and the very rapid increase in the number of user subscribers makes it necessary to correspondingly reduce the coding speed, while the requirements of users in terms of speech quality keep growing.
• Other fields of application of these coding techniques relate, for example, to the storage of digital data representative of these signals on storage media, high quality telephony for applications of video or audio conference, multimedia, or digital satellite transmissions.
• linear prediction filters used in the abovementioned techniques are obtained using an analysis module called "LPC analysis" operating on successive blocks of the digital signal.
• LPC analysis an analysis module operating on successive blocks of the digital signal.
• 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 Lovr 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.
• Coding technique MP-LPC for example, 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 extracted from appropriate repertoires .
• the general envelope of the frequency spectrum is odelized 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) ⁇ :
• a (z) • 1 - ⁇ az "1
• p denotes the number of coefficients a 1 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 1 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):
• Ep L (s (n) - L a.-sCn-i)) 2 where s (n) denotes the sample of rank n in the frame of N samples.
• the coding frame can advantageously be divided into several adjacent LPC sub-frames or blocks.
• the analysis length N then exceeds the length of each block in order to allow taking into account a certain number of past and, where appropriate, future samples, by means and at the cost of appropriate coding delays.
• 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 "backward" 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.
• a good functioning of the "back” LPC analysis requires the following conditions:
• the frame and block length must therefore be small compared to the average stationarity time of the speech signal to be coded;
• LPC frames coded by "forward" LPC analysis allows the coder and the decoder to converge again towards 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 aforementioned "front” - “rear” mixed LPC analysis consists in carrying out two LPC analyzes, a "front” LPC analysis on the speech or audio frequency signal to be coded and a “rear” LPC analysis on the synthesis signal.
• Second criterion For a current analysis in "back" LPC analysis mode, prohibition of switching from “back” LPC analysis mode to "front” LPC analysis mode if the distance calculated on the parameter vectors representing two filters LPC "before" consecutive is less than a second threshold value, a too small distance characterizing a substantially stationary area for which it is advisable to avoid any change of LPC analysis mode.
• the calculated distance is a Euclidean distance between the spectral lines of the speech or audio frequency signal to be coded.
• the encoder is then no longer able to correct the phenomena caused by the discontinuity introduced by the tilting of the filters; - the LPC filter which gives the best subjective quality and therefore models the best the spectrum of the signal to be coded is not always the one with the best prediction gain. Certain switches from one LPC analysis mode to another, linked to an instant decision, are therefore unnecessary.
• 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 choice function between the "forward” LPC analysis and the "back” 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 dynamic adaptation process 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, in order to allow the most appropriate coding processing by LPC "backward analysis "and” before “respectively.
• the method and the device for coding a digital audio signal implement a double analysis on criteria of choice of LPC analysis analysis "before” and “rear” respectively to generate a transmitted coded signal consisting of LPC filtering parameters accompanied by analysis decision information and a coding residue signal, not transmitted.
• the digital audio signal is subdivided into frames, a succession of blocks of a determined number of samples and the coding of this digital audio signal is carried out on this signal using "forward" LPC filtering for non-stationary areas and on a synthesis signal respectively, this synthesis signal being obtained from the residual coding signal, from a “backward” LPC filtering for the stationary zones. They are remarkable in that they consist of and allow, respectively:
• LPC for coding the digital audio frequency signal by "forward" LPC filtering for non-stationary areas on the digital audio frequency signal and by "rear” LPC filtering for stationary areas on the synthesis signal.
• This operating mode makes it possible to favor the maintenance in one of the LPC filtering modes "front” and “rear” respectively, in connection with the degree of stationarity of the digital audio signal and to limit the number of toggles from one to the 'other filtering methods and vice versa.
• the method and the device, objects of the present invention find application not only in the field of mobile telephony but also in the industry of creation and reproduction of phonograms, in satellite transmission and in high quality telephony for video or audio conference, multimedia applications.
