SE517993C2 - Excitation pulse position determination method for speech frame - Google Patents

Abstract

The method involves calculating positions of excitation pulses in several calculation stages in accordance with a first method. The positions of the excitation pulses in several calculation steps are calculated in accordance with a second method. Each stage has a starting point from one of several positions (mi,mk,mr) calculated in accordance with the first method to obtain one of the pulse placements. The proportion between the number of calculation stages is chosen according to the first and second methods respectively to obtain the least complexity for a certain speech quality.

Description

- |., | ø | u | ao 517 993 šïïíïfï ._: I- "2 and phase mode (time mode) depending on the predictive parameters ak, the pattern and the speech copy. partly depending on the prediction remainder dk between speech input- Each of the pulses may affect the speech pattern copy so that the prediction remainder becomes so small as a relatively low bit rate and, therefore, can be coded and transmitted narrowly improved as possible.The generated excitation pulses have like the predictive parameters, thereby obtaining a quality of the recovered speech signal.

In the above-mentioned known method, the excitation pulses are generated within weighting the residual signal dk and feedback and weighting the generated values for each frame interval of the speech input pattern through the excitation pulses Then in each a predictive filter. a correlation is performed between the output signals from the two filters and a maximization of the correlation of a number of signal elements from the correlated signal, in order to form the parameters of the excitation pulses (amplitude and phase position). The advantage of this multipulse algorithm for generating the excitation pulses is that you are able to generate different types of sounds with a small number of pulses (for example 8 pcs / frame interval). The pulse search algorithm is general with respect to the pulse placements in the frame. It is possible to recreate non-toning sounds (consonants), which generally require randomly placed pulses and toning sounds (vowels) that require a more cohesive pulse placement.

These known methods calculate the correct phase positions of the excitation pulses within a frame and subsequent frames of the speech signal and the placement of the pulses is done only in depending on a complex treatment of the speech signal parameters (prediction residue, residual signal and excitation pulse parameters in the previous frame).

A disadvantage of the original pulse placement methods according to the above-mentioned US patent is that the coding performed after calculation of the pulse positions is complex in terms of calculations and storage. It also requires a large number of bits per pulse position in the frame interval. In addition, the bits of the codewords obtained from the optimal combinatorial pulse coding algorithms are. 1 »o n nu. . I | .n 517 993 3 bit error sensitive. A bit error in the codeword during the transmission from the transmitter to the receiver can have fatal consequences on the pulse placement during decoding in the receiver.

To remedy this, restrictions can be imposed on how many excitation pulses need to be exposed within each speech frame. This is possible due to the fact that the number of pulse positions for the excitation pulses within a frame interval is so large that one can waive an exact placement of one or more excitation pulses within the frame and still obtain an acceptable quality of the recovered speech signal after coding and transmission. .

It has therefore been proposed, see U.S. Patent 5,193,140, a method according to which certain phase position limitations in the placement of the pulses are introduced by prohibiting a certain number of already determined phase positions for the pulses following the phase position of an already calculated excitation pulse. When the position of a first pulse within the frame has been calculated and when this has been placed in the calculated phase position, this phase position is prohibited for subsequent pulses within that frame.

This rule preferably applies to all heart rate placements within the framework. When the deployment for a new subsequent frame is to begin, all positions within the frame are again vacant.

Recently, it has been proposed to use so-called codebooks in speech encoders in generating the synthetic speech signal, see Such a codebook stores a number of codewords for the speech signal which are used in the creation of, for example, U.S. Patent 4,701,954. synthetic speech copy. ie contain the Codebook thereby be fixed, fixed codewords or be adaptive, ie. it can be updated as the speech copy is formed. You can also use a combination of a fix and an adaptive codebook.

DESCRIPTION OF THE INVENTION The above method of prohibiting all phase modes within a speech frame, of which a mode has already been assigned an excitation pulse leads to a more limited number of transmitted excitation pulses than if 1 | n o n n.

Mm: n n e. »U n 517 993 4 the restriction is not used. In addition, it is possible to more easily encode the phase modes for the excitation pulses in the transmission side, and to better distinguish the phase modes when decoding in the receiver side.

Thus, according to the present invention, the most sensitive phase position positions are coded separately and the less sensitive phase positions are coded together.

The method according to the invention is then characterized as appears from the characterizing part of claim 1.

Advantageous embodiments of the coding method according to claim 1 are characterized as it appears from claims 2-3.

The proposed method can be applied to a speech encoder operating according to the multipulse principle with correlation of an original speech signal and an impulse response of an LPC synthesized signal and with or using Icode books as above. The procedure. however, it can also be applied to a so-called RPE speech encoder in which several excitation pulses are simultaneously placed in the frame interval.

