Excitation pulse positioning method in a linear predictive speech coder

Abstract: A method for positioning excitation pulses for a linear predictive coder (LPC) operating according to the multi-pulse principle, i.e. a number of such pulses are positioned at specific time points and with specific amplitude. The time points and the amplitudes are determined from the predictive parameters (a k ) and the predictive residue signal (d k ), by correlation between a speech representa­ tive signal (y) and a composed synthesized signal (y and). This can provide all possible time positions for the excitation pulses within a given frame interval. According to the proposed method, the possible time positions are divided into a number (n f ) of phase positions and each phase- position is divided into a number of phases (f). These phases are vacant for the first excitation pulse. When this pulse has been positioned, the phase determined for this pulse is denied to the following excitation pulses until all pulses in a frame have been positioned.

Description: The present invention relates to a method of positioning excitation pulses in a linear predictive speech coder which operates according to the multi-pulse principle. Such a speech coder may be incorporated, for instance, in a mobile telephone system, for the purpose of compressing speech signals prior to transmission from a mobile. BACKGROUND ART Linear predictive speech coders which operate according to the aforesaid multi-pulse principle are known to the art, from, for instance, US-PS 3,624,302, which describes linear predictive coding of speech signals, and also from US-PS 3,740,476 which teaches how predictive parameters and predictive residue signals can be formed in such a speech coder. When forming an artifical speech signal by means of linear predictive coding, there is generated from the original signal a number of predictive parameters (a k ) which characterize the synthesized speech signal. Thus, there can be formed with the aid of these parameters a speech signal which will not include the redundancy which is normally found in natural speech and the conversion of which is unnecessary when transmitting speech between, for instance, a mobile and a base station included in a mobile radio system. From the aspect of bandwidth, it is more appropriate to transfer solely predictive parameters instead of the original speech signal, which requires a much wider bandwidth. The speech signal regenerated in a receiver and constituting a synthetic speech signal can, however, be difficult to apprehend, due to a lack of agreement between the speech pattern of the original signal and the synthetic signal recreated with the aid of the prediction parameters. These deficiencies have been described in detail in US-PS 4,472,832 (SE-A--456618) and can be alleviated to some extent by the introduction of so-called excitation pulses (multi-pulses) when forming the synthetic speech copy. In this case, the original speech input pattern is divided into frame intervals. Within each such interval there is formed a given number of pulses of varying amplitude and phase position (time position), on the one hand in dependence on the prediction parameters a k , and on the other hand in dependence on the predictive residue d k between the speech input pattern and the speech copy. Each of the pulses is permitted to influence the speech pattern copy, so that the predictive residue will be as small as possible. The excitation pulses generated have a relatively low bit-rate and can therefore be coded and transmitted in a narrow band, as can also the prediction parameters. This results in an improvement in the quality of the regenerated speech signal. DISCLOSURE OF THE INVENTION In the case of the aforesaid known methods, the excitation pulses are generated within each frame interval of the speech input pattern, by weighting the residue signal d k and by feeding-back and weighting the generated values of the excitation pulses, each in a separate predictive filter. The output signals from the two filters are then correlated. This is followed by maximization of the correlation of a number of signal elements from the correlated signal, therewith forming the parameters (amplitude and phase position) of the excitation pulses. The advantage of this multi-pulse algorithm for generating excitation pulses is that various types of sound can be generated with a small number of pulses (e.g. 8 pulses per frame interval). The pulse searching algorithm is general with respect to the positioning of pulses in the frame. It is possible to recreate non-accentuated sounds (consonants), which normally require randomly positioned pulses, and accentuated sounds (vowels), which require more collected positioning of the pulses. One drawback with the known pulse positioning method is that the coding effected subsequent to defining the pulse positions is complex with respect to both calculation and storage. Furthermore, the method requires a large number of bits for each pulse position in the frame interval. The bits in the code words obtained from the optimal combinatory pulse-coding algorithms are also prone to bit-error. A bit-error in the code word being transmitted from transmitter to receiver can have a disastrous consequence with regard to pulse positioning when decoding the code word in the receiver. The present invention is based on the fact that the number of pulse positions for the excitation pulses within a frame interval is so large as to make it possible to forego exact positioning of one or more excitation pulses within the frame and still obtain a regenerated speech signal of acceptable quality subsequent