• FIG. 1 shows, in the form of a general flowchart, an illustrative diagram of the steps allowing the implementation of the coding method, object of the present invention
• FIG. 2a shows a general flowchart steps for calculating the stationarity parameter for each current LPC block
• FIG. 2b shows a particular advantageous embodiment of the essential steps of calculating the stationarity parameter according to Figure 2a;
• FIG. 2c represents a detail of embodiment of FIG. 2b, more particularly a detail of the process of refining the value of the intermediate stationarity parameter for obtaining the stationarity parameter;
• FIGS. 2d and 2e represent a first, respectively a second nonlimiting example of implementation of a refining function making it possible to calculate a refining value of the intermediate stationarity parameter as a function of the relative values of the gain of LPC filtering "front" and “rear” ";
• Figure 2f shows, by way of illustrative example, a flowchart of steps for implementation of the decision function and the value of the LPC analysis choice" front " or "rear”;
• FIG. 3 represents, in the form of functional blocks, the general diagram of an encoder making it possible to carry out the coding of an audio frequency signal in accordance with the object of the present invention; - FIG.
• FIG. 4 represents, in the form of functional blocks, the general diagram of a decoder making it possible to decode a coded audiofrequency signal thanks to the use of an coder as shown in FIG. 3.
• a description a more detailed description of the coding process for a digital audio signal by double analysis, on the basis of the LPC analysis selection criterion "front” or “rear” respectively into a transmitted coded signal, object of the present invention, will now be given in conjunction with figure 1.
• the signal coded transmitted denoted s_c. (t)
• s_c. (t) partly consists 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 "back" 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 strong signal strongly non-stationary.
• 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, 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 "before”, respectively “rear”, according to the degree of stationarity of the digital audio signal and of thus limit the number of toggles ents from one to the other of the 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" back "or” before ".
• LPC filtering mode priority value it is indicated that 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".
• 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 ⁇ specifically.
• 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 .
• 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 parameter of stationarity, as shown in step 110 of FIG. 2a, this arbitrary value being denoted STAT (O), to be calculated in a step 111 for this current block B n an intermediate stationarity parameter value, denoted STAT * (n ), function of a determined number of successive analysis choice values, these LPC analysis choice values, denoted d ⁇ Cn-l), ..., to d n .
• step 111 representing in FIG. 2a, it is indicated that the function of the determined number of previous analysis choice values is given in relation to these previous values, denoted d n. ⁇ -l) to d p _ p (np).
• STAT the stationary parameter
• this can, by way of nonlimiting example, be taken equal to the average value between the maximum value and the minimum value of the stationarity parameter previously mentioned in the description, STAT M and STAT m .
• 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 "before" and "back” 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 LPC filter "back” for the frame preceding the current frame.
• step 111 consists, starting from an initialization step 1110 in which the value of the stationarity parameter STAT (nl) and the value of analysis choice d n -. (Nl) relative at block LPC B n . 1 prior to the current block B n is available, to be performed, in a step 1111, a step consisting in discriminating the LPC analysis mode "front" or LPC "rear” of the 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 choice of analysis value d n . : (nl) with respect to the symbolic value "fwd" or to the logical value zero corresponding to the value complemented by the true logical value.
• the step of calculating the stationarity parameter value intermediate consists, in a step 1113, in determining the number of anterior frames analyzed consecutively in LPC analysis mode "back", number noted N_BWD, then, in a step 1114, in comparing on number of comparison criteria 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 (nl), increased by a determined value as a function of the first arbitrary value representative of a number of frames successive analyzed, that is to say in fact the number of anterior 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) for the current LPC block B n is thus increased relative to the corresponding value of the same stationarity parameter for the previous block B n . 1 .
• 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 r .
• the value of the stationarity parameter STAT (n- 1) of the preceding block the current block B r On the contrary, for any preceding 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 n , lf while a negative response to test 1112 above indicates the absence of such a transition.
• the calculation step 111 Upon a positive response to the aforementioned occurrence test 1112, the calculation step 111 then consists in comparing, on the basis of an inferiority comparison criterion, the number of above-mentioned anterior frames N_B D to a second arbitrary value N_ representative of a number of successive frames analyzed in LPC "back" mode preceding block B - ,. ⁇ preceding the current block.
• this test is followed by a 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 (nl) 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 then consists in assigning, in a step 1118b, to the value of the intermediate stationarity parameter STAT * (n) the value of stationarity parameter of the block preceding- the current block, that is STAT (nl).
• step 1118a and 1118b are then followed by a step for resetting to zero the number of successive blocks processed in "backward" LPC analysis mode, this step of zeroing carrying the reference 1118c and making it possible to update the entire process for calculating the value of the intermediate stationarity parameter.