LIST OF FIGURES The proposed method will be described in more detail with reference to the accompanying drawings, in which Figure 1 shows a simplified block diagram of a known LPC speech encoder; Figure 2 shows a timing diagram of certain signals appearing in the speech encoder of Figure 1; Figure 3 shows a diagram of a speech frame for explaining the principle of the prior art method of restrictiveness in determining the excitation pulses; Figure 4 shows a block diagram of a part of a speech encoder where the method according to the invention is applied; Figure 5 shows a block diagram of a part of a known speech encoder with adaptive codebook in which the method according to the invention can be applied; «E e o ao .wwøn ~ n | . Figure 6 is a flow chart for explaining the determination of pulse positions; Figure 7 is a diagram showing the pulse exposure; Figure 8 is a diagram showing pulse exposure by phase position adjustment; Figures 9-10 show block diagrams of a portion of a speech encoder in which the proposed coding method is applied.

EMBODIMENTS A simplified block diagram of a known LPC speech encoder according to the multipulse principle with correlation is shown in Figure 1. Such an encoder is described in detail in U.S. Patent 4,472,832 (SE-B-456618). appears at a microphone 110.

The prediction analyzer 110 contains, in addition to an analog-digital converter, an analog speech signal from, for example, the input of a prediction analyzer, an LPC computer and a residual signal generator, which form predictive parameters ak and a residual signal dk. The predictive parameters characterize the synthesized signal resp. the original speech signal over the analyzer input.

An excitation processor 120 receives the two signals ak and dk and operates during one of successive frame intervals determined by the frame signal FC, to emit a certain number of excitation pulses during each of the intervals. Each pulse is then determined by its amplitude Amp and its time position (position), mp within the frame. The excitation pulse parameters Amp, mp are passed to an encoder 131 and then multiplexed with the predictive parameters ak before transmission, for example from a radio transmitter.

The excitation processor 120 contains two predictive filters with k and (Ai, mi) depending on the predictive parameters ak during a certain calculated impulse response to weight the signals serving step gu. Furthermore, a correlation signal generator is performed which performs a correlation between the weighted original signal (y) and the weighted artificial signal (y) each time an excitation pulse is to be generated. For each correlation, a number is obtained. »N ~ v. - \. = U | u n. 517 993 6 "Candidates" of pulse elements Ai, mi (0 5 i <I), of which one gives the least squared error or the least absolute amount. In the excitation signal generator, the amplitude Amp and time position (position) mp are calculated for the selected "candidate". The contribution from the selected pulse. Amp, mp is then subtracted in the correlation signal generator from the desired signal to obtain a new sequence of "candidates" and the procedure is repeated as many times as the desired number of excitation pulses within a frame. This is described in detail in the aforementioned U.S. patent.

Figure 2 shows a time diagram of speech input signal, predictive residual signal dk and excitation pulses. The number of excitation pulses is here == 8 of which the pulse Am is selected first m (gave the least l '1 error), then pulse Amz, m2 etc. within the frame.

In the previously known method for calculating amplitude A and phase position mp for each excitation pulse from a number of candidates Ai, mi was calculated mp = mi for the candidate i which gave the maximum value of ai / oij and the associated amplitude Amp was calculated, where ai is cross-correlated. the relation vector between the signals yn and yn according to above and øij (hereinafter referred to as Cij, i = j = m1) is the autocorrelation matrix for the impulse response of the predictive filters. Any position mp was accepted if only the above conditions were met. Index p indicates the step during which the calculation of an excitation pulse as above takes place.

According to the previously known method of restrictiveness in determining the positions of the excitation pulses, a frame is divided according to Figure 2 as shown in Figure 3. It is assumed as an example that the frame contains N = 12 positions. The N positions then form a search vector (n). The entire frame is divided into so-called subblocks. Each sub-block will then contain a certain number of phase modes. For example, if, as shown in Figure 3, the entire frame contains N = 12 positions, 4 sub-blocks and 3 different phase positions are obtained within each sub-block.

The sub-block has a certain position within the whole frame, which is called (05n belong to a certain sub-block nf (05nf (O5f5F) in this sub-block. The phase position position. Each position n.. - »n;: 25 | v» -v | nu - .nn 517 993 7 In general, the positions n (05n5N) in the total search vector, which contains N positions, are n = nf'F + f ... (1), where f = the pulse phase position within a subblock nf and F = within the block nf the number of phase positions nf = 0, ..., (Nf-1), f = n = 0, (N - 1).

Furthermore, the following relationships apply fp = n MOD F and nfp = n DIV F ... (1). 0, ... (F-1) and 000 'The diagram according to Figure 3 shows the distribution of phase positions fp and subblocks nfp for a certain search vector containing N positions. In this case, N = 12, F = 3 and NF = 4.

The known method with restrictions means to limit the pulse search to positions which do not belong to an already occupied phase position fp for those whose excitation pulses, positions n have been calculated in the previous step.