to coding and transmission. According to the known methods, the correct phase positions are calculated for the excitation pulses within one frame and following frames of the speech signal and positioning of the pulses is effected solely in dependence on complex processing of speech signal parameters (predictive residue, residue signal and the parameters of the excitation pulses in preceding frames). According to the present inventive method, certain phase position limitations are introduced when positioning the pulses, by denying a given number of previously determined phase positions to those pulses which follow the phase position of an excitation pulse that has already been calculated. Subsequent to calculating the position of a first pulse within the frame and subsequent to placing this pulse in the calculated phase position, said phase position is denied to following pulses within the frame. This rule will preferably apply to all pulse positions in the frame. Accordingly, the object of the present invention is to provide a method for determining the positions of the excitation pulses within a frame interval and following frame intervals of a speech-input pattern to a linear predictive coder which requires a less complex coder and a smaller bandwidth and which will reduce the risk of bit-error in the subsequent recoding prior to transmission. The inventive method is characterized by the features set forth in the characterizing clause of Claim 1. The proposed method can be applied with a speech coder which operates according to the multi-pulse principle with correlation of an original speech signal and the impulse response of an LPC-synthesized signal. The method can also be applied, however, with a so-called RPE-speech coder in which several excitation pulses are positioned in the frame interval simultaneously. BRIEF DESCRIPTION OF DRAWINGS The proposed method will now be described in more detail with reference to the accompanying drawings, in which Figure 1 is a simplified block schematic of a known LPC-speech-coder; Figure 2 is a time diagram which covers certain signals occurring in the speech coder according to Figure 1; Figure 3 is a diagram explaining the principle of the invention; Figure 4a,4b are more detailed diagrams illustrating the principle of the invention; Figure 5 is a block schematic illustrating a part of a speech coder which operates in accordance with the inventive principle; Figure 6 is a flow chart for the speech coder shown in Figure 5; and Figure 7 is an array of blocks included in the flow chart of Figure 6. BEST MODE OF CARRYING OUT THE INVENTION Figure 1 is a simplified block schematic of a known LPC-speech-coder which operates according to the multi-pulse principle. One such coder is described in detail in US-PS 4,472,832 (SE-A-456618). An analogue speech signal from, for instance, a microphone occurs on the input of a prediction analyzer 110. In addition to an analogue-digital converter, the prediction analyzer 110 also includes an LPC-computer and a residue-signal generator, which form prediction parameters a k and a residue-signal d k respectively. The prediction parameters characterize the synthesized signal, whereas the residue signal shows the error between the synthesized signal and the original speech signal across the input of the analyzer. An excitation processor 120 receives the two signals a k and d k and operates under one of a number of mutually sequential frame intervals determined by the frame signal FC, such as to emit a given number of excitation pulses during each of said intervals. Each of said pulses is determined by its amplitude A mp and its time position, m p within the frame. The excitation-pulse parameters A mp , m p are led to a coder 131 and are thereafter multiplexed with the prediction parameters a k , prior to transmission from a radio transmitter for instance. The excitation processor 120 includes two predictive filters having the same impulse response for weighting the signals d k and A i , m i in dependence on the prediction parameters a k during a given computing or calculating stage p. Also included is a correlation signal generator which is operative to effect correlation between the weighted original signal (y) and the weighted synthesized signal (y) each time an excitation pulse is to be generated. For each correlation there is obtained a number q of "candidates" of pulse elements A i , m i (0≦i<I), of which one gives the smallest quadratic error or smallest absolute value. The amplitude A mp and time position m p for the selected "candidate" are calculated in the excitation signal generator. The contribution from the selected pulse A mp , m p is then subtracted from the desired signal in the correlation signal generator, so as to obtain a new sequence of "candidates", and the method is repeated for a number of times which equals the desired number of excitation pulses within a frame. This is described in detail in the aforesaid US-patent specification. Figure 2 is a time diagram over speech input signals, predictive residues d k and excitation pulses. The number of excitation pulses in this case is also eight (8), of which the pulse A ml , m l was selected first (gave the smallest error), and thereafter pulse A m2 , m₂, etc. within the frame. In the earlier known method for calculating amplitude A i and phase position m i for each excitation pulse, m i =m p is calculated for that pulse which gave maximum value of αi/ij, and associated amplitude A mp was calculated, where αm is the cross-correlation vector between the signals y n and y and n according to the above and mm is the auto-correlation matrix for the impulse response of