• the value of the stationary parameter STAT * (n) is assigned the value of the stationary parameter STAT (nl) of the preceding block B ⁇ in a step 1119.
• 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 above-mentioned intermediate stationarity parameter
• this can advantageously consist of a step 1120, to discriminate the prediction gains of the “backward” LPC filtering and of the “forward” LPC filtering, these gain values being denoted Gpb and Gpf respectively.
• the aforementioned discrimination step simply consists in memorizing and reading the gain values calculated for the LPC filtering "before” respectively "back” above.
• 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".
• 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 representative of the refinement value ⁇ S is noted:
• the modification, by augmenta- tion or by reduction, of the value of the intermediate stationarity parameter of the refinement value ⁇ S is proportional to this relative value of the gains.
• step 1121 the value of the stationarity parameter STAT (n) is thus available 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 "before" and "back" LPC prediction gain 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.
• the refinement value ⁇ S is subjected to a comparison test of superiority to the value 0, in a step 1121b, this comparison test in fact making it possible 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 “rear” LPC filtering gain Gpb with respect to this first determined positive value, denoted S.
• the value of the stationary parameter STAT (n) is assigned the value of the intermediate stationary parameter STAT * (n) in a step 1121g.
• 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 with respect to a second determined positive value STA ⁇ of course representing a stationarity value. This inferiority condition test is carried out in step 1121e.
• the value of the intermediate stationarity parameter STAT (n) is assigned the value of the intermediate stationarity parameter STAT * (n) in the aforementioned step 1121g.
• the value of the intermediate stationarity parameter STAT (n) is assigned the value of the intermediate stationarity parameter STAT * (n) increased by the positive value ⁇ S by the ripening value at step 1121i.
• the refinement value ⁇ S being negative
• the step of decreasing the intermediate stationarity parameter by the refinement value ⁇ S is also subjected to a test. of inferiority condition of the gain value of the "back" LPC filtering Gpb with respect to a third determined positive value denoted S d in a comparison step 1121d.
• This third determined positive value is of course representative of an LPC filtering gain value.
• the value of the intermediate stationary parameter STAT * (n) is assigned in step 1121g.
• the step of decreasing the value of the intermediate stationarity parameter by the ripening value ⁇ S is also subject to a condition of superiority of the value of the intermediate stationarity parameter STAT * (n) with respect to a fourth determined positive value, denoted STATd in a comparison test denoted 1121f.
• the fourth positive value determined is representative of a chosen stationarity parameter value.
• the value of the stationary parameter STAT (n) is assigned the value of the intermediate stationary parameter STAT * (n) in step 1121g.
• the value of the stationary parameter STAT (n) is assigned the value of the intermediate stationary parameter STAT * increased by the algebraic value of the refinement value ⁇ S, negative, the value of the parameter of intermediate stationarity thus being reduced to establish the stationarity parameter value STAT (n) in step 1121h.
• the parameter of STAT (n) stationarity At the end of steps 1121g, 1121h and 1121i, there is thus at step 1122 of FIG. 2b 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.
• the case where the relative value of the LPC filtering prediction gains "before” and "back” no longer corresponds to the ratio of gains p but to the difference of the aforementioned gains.
• 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 (nl) in the allocation step 1111b shown in FIG. 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 n ⁇ . 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. We notice :
• the value of S_TRANS is chosen so as to strongly favor the choice of the LPC filter "before" in the presence of a spectrum transition measured with the aid of the distance d LPC ; otherwise, in all other cases, if Gpb> Gpf-S PRED and Gpi> Gpf-S_PRED, then the LPC filter used is the interpolated "rear" LPC filter, provided that the gain of the latter and that of the LPC filter pure "rear” exceeds the threshold value G : previously mentioned. If the condition on the aforementioned prediction gain values is not fulfilled, then the “before” LPC filtering is chosen.
• 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 r 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 ... very little stationary, to 100, corresponding to the value STAT M very stationary. According to the value of the stationarity parameter
• 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.
• G x keep a fixed value, these values can for example 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 FUD previously cited in the description.
• the value of choice of analysis d n (n) is taken equal to 0, that is to say symbolic value "fwd" in step 122.