The sequence number for a certain calculation cycle of an excitation pulse is hereinafter referred to as p as above. The known method with restrictions then gives the following a calculation run for frame intervals: 1. Calculate the desired signal yn. Calculate the cross-correlation vector ai 3. Calculate the autocorrelation matrix Ci j (= $ ij i = j = m) 4. For p = 1. Search for the mp, ie the pulse position that gives the maximum ai / Cij in non-occupied phase positions f.

. Calculate the amplitude Amp of the found pulse position mp. 6. Update the cross-correlation vector ai and n 7. Calculate fp according to the relationships (1) above fp 8. For p = p + 1 perform the same steps 4-7.

Flowcharts further illustrating the known method are shown in Figures 4a and 4b of the aforementioned U.S. Patent 5,193,140. n | ~ | n ø nu phase min: »uu Q nu nn 517 993 8 According to the proposed method, the obtained phase positions fl, ..., fp are coded jointly and the obtained phase position positions (subblocks) nfl, ..., nfp the coding of the phase modes can combinatorial coding is used. Phased separately before transmission. For the position positions are coded with code words separately.

In one embodiment, the known speech processor circuit without restrictions at the pulse exposure may be modified as shown in Figure 4, which shows the part of the speech processor which comprises the excitation signal generating circuits 120.

The predictive residual signal dk and the excitation generator 127 are input to their respective filters 121 and 123, respectively, in step with a frame signal FC via the gates 122, 124. From the filters 121, 123, the signals yn and yn are obtained, which are correlated in the correlation generator 125.

The signal yn represents the real speech signal while yn represents the artificial speech signal. From the correlation generator 125 a signal Ciq is obtained containing the components ai and øij as above. In the excitation generator 127 a calculation is performed of the pulse position mp which gives a maximum of a. Whereby $ - 1 1/13 in addition to the pulse position mp also the amplitude Amp as above is obtained.

From the excitation generator 127 the excitation pulse parameters mp, Amp are output to a phase position generator 129. This calculates the current (subblocks) nfp phase positions fp and the phase position positions from the excitation generator 127, according to the incoming values m, AP mp the relationship Mo = (m-1 = m - 1) DIV F where F = the number of possible phase modes.

The phase position generator 129 may be a processor comprising a read only memory which stores instructions for calculating the phase position and the phase position position according to the above relationship. a n r: | § 25 mm »517 993 9 Phase position fp and phase position position nfp is then applied to the encoder 131, figure 1. This encoder is of the same basic structure as the known encoder but encodes phase position and phase position position instead of pulse positions m.

P phase position positions and the decoder then calculates the pulse position m In the receiver side, the phase positions and P are decoded according to m = (n P fp - 1) 'F + f P, which unambiguously determines the excitation pulse position.

The phase position fp is also supplied to the correlation generator 125 and the excitation generator 127. The correlation generator stores this phase position and takes into account that this phase position fp is occupied.

No values of the signal Ciq are calculated where q is included in the positions belonging to all previous fp calculated for an analyzed sequence. The occupied positions are = -Ff qn + p where n 0, ..., (Nf - 1) and fp are all previous phase positions within a occupied frame. The excitation generator 127 similarly takes into account the occupied phase positions when comparing the signals Ci and Ci *. q q When all pulse placements for one frame have been calculated and executed and when the next frame is to be started, of course all phase positions are again available for the first pulse in the new frame.

Figure 5 shows another type of speech coding by means of a so-called adaptive codebook. The prediction analyzer 110 gives the two parameters ak and dk and these parameters are used as quantities for a block here designated 111, which is introduced before the excitation processor 120 in Figure 4.

Block 111 contains an adaptive codebook 112, which stores a number of code words c1, ... cn and which can be updated by a control signal. This is symbolized in Figure 5 by means of a selector 113, which points out a certain code word ci in dependence on the value of the control signal.

I. n ø o. - .. «| ø n ø. of u 517 993 A scale unit 114 scales the codeword from the codebook 112 in a suitable manner and the scaled codeword is applied to the minus input of a summator 115, the plus input of which receives the predictive residue dk from the analyzer 110. The predictive residue supplied to the summator 115 is here denoted dkl and the summator 115 is denoted dk2. from the analyzer 110) and. the new. the prediction residue dk2 is applied to the Predictive parameters ak (unchanged excitation processor 120 according to Figure 4.

The control signal to the selector 113 is derived from a loop including a weighting filter 117 and an extreme value generator 118. The residual signal dk2 is supplied to the filter 116 and the filtered signal is weighted in the filter 117. They have an adaptive filter 116 with the filter parameters ak. is filtered and weighted in. the filters 116,117 and the least squares error E are formed in the extreme value generator 118. The above is done for all selected codewords and the codeword that gave the smallest error is subtracted after scaling in 114 from the residual signal dkl to give a new residual signal dk2. This is a so-called "closed loop" search for the best password in the event that an adaptive codebook is used. Thus, the circuit of Figure 5 may or may not be used. It gives an improved value of the prediction remainder dk according to figure 1. In figure 4, dk = dkl if adaptive codebook is not used, and dk - dk2 if adaptive codebook is used.