the prediction filters. Any position m p whatsoever is accepted when solely the above conditions are fulfilled. The index p signifies the stage under which calculation of an excitation pulse according to the above takes place. In accordance with the invention, a frame according to Figure 2 is divided in the manner illustrated in Figure 3. It is assumed, by way of example, that the frame contains N=12 positions. In this case, the N-positions form a search vector (n). The whole of the frame is divided into so-called sub-blocks. Each sub-block will then contain a given number of phases. For instance, if the whole frame contains N=12 positions, in accordance with Figure 3, four sub-blocks are obtained and each sub-block will contain three different phase. The sub-block has a given position within the full frame, this position being referred to as the phase position. Each position n(0≦n<N) will then belong to a given sub-block n f (0≦n f <N f ) and a given phase f (0≦f<F)) in said sub-block. In general the positions n (0≦n<N) in the total search vector, which contains N positions, will be n=n f ·F + f n f =0, ..., (N f -1), f = 0,...(F-1) and n = 0, ..., (N - 1). Furthermore, the following relationship will also apply f = n MOD F and n f = n DIV F The diagram of Figure 3 illustrates the distribution of the phases f and sub-blocks n f for a given search vector containing N positions. In this case, N = 12, F = 3 and N F = 4. The inventive method implies limiting the pulse search to positions which do not belong to an occupied phase f p for those excitation pulses whose positions n have been calculated in preceding stages. In the following, the order or sequence number of a given calculating cycle of an excitation pulse is designated p, in accordance with the aforegoing. The proposed method will then result in the following calculation stages for a frame interval: 1. Calculate the desired signal Y n 2. Calculate the cross-correlation vector α i 3. Calculate the auto-correlation matrix  ij 4. When p=1. Search for m p , i.e. the pulse position which gives maximum α i / ij =α m /mm in the unoccupied phases f. 5. Calculate the amplitude A mp for the discovered pulse position m p . 6. Update the cross-correlation vector α i . 7. Calculate f p and n fp in accordance with the relationship (1) above, and 8. Carry out steps 4-7 above when p=p+1. Figures 4a and 4b are diagrams which illustrate the proposed method. Figure 4a illustrates an example in which the number of positions in a frame are N=24, the number of phases are F=4 and the number of phase positions are N F =6. It is assumed that no phases are occupied at the start p=1, and it is also assumed that the above calculating stages 1-4 gave the position m₁=5. This pulse position is marked with a circle in Figure 4 a. This gives the phase 1 in respective phase positions n f = 0,1,2,3,4 and 5, and corresponding pulse positions are n = 1, 5, 9, 13, 17 and 21 in accordance with the relationship (1) above. The phase 1 and corresponding pulse positions are thus occupied when calculating the position of the next excitation pulse (p=2). It is assumed that the calculating stage 4 for p=2 results in m₂=7. Possibly m₂=9 can have given the maximum value of α i / ij , although this gives an occupied phase. The pulse position m₂=7 gives phase 3 in each of the phase positions n f =0,...5, and means that the pulse positions n=3,7,11,15 and 22 will be occupied. The positions 1,3,5,7,9,11,13,15,17,19,21 and 23 are thus occupied before commencement of the next calculating stage (p=3). It is assumed that the calculating stages 1-4 above for p=3 will give m₃=12, and that for p=4 the calculating stages result in the last position m₄=22. All positions in the frame are herewith occupied. Figure 4a illustrates the excitation pulses (A m1 , m₁), (A m2 , m₂) etc., obtained. Figure 4b illustrates a further example, in which N=25, F=5 and N F =5, i.e. the number of phases within each phase position has been increased by one. Pulse positioning is effected in the same manner as that according to Figure 4a and finally five excitation pulses are obtained. The maximum number of excitation pulses obtained is thus equal to the number of phases within one phase position. The obtained phases f₁, ..., f p (p=4 in Figure 4a and p=5 in Figure 4b) are coded together and the resultant phase positions n f1 ,..., n fp are each coded per se prior to transmission. Combinatory coding can be employed for coding the phases. Each of the phase positions is coded with a code word per se . In accordance with one embodiment, the known speech-processor circuit can be modified in the manner illustrated in Figure 5, which illustrates that part of the speech processor which includes the excitation-signal generating circuits 120. Each of the predictive residue-signals d k and the excitation generator 127 are applied to a respective filter 121 and 123 in time with a frame signal FC, via the gates 122, 124. The filters 121, 123 produce the signals y n and y and n which are correlated in the correlation generator 125. The signal y n represents the true speech signal, whereas y and n represents the synthesized speech signal. There is obtained from the correlation generator 125 a signal C iq which includes the components α i and  ij in accordance with the aforegoing. A calculation is made in the excitation generator 127 of the pulse position m p which gives maximum α i / ij , wherein the amplitude A mp according to the aforegoing is obtained in addition to the pulse position m p . The excitation pulse parameters m p , A mp produced by the excitation generator 127 are sent to a phase generator 129. This generator calculates the