• step a new test is carried out relative to the choice value of ana- lyse d n _ x (nl) to the logical value 1, that is to say to the symbolic value "bwd".
• 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 "back" LPC filtering prediction gain to the "front” LPC filtering prediction gain 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".
• test 126b On negative response to test 126b, the value of analysis choice d n (n) is assigned the logical value 0, that is to say the symbolic value "fwd", step 128.
• a new test is carried out, in a step 127, this test consists as long as verifying the conditions for comparing the "back" LPC filtering gain Gpb to the "forward" LPC filtering prediction gain minus the threshold value S_PRED (n), for comparing the superiority of the intermediate LPC filtering prediction gain Gpi to the "forward" LPC filtering prediction gain value minus the aforementioned threshold value S_PRED (n) and to the comparison of the "back” filtering prediction gain Gpb superiority to the threshold value G x , as well as comparison of the value of the intermediate filtering prediction gain Gpi with the threshold value G x .
• the negative response to test 123 previously mentioned in the description also leads to the carrying out of the aforementioned test 127.
• 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 mode of constitution of the digital audio-frequency signal to be coded in successive blocks of samples B n has not been shown because this operating mode is perfectly known from the state of the art and can be carried out from 'a simple buffer memory, for example read periodically at the frame frequency and the block frequency.
• 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 make it possible to deliver a transmitted coded signal consisting of LPC filter parameters accompanied by an analysis decision indication, as well as parameters Pr r relating to the harmonic analysis and to the CELP excitation signal.
• the analysis decision indication corresponds to the analysis choice value d r ⁇ (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 in order to calculate the residual signal which will then be coded by the IF 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 IF, of a non-transmitted coding residual signal, the coding residual 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 abovementioned synthesis signal s_syn n (c , to deliver the abovementioned 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, as shown in FIG. 3 makes it possible to code the digital audio signal on the aforementioned digital audio signal from the "forward" LPC filter for non-stationary areas and on the aforementioned synthesis signal s_syn n . 1 (t) from the "back" LPC filter 1B for stationary areas , 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 object coding method. of the present invention.
• the maximum and minimum stationarity values are also defined above. As shown in addition in FIG.
• the coding device which is the subject of the invention comprises a module, denoted 1D X , 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.
• 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 current logic value, 0 or 1, allowing the addressing in reading of the filtering parameter values 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 of the residue IF, which then makes it possible to deliver the residual signal LPC synthesis designated by res_syn n (t).
• the 1G module is a filtering module whose transfer function is the inverse of the transfer function of the 1E module obtained from the parameters memorized from 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 IF module.
• 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 'LPC analysis “back” 1B, the coded signal s_c n (t) consisting of the transmission of the LPC filtering parameters "before” when the value of analysis choice d n (n) has the symbolic value "fiv'd” 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 signal and to privilege the maintenance in one of the LPC filtering modes "before”, 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 "before”, 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
• the decoding device comprises at least one module for synthesis, referenced 2A, of the filtering residue signal receiving the LPC residue coding parameters delivered by the IF module.
• the module 2A decodes the coding parameters supplied by the module IF 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 “backward” 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 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 "front", 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 coder proper consisted of a telephone band coder from 300 to 3400 Hz, at a rate of 12 kb / s of the CELP type.
• the frames were formed over a duration of 10 ms for an excitation provided by algebraic dictionary according to the so-called ACELP technique previously mentioned in the description.
• the LPC analysis "before” was a 10 order analysis and the LPC analysis "back” was 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.
• S_PRED is adapted as follows:
• 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 4.0 dB.
• the threshold S_LSP_H was not used in this embodiment.
• the value of the threshold G ⁇ 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 :
• ENER SIL 10.Log ⁇ L s (i With regard to the value of the Spy- threshold, mentioned previously and intended to further limit the risk of switching by imposing the LPC filtering mode "before" when the value STAT (n) is less than this threshold, this value S FUD 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 b (N_BWD) are such that:
• S_PRED is adapted as follows:
• the threshold S_LSP_L is adapted to the idea of the following staircase function: 5 d0.02 if STAT (n)> 100
• the threshold S_LSP_H is adapted using the following step function: 0 (0.08 if STAT (n)> 100
• 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 50 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 G : threshold has been set at OdB.

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