As will be described in connection with Figure 9, the circuit of Figure 5 is also used in some blocks (132a-d) to search for the "closed loop" error, but the codebook is then replaced with a memory space to store only a set of restricted pulse sets.

The predictive parameters ak and index i for the codewords ci selected (minimum error) are output to the multiplexer 135 and transmitted in a known manner. x: | n now oooo: o ooo ou oo »o oo oo oo oo oo oooo oo oooooo oo oo I ooooo ooo ooo ooo oo .ooo ooo o oo ooo oo oo oo oo oo ooooo o. oo oo oo oo 11 with reference to Figure 6.

At the start of the pulse exposure, a number of excitation pulses are first determined by means of the known method without restrictions, block 1. described above. Both the pulse positions mp (lgpsj) and the amplitudes The calculation of these thus takes place in a known manner as Amp within the framework is determined in this way. However, the amplitude Amp of these need not be used in the determination with restrictions, since each of the pulse positions calculated according to the above is still used, the amplitude not being of interest. However, in an alternative method of phase adjustment of the starting pulses, the amplitudes must also be used (unless these are recalculated).

When the number j of pulse positions mp (p = 1, ..., j) is determined in this way, the calculation of excitation pulses is started according to the known method with restrictions, block 2. Both pulse position mp and the amplitude Amp of each excitation pulse are determined and according to with (lšišj) excitation pulse determined according to block 1 (without restrictions) the known method described above. starting point from the position mi of an arbitrary within the same frame. The calculation of the new excitation pulses (with restrictions according to the known method) is made for a certain number, Nl.

After this first calculation step with restrictions (block 2) the position mk (k = i) is selected for a new excitation pulse calculated (block 1) then the same calculation routine is performed with restrictions as above according to the known method without restrictions and mentioned for the first calculation step also for this second calculation step, block 3 in figure 6, but now starting from another position mk (15k5j; k = i). The calculation is made for a certain number of N2 excitation pulses, where, however, N2 can be = N1.

The calculation steps by the known method of restriction are then continued a number of times until the last step, block 4 in Figure 6. The number L of such steps need not be equal to oooooo oo 517 993 šïï fi ïïí ë- “12 the number of positions j = P for the excitation pulses, obtained (block 1). suitable to interrupt after a small number of steps if the talc quality according to the method without restrictions It may be may be acceptable, whereby the calculations will be fewer. It may also be appropriate to improve the accuracy of providing more starting positions than the original number of positions P for the excitation pulses calculated without restrictions. The resulting number of positions will then be L = P + Pextra, where Pextra are the positions that have been added. This will be shown in more detail in Figure 7.

In order to reduce the complexity of the subsequent pulse searches with restrictions, the above-mentioned resulting pulse positions L = P + Pextra can be used to further impose restrictions. One can thus prohibit the phase positions fp (p = 1, ..., j) which is far from the tions on which pulses are to be allowed. pulse positions calculated without restrictions.

According to Figure 6, the pulse exposure is interrupted according to the method with restrictions after a certain number of steps (N1 steps starting from pulse p1, N2 steps starting from pulse p2, etc.). After this, L pcs. pulse sets from each of the originally calculated positions (without restrictions) including any. extra positions Pextra. It is then examined, block 5, which of the L sets is to be used according to a certain criterion. The set of pulses that best meets the criterion is maintained while the others are rejected. How this criterion, so-called, is formed, the block diagram according to Figure 9, which shows the case L = 4. "closed loop" is explained in more detail in connection with The set of pulses (Amp, mp, p = 1,2, ..., i, k, r) with restrictions selected thereby forming the final excitation pulses for the frame and corresponding values of phase position and phase position position were transmitted to the receiver. The positions (mp, p = 1, ..., j) that were initially calculated without restrictions are not transferred.

An algorithm for the calculation steps as above is shown in Appendix 1. ø o: u a. Mrv »13 Figure 7 shows a diagram of excitation pulses and extra pulses that are calculated without restrictions and the pulse sets, which are calculated with restrictions. The pulses P1, P2, P3 and P4 are the excitation pulses calculated according to the original known method (block 1, figure 6). These pulses have the phase positions n1, n2, n3 resp. n4. In addition to these pulses, in this case two further pulse positions Pel and Pe2 are calculated with the phase positions n5 and n6 according to the same known method. The phase positions n1-n6 thus give the starting positions for calculating a number of L = 6 pulse sets calculated according to the known method with restrictions (blocks 2-4, figure 6).