current phases f p and the phase positions n fp from the values m p , A mp arriving from the excitation generator 127, in accordance with the relationship f = (m - 1) MOD F + 1 n f = (m - 1) DIV F + 1 where F = the number of possible phases. The phase generator 129 may consist in a processor which includes a read memory operative to store instructions for calculating the phases and the phase positions in accordance with the above relationship. Phase and phase position are then supplied to the coder 131. This coder is of the same principle construction as the known coder, but is operative to code phase and phase position instead of the pulse positions m p . On the receiver side, the phases and phase positions are decoded and the decoder thereafter calculates the pulse position m p in accordance with the relationship m p =(n fp - 1) ·F + f p which gives a clear determination of the excitation-pulse position. The phase f p is also supplied to the correlation generator 125 and to the excitation generator 127. The correlation generator stores this phase and takes into account that this phase f p is occupied. No values of the signal C iq are calculated where q is included in those positions which belong to all preceding f p calculated for an analyzed sequence. The occupied positions are q = n·F + f p where n = 0, ..., (N f - 1) and f p signifies all preceding phases occupied within a frame. Similarly, the excitation generator 127 takes into account the occupied phases when making a comparison between the signals C iq and C iq *. When all pulse positions in respect of one frame have been calculated and processed and when the next frame is to be commenced, all phases will, of course, again be vacant for the first pulse in the new frame. Figure 6 illustrates a flow chart which constitutes the flow chart illustrated in Figure 3 of the aforesaid US-patent specification which has been modified to include the phase limitation. Those blocks which are not accompanied with explanatory text are described in more detail with reference to Figure 7. Introduced between the blocks 328 and 329, which concern the calculation of the output signal m p , A mp of the phase generator 129 and recitation of position index p, is a block 328a which concerns the calculations to be carried out in the phase generator, and thereafter a block 328b which concerns the application of an output signal on the coder 131 and the generators 125 and 127. f p and n fp are calculated in accordance with the above relationship (1). There is then carried out in the generators 125 and 127 a vector allocation u fi = 1 which is used when testing the obtained q-value = q* which gave the maximum value α m / mm with the intention of ascertaining whether a corresponding pulse position gives a phase which is occupied or vacant. This test is carried in blocks 308a, 308b, 308c (between the blocks 307 and 309) and in the blocks 318a, 318b (between the blocks 317, 319). The instructions given by the blocks 308a, b and c are carried out in the correlation generator 125, whereas the instructions given by the blocks 318a, b are carried out in the excitation generator 127. Firstly the signal f, i.e. the phase, is calculated from the index q in accordance with the aforegoing, whereafter a test is carried out to ascertain whether the vector position for the phase f in the vector u f is equal to 1. If u f = 1, which implies that the phase is occupied for precisely this index q*, no correlation-calculations are carried out in accordance with the instruction from block 309 and similarly the comparisons in block 319. On the other hand, when u f = 0 this indicates a vacant phase and the subsequent calculations are carried out as earlier. The occupied phases shall remain during all calculated sequencies relating to a full frame interval, but shall be vacant at the beginning of a new frame interval. Consequently, subsequent to block 307 the vector u i is set to zero prior to each new frame analysis. When coding the positions m p for the various excitation pulses within a frame, both the phase position n fp and the phase f p shall be coded. Coding of the positions is thus divided up into two separate code words having mutually different significance. In this case, the bits in the code words obtain mutually different significance, and consequently the sensitivity to bit-error will also be different. This dissimilarity is advantageous with regard to error correction or error detection channel-coding. The aforedescribed limitation in the positioning of the excitation pulses means that coding of the pulse positions takes place at a lower bit-rate than when coding the positions in multi-pulse without said limitation. This also means that the search algorithm will be less complex than without this limitation. Admittedly, the inventive method involves certain limitations when positioning the pulses. A precise pulse position is not always possible, however, for instance according to Figure 4b. This limitation, however, shall be weighed against the aforesaid advantages. The inventive method has been described in the aforegoing with reference to a speech coder in which positioning of the excitation pulses is carried out one pulse at a time until a frame interval has been filled. Another type of speech coder described in EP-A-195 487 operates with positioning of a pulse pattern in which the time distance t a between the pulses is constant instead of a single pulse. The inventive method can also be applied with a speech coder of this kind. The forbidden positions in a frame (compare for instance Figures 4a, 4b above) therewith coincide with the positions of the pulses in a pulse pattern.