You get two "extra" heart rate sets that can be included in the test of "best" heart rate set according to the above criteria. In Figure 7, the starting pulse in resp. pulse set 'is marked with a thick solid line with a square, while the calculated pulses in each pulse set are marked with dashed lines and a ring.

The different pulse sets calculated with restrictions belonging to all starting pulses P1-P4 and extra pulses Pe1, Pe2 are then tested according to the criterion "closed loop". The pulse set that was "best", ie the one with the smallest error, is selected and transmitted. Other pulse sets are not used for the current frame.

As an alternative to the above calculation of the positions of the excitation pulses, one can adjust the phase positions of the excitation pulses calculated without restrictions with regard to the restrictions. You select the phase modes fp for the set that was the best according to the above criteria.

For each starting pulse of the total calculated starting pulses without restrictions in a frame, a search area is defined which consists of a time interval of determined size around the position of the starting pulse in the frame. Then, based on each of the starting pulses, a pulse set is calculated with the restriction that none of the positions of the calculated pulses may be outside the search range.

In addition to the position of the current start pulse, you also get ~ | «N u: i.» Ww- mun: 517 993 14 a small number of positions for the pulses that are within the search range for the other start pulses. This is repeated for each of the other start pulses. As a result, you get a number of pulse sets where, in each, a pulse always corresponds to the exact position of resp. starting pulse and where the positions of the other pulses are within resp. the search area for other start pulses.

In addition to the above-mentioned restriction on the pulse exposure, further restrictions are placed on the coding of the various obtained pulse positions mp. This condition is applied to the different positions tested above before the "closed loop" test is performed. The code restriction means that certain so-called codeable vectors are selected where each vector corresponds to a pulse set calculated with restrictions as above.

In general, the positions n (Osn5N) in the total search vector, which contains N positions, are n = nf'F + f; f = pulse phase position within a subblock (phase position position) nf and F = where the number of phase positions within block nf.

The restriction is now imposed that the pulse phase mode for all positions in a certain pulse set (vector) must be different for the vectors selected. Others are rejected.

All the vectors thus obtained are considered codable and are then tested "closed loop", the values of the phase positions of the "best" pulse set relative to each of the positions of the starting pulses being transmitted to the receiver.

In order to keep the complexity of the calculations and tests unchanged, you can set up and use a list of the different candidates in descending order with regard to the total phase position adjustment, whereby the candidate who was "second best" in one test is examined first in the next test, etc. . for an entire frame. a o c. now 517 993 An algorithm for the above-mentioned phase position adjustment is shown in the attached Appendix 2.

Figure 8 shows a diagram illustrating the phase position adjustment described above of the excitation pulses produced without restrictions as above. Figure 8a) shows the excitation pulses P1, P2, P3 0Ch P4, corresponding to the pulses P1-P4 shown in Figure 7 at the top. obtained without restrictions and as For each starting pulse P1-P4 of the total calculated starting pulses without restrictions in a frame, a search area S1, S2, S3, S4 is defined which consists of a time interval around the position of the starting pulse in the frame, Figure 8b). Pulse positions outside each of these search areas are impermissible pulse positions and. thus constitutes the restriction. Within resp. search area there are a small number of allowed pulse positions. For example, there are the two pulse positions which consist of those which are closest to the pulse position mi which has been calculated without restrictions. A number of pulse sets can therefore be calculated which can be formed from these "side positions" permitted around each starting pulse position.

Figure 8c) shows a pulse set calculated with the pulse P1 as the starting pulse, whereby three more pulses have been formed in that two, P2 and P4, have a step phase deviation from the one to the original positions of these later starting pulses, while the third P3 has not given any phase deviation from its associated starting pulse position according to the example. This gives a total phase shift = 2.

Figure 8d) shows, based on the starting pulse P2, how two POs and P4, have been shifted one step to the left from their associated starting pulse positions and the fourth pulse P1 has been shifted one step to the right. This gives a total phase shift = 3. pulses, Figure 8e) shows, starting from the starting pulse P4, how two pulses, P2 and P3, have been shifted one step to the right from their - «u ø u u. .1-1; 517 993 16 associated start pulse positions and the first pulse P1 is shifted one step to the right from its associated start pulse position. This also gives a total phase shift = 3.

The following table can then be set up: As an example, only two starting pulse positions are taken, which have the sequence numbers 2 and 5 and where f fpz below has been calculated according to the P1 'relationship page 9, line 20 with F = 3.

TABLE: Pulse position mp shift fpl fpz codable? Start pulse- 2 5 0 2 2 No position 2 6 1 2 3 Yes shifted 2 4 1 2 1 Yes versions 1 5 1 1 2 Yes of 1 6 2 1 3 Yes start- 1 4 2 1 1 No pulse- 3 5 1 3 2 Yes position 3 6 2 3 3 No tion 3 4 2 3 1 Yes In general, none of the phase modes fp must be equal for codability, ie. f P 2 pl = fpz. If several starting pulse modes mp are used, ie 3 for the calculation, it applies that f = f pl P2 = fp3 = fp4 ... for one and the same pulse set.