Claims: A method for positioning excitation pulses for a linear predictive coder (LPC) which operates according to the multipulse principle, wherein a synthesized signal is formed from the given speech signal, by a) forming a number of predictive parameters (a k ) within a given frame interval which constitutes a time section from the given speech signal; b) forming a residue signal (d k ) which gives the error between the given speech signal and the synthesized signal within the frame interval, and for the purpose of determining an array (p) of excitation pulses within the frame interval; c) weighting said residue signal (d k ) through a filter (121) so as to form a speech-representative signal (y) weighted in dependence on the predictive parameters (a k ), and d) weighting a signal which represents the amplitude (A i ) and time position (m i ) of the excitation pulses in the frame through a filter (123) so as to form a synthesized speech signal (y and) weighted in dependence on the predictive parameters (a k ), and by e) correlating the representative speech signal (y) with the synthesized speech signal (y and) so as to obtain an expression (C iq ) for the error between said signals, and then f) determining an extreme value of said expression (C iq ) so as to obtain a given amplitude (A mp ) and a given time position (m fp ) of one of said excitation pulses during a given number of stages (p), said weighted synthesized speech signal according to step d) being formed by subtracting the contribution from preceding stage (p-1), characterized by dividing the frame into a number n f of sub-blocks, thereby dividing the number of possible time positions n (0≦n<N) for the excitation pulses within a frame into a number n f of phase positions (0≦n f <N F ) of which each phase position includes a number of phases f (0≦f<F), so that n = n f · F + f where F = the total number of phases in a phase position; and in that at the beginning of said positioning process and when determining the amplitude (A m1 ) and position (m₁) of the first excitation pulse within the frame, all positions n within the frame are vacant for positioning said first excitation pulse in accordance with said steps d)-f), whereas with respect to subsequent positioning of said excitation pulses, the phase f determined for the first excitation pulse position according to (1) is denied in all phase positions n f , to the excitation pulse (A m2 , m₂) subsequently calculated, and in that the search for determining the amplitude and position of subsequent excitation pulses in accordance with said steps d)-f) is limited to positions which do not belong to an occupied phase f p for those excitation pulses whose positions n have been calculated in preceding stages and in that the thus obtained phase positions n f are each coded separately to form separate code words, whereas the obtained phases f are coded together to form a single code word prior to transmission via a transmission medium. A method according to Claim 1, characterized by calculating the amplitude (A mp ) and the position (m p ) of a given excitation pulse and subsequent hereto calculating the associated phases f p and phase position n fp in accordance with the relation-ships f p = (m p - 1) Mod F + 1 n fp = (m p - 1) Div F + 1, wherein only the value of the phase f p determines which position (m p+1 ) of the pulse following said excitation pulse shall be forbidden, and wherein this procedure is repeated for all the phases f p+1 , f p+2 ... of subsequently calculated excitation pulses, until the desired number of excitation pulses has been obtained within the frame. A method according to Claims 1-2, characterized in that when calculating the phase of the pulse position (q) calculated in the correlation step e) from a total number (Q) of possible positions there is assigned a test vector (u f ) which represents the state, occupied or vacant, of the different phases within the frame; and in that a calculated phase f i is investigated with the aid of the test vector to as certain whether this phase is occupied or vacant, wherein if the phase f is occupied the correlation step is counting and continues upwards to the next possible position (q+1), whereas if the phase is vacant, step e) is carried out and repeated for all such positions, and that when determining an extreme value in accordance with step f) a new calculation of the phase f i for a given pulse position (q) is carried out whereafter an investigation with the aid of said test vector (u f ) is effected, wherein if the phase is vacant, the step f) is omitted and counting upwards to the next pulse position (q+1) is effected, and if the phase is occupied, said step f) is carried out in order to calculate a new value (q) of the pulse position which gives maximum value of the correlation α m / mm ) until the thus calculated new position (q+1) obtains a phase which constitutes a vacant phase in the phase vector (u f ). The method according to Claim 1, characterized in that the excitation pulse position during said steps d)-f) is included in a regular pattern of excitation pulses each of which has the same amplitude (A mp ) and a mutually similar time distance (t a ) within the frame, whereby the forbidden positions in a frame therewith coincide with the positions of the pulses in the pulse pattern.

Inventors: MINDE TOR BJOERN, MINDE, TOR BJOERN, Minde, Tor Björn
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET L M ERICSSON
Application year: 1990
Main classification: G10L9
IPC codes: G10L19