The obtained codable pulse sets are then compared as above "best". The codable pulses amplitude Am can be taken from said "closed loop" and the phase values for that set are transmitted. the amplitude value of the starting pulse based on resp. pulse set or they can be calculated again, to take into account the phase change of resp. calculated heart rate. «1 | a av o o a q -o 517 993 17 e = | n now The values of the phase modes fp and the phase mode positions nfp of the "best" codeable set are transmitted.

Figure 9 shows a block diagram of a part of a speech encoder which uses the coding method according to the invention.

Block 125, as in Figure 4, represents a correlation generator, which forms the quantity Ciq = (C¿j, ai) representing the correlation between the signals y and y Then follows the excitation generator 127 which selects the amplitude Amp and pulse phase position mp of the excitation pulse which gave the best correlation (= least square mean error) of in candidates.

Total I correlations are performed before the position and amplitude of a particular excitation pulse are determined. In the prior art embodiment, the excitation generator 127 was followed by a phase position generator (129, Figure 4). In Figure 9, a memory unit 126 is connected instead. This memory unit stores the amplitude of the selected excitation pulses Amp and phase positions mp (p = 1, ..., j) obtained according to the method without restrictions (block 1, figure 6).

The memory unit 126 is followed by a selector unit which in Figure 9 is symbolized by the block 228a and a controllable contact 128b.

The selector unit 128a controls the contact 128b to scan a number of branches to connect a certain branch to the memory unit 126 so that a certain position mp (p = 1, ..., j) stored in the memory unit 126 may constitute the starting value as above, block 2, figure 6.

The upper branch (a) shown in the figure contains V 25 an excitation generator 127a of the same design as the excitation fV * generator 127; a phase position generator 129a of the same design as the phase position generator 129 according to Figure 4 and which has a feedback to the excitation generator 127a for updating, cf. Figure 4; ¿”3p a storage unit 130a, as well as;;; a calculation unit 132a for calculating the error E1 "closed loop" as above and which thus has the same function as the circuit according to Figure 5 but without a codebook. Instead of the codebook, there is a memory in whose positions the values of the associated pulse set Q are calculated mßu; . n. . .e 517 993 18 with restrictions in units 127a, 129a and 130a can be stored.

The predictive residue dkz 'is supplied to the unit l32a if adaptive codebook. In addition, in other cases the predictive residue dkl is supplied. is added to the predictive parameters ak. Other branches (b), (c), and (d) contain corresponding units as branch (a). Each branch thus contains units which can perform position determination according to the method of restrictions. Each branch thus gives a pulse set calculated with restrictions, ie a total of four pulse sets (cf. figure 7) with restrictions are performed according to figure 9 based on four start values mp retrieved from the memory unit 126. If more start values than four, likewise the units 132b, 132c and 132d The associated number of branches is of course increased. there is settlement from the branches (b), (c) resp. felet "closed loop" E2, E3 resp E4. (d) and for calculating the Selector Unit 128a controls the contact 128b to the uppermost branch (a) and gives the positions mp (p = 1, already occupied from the pulse search without restrictions (block 1, to ex- This also receives the updated figure 6) ..., j) citation generator 127a. the value (Cij, ai) from the correlation generator after said pulse search without restrictions. The excitation generator 127a and the phase position generator 129a can now perform a pulse search with restrictions because these units know The result after a certain number of searches gives a number of excitation pulses, starting from a certain position in, which positions are already occupied. whose amplitude Amp and pulse position mp are stored in the unit 130a. In this case, the phase position fp and the phase position position nfp are stored instead of the pulse position mp.

Thereafter, the contact 128b is advanced by the selector unit 128a to the excitation generator 127b in branch (b) and pulse search starting from a second starting value with the position m2 is started by the next branch (b). This takes place in the same way as for branch (a) as above and in the same way pulse search then takes place in branches (c) and (d). The same value of Cij is fed to all the branches from the beginning of - a e | ø; en ».: uv u n -. now 517 993 19 the pulse search, since this value is used in testing those in the "candidates" when searching for the best excitation pulse (with restrictions).

After a certain number of parallel steps M, all branches (a) - (d) have calculated the amplitude of their excitation pulses and phase positions / phase position positions and stored these in the storage unit 134. Thereafter, the calculation units 132a-132d perform their respective calculations of the error between the incoming speech frame and the synthesized the speech frame in accordance with the code words used and the excitation pulses produced from the respective branch. The incoming speech signal is therefore applied to each of the units 132a-132d. Each of these units calculates the so-called closed loop error and outputs the value E1, E2, E3 or E4 on the error as an output signal. For example, one can select the quadratic weighted error E == Z: ew (n) 2, where ew (n) is the difference between incoming (real) speech signal and synthesized speech signal for the values y (n) and y ^ (n) within the speech frame.

The function of the calculation units 132a-132d is the same if adaptive codebook is not used and the calculation of the error E1-E4 takes place in the same way.

A selector unit 133a with associated contact 133b senses the different calculated values E1, E2, E3 and E4 on the error of the different pulse sets and outputs these values one by one to a storage unit 134. This unit receives the values one by one, selects and saves an incoming value if this is "better" ie is a smaller error E than the immediately preceding value.

At the same time as the unit 134 receives the values E1-E4, it registers which value is the least, ie the pulse set which is "best".

After the "best" pulse set is thus identified, the storage unit 134 retrieves the values of amplitude Amp, phase position fp and phase position position nfp for this "best" pulse set. These values are obtained via one of the connections to the respective storage unit 130a-130d, and these values are then output to the encoder 131.

This is connected to a multiplexer 135 as shown in Figure 1. nnn nnn nn nn nn I: n »n en oonn no nonn oo 0 o nn onno nnnn onnnnn nn non .n" nn non n no nnonon nn no no no nnnnnoo The encoder 131 thus receives the quantities: the amplitude Amp and the phase positions / phase position positions fp, nfp for the "best" excitation pulses produced with restrictions. The obtained phase positions fl, ..., fp can be coded as mentioned above. jointly and the obtained phase position positions n are coded separately before fllill 'transmission.The important thing is that the phase positions and the phase position positions are coded in different message words.This gives better separation and thus reduced error probability.

Figure 10 shows the block diagram of Figure 9 but modified to perform the phase adjustment of the start pulses as described above (Figure 8). The selector 128b together with subsequent blocks 127a-d, 129a-130a-d have for the calculation of pulse positions with restrictions been removed and instead replaced with a unit 100 which defines the above-mentioned search area for each of the starting pulses P1-P4 (figure 8). Outside this search area, it is not permitted to place any excitation pulses and the introduction of the search area around each starting pulse and associated calculation can therefore be said to replace the previously used calculation with restrictions according to Figure 9.

The unit 100 further calculates the pulse positions for the possible number of pulse sets as previously described (Table above) and produces the possible pulse sets with regard to the code restrictions. These are thus obtained via the outputs a, b, c and d and the obtained pulse sets are then applied to the units 132a-132d, which calculate resp. errors E1, E2, E3 and E4 "closed loop" in the manner previously described.

Since no value of the amplitudes of the pulse set selected for the coding in the encoder 131 was supplied to the unit 100, the amplitude values Amp of each cx fl are supplied in one of the pulse sets from the memory unit 126, partly to the calculation units 132a-132d, and partly to the storage unit 134.

When coding the phase modes resp. the phase position positions, these can be combined separately, two or more in a message block including associated parity words in a manner known per se.

.U H '39 _.

. N n n n n n n n n n. n nn 21 Also coding of a single word for phase mode resp. phase position position with associated parity words can be performed. The advantage in the former case with several values in a message word is that you save bandwidth, but you must then use "harder" coding to get better protection. In the latter case with a single value of the phase mode or the phase mode position, a simpler coding with less protection can be used, but bandwidth is lost. For the coding of the phase modes, combinatorial coding can be used.

It is understood that Figures 9 and 10 are only intended to show the principle of how associated circuits in a speech encoder can be constructed. In reality, all units can be integrated in a microprocessor that is programmed to perform the functions in accordance with the flow chart in Figure 6 and attached to Appendix 1 resp. 2.

Since both methods with and without restrictiveness are used, one can choose the proportion between the number of calculated excitation pulses according to the method without resp. with restrictions and thus get an optimal distribution that provides the lowest calculation complexity for a certain desired speech quality. The calculation complexity is significantly reduced compared to performing extreme value calculation for all possible positions within a speech frame.

The method of pulse exposure is as described above in connection with a speech encoder in which the placement of the excitation pulses is performed one pulse at a time until a frame interval has been filled. Another type of speech encoder described in EP-A.195,487 works with the placement of a pulse pattern in which the time interval between pulses is constant instead of a single pulse.

The pulse rate method and coding method of the present invention can also be applied to such a speech encoder.

The prohibited positions in a frame (compare, for example, Figures 4a, 4b above) then coincide with the positions of the pulses in a pulse pattern. ...> @ .17 993 22 Appendix 1 Algorithm for the calculation steps according to the flow chart, figure 6.

Modified calculation steps 1-8 shown in US 5,193,140.

US 5,193,140 is hereinafter referred to as [2].

The autocorrelation matrix qbij i [2] is hereinafter referred to as C (i, j) = Cij in the description part.

The pulse positions mp and mq in [2] are here denoted msp and msq, respectively.

The quantity ai in [2] and in the description part is denoted here by a (i).

Analogous to am i [2] relative to a (m) below. 1. Calculate the desired signal y (n) 2. Calculate the cross-correlation vector a (i) and copy to asave (i) 3. Calculate the covariance (or autocorrelation) matrix C (i, j) 4. For p = l to P + extra 4.1 Search for ms, ie the pulse position which gives maximum a (i) * a (í) / C (i, j) = a (m) * a (ms) / C (nzs, rns) in the unoccupied positions. 4.2 Calculate the amplitude Aünsp) for the discovered pulse position msp 4.3 Update the cross-correlation vector a (i) 4.4 Discard the found position msp fmrn the possible positions. For q = 1 to P-Hxtra .1 Copy asave (í) to a (i) .2 Assign m, the value ofmsp I. .3 Caiculate the amplitude A (m¿) for the starting pulse position m1 .4 Update the cross-correlation vector a (i) .5. Forp = 2 to P .5.1 Search for mp, i.e. the pulse position which gives maximum a (í) * a (i) / C (i, j) = a (m) * a (m) / C (m, m) in the unoccupied phases. .5.2 Calculate the amplitude A (mp) for the detected pulse position mp .5.3 Update the cross-correlation vector a (i) .5.4 Exclude the positions with the same phase as mp .6 Caiculate closed-loop error E, _ .7 If the error E is lower than the error for the previously saved set of positions and amplitudes, save the positions mp as mwp and the amplitudes A (mp) as A (mwp) »6 Calculatefp and nfp in accordance with the relationship (1) in [2] for the saved (winning) set of positions mwp. «| | u e. .. »», 517 993 23 Appendix 2 A1 orithm for faslä es'usterin.

Designations according to Appendix 1. 1 Calculate the optimal positions msp and arnplitudes A011; as in step 1 through 4 in the previous section. (kffamålz i). 2. Construct the n = ((P + extra) over P) combinations of P positions out of the msp optimal positions. 3.0 For combínation¿ = combination 1 to combination n 3.1. Shake all positions in combination around by shifting each position l step in each direction. If the resulting set of positions are encodable using the restricted positioning code, save them in a list min_shift__líst with an ordering in respect to the total phase shift from the unshifted combination¿. 4.0 Forj = 1 to nb_to_test, 4.1 Copy the positions at the top of the min_shijft__list to mvp. 4.2 Remove the top positions in the list rnin_shift_list 4.3 Copy the amplitudes from A (msp) of the corresponding unshifted combinations to A (mvp). 4.4 Calculate closed-loop error No, using mvp and A (mvp). 4.5 If the error Ej is lower than the error for the previously saved set of positions and amplitudes, save the positions mvp as mwp and the amplitudes A (mvp) as A (mv; p) Calculateß, and nfp in accordance with the relationship ( 1) in [2] for the saved (winning) set of positions rnwp _ nn. . and s n | | | u oo

Claims (2)

A method for use with a speech encoder, which operates with a speech signal divided into speech frames, for encoding a sequence of a first and a second kind of excitation pulse parameters (fp and nfp, respectively), which jointly indicates the positions (mp) of the excitation pulses calculated in such a way that one a) performs a number of calculation steps (j) of the positions of the excitation pulses according to a first method, which means that a speech signal, divided. in speech frames are analyzed and. the analyzed (110) to form a (ak), synthesized (dk), the speech signal prediction residue and a number of predictive parameters which are applied (120), an excitation processor which filters the prediction residue (dk) and parameters obtained from the excitation processor (Ai, mi) for each of the desired excitation pulses depending on said predictive parameters (ak), b) performs a number of calculation steps (N1, N2, ..., NL) of the positions of the excitation pulses each starting from one of a number according to the first method calculated positions (mi, mk, mr) according to a second method, which means that a number frame is further divided into a number of phase position positions (nf) and each phase position position is divided into a number of phase positions (n), restrictions being introduced meaning that the phase position, which is taken when placing an excitation pulse is prohibited for each subsequent excitation pulse and for each phase position position (nf) within the speech frame, so that one of a number (L) of pulse sets is obtained, s amt 10 15 20 25 30 517 993 25 O Il Ideon: 0 Count! c) selects the ratio between the number of calculation steps (j or max [N1, N2]) according to the first and the second respectively. the method so that the minimum computational complexity is obtained for a given given speech quality, characterized in that the first type (fp) parameters are combined in one or more message words, that the second type parameters (nfp) are divided separately into separate message words, each of which one is separate from the first-mentioned message words, each of the latter-mentioned message words being coded separately. Method according to claim 1, characterized in that some of said message words contain two or more of only one type of said parameters (fp or nfp) and a parity word for the coding of the message word in a manner known per se.