Method of and arrangement for restoring samples of an equidistantly sampled signal which are considered invalid.

0001The invention relates to a method of and an arrangement for restoring a sequence of samples which are considered as in­valid, of an equidistantly sampled signal, on the basis of re­placement values derived from sequences of valid samples.

0002Such a method and arrangement are described in Netherlands Patent Application PHN.11.716 of equal rank.

0003The restoration technique described therein requires the availability, immediately after a sequence of samples to be restored, of a relatively long subsequent sequence of valid samples. More specifically, the length of such a subsequent sequence must correspond to the maximum anticipated instantaneous periodicity of the signal segment of which the sequence of sample to be restored from part. For, for example, speech, sampled at a rate of 8000 Hz this implies that such a subsequent sequence must comprise approximately 160 samples. In other words in that case a delay of 20 msec after the appearance of the sequence of samples to be restored is introduced before the result of the restoration procedure can be made available.

0004In addition, the implementation of the above-described restoration technique implies that the periodicity of a signal segment containing the sequence of samples to be restored must be estimated. Then the replacement values used for restoring invalid samples are determined by reversing a system of equations in which the replacement values are used as unknown factors.

0005The invention has for its object to provide an alter­native restoration technique, the delay such as in introduced by the necessity of a subsequent sequence of valid samples, as defined above, is reduced.

0006To that end, a method according to the invention is characterized in that an environment of the relevant sequence of samples to be restored is compared with an environment of equally large sequence of valid samples which occurred a predetermined period of time earlier; that such a comparison is made for a plurality of dif­ferent predetermined periods of time; that the environment which most closely approximates the sequence of samples to be restored is selected; and that the valid samples bounded by the environment thus selected are used for forming the replacement values.

0007More specifically, a method of restoring a sequence of samples s_t+i, wherein i = 0, ..., m which are considered invalid, is characterized in accordance with the invention by the following steps: <ul id="ul0001" list-style="none"><li>a) values of samples immediately preceding the sample s_t, in combination with values of samples in a immediately subse­quent environment of the samples s_t+m-1 containing a number of p samples are correlated with values of a number of samples prefer­ably equal to p which are present in an environment immediately preceding sample s_q, wherein q represents a variable sample position in the range p ≦ q ≦ t-p in combination with values of samples present in an environment immediately subsequent to the sample s_q+m-1;</li><li>b) the correlation result of step a) is determined and recorded;</li><li>c) the steps a) and b) are repeated in that sequence for a plurality of differently chosen q-values;</li><li>d) from the set of correlation results determined after step c) the result having the lower value is determined;</li><li>e) the value q corresponding to the correlation result obtained after step b) is determined, more specifically q_opt; and</li><li>f) a sequence of value samples in the positions q_opt, ..., q_opt+m-1 is selected, the replacement values for the samples s_t, ..., s_t+m-1 being formed from the values of the samples thus selected.</li></ul>

0008A preferred embodiment of a method according to the invention, in which the correlation procedure is normalised, is characterized in that each environment determined by a value q opted for is scaled such by a factor a_q that the means square error relative to the environment of the samples to be restored is minimal and each of the values of the samples in the indicated positions q_opt, ..., q_opt+m-1 is multiplied by the factor a_qopt.

0009It is illustrative of the invention that in the case of speech which is sampled at a rate of 8000 Hz, the time interval corresponding to said subsequent sequence can be reduced to ap­proximately 25 samples, <u style="single">i.e.</u> to approximately 3 msec.

0010An arrangement for performing the method according to the invention is characterized by comparator means for comparing values of samples which constitute an environment for the relevant sequence of samples to be restored, the value constituting a test environment of the relevant sequence of samples to be restored, which values or samples constitute a test environment of an equally large sequence of valid samples which appeared a predetermined period of time earlier; indicating means for indicating several groups of samples shifted in time relative to each other, each group forming one of said test environments; control means controlling the mode of operation of said comparator means in dependence on the indication result of the indicator means; selection means which on the basis of the results pro­vided by the comparator means select that test environment which approximates the environment of the sequence of samples to be restored most closely; and arithmetic means deriving the replacement values from the values of the samples which are bounded by the selected test environment.

0011More specifically, an arrangement for performing the method according to the invention, is characterized by an arith­metic unit arranged for always calculating for a plurality of chosen values q, wherein q = p, ..., t-p the quantities a_q and ε(q, aq) in accordance with the formulae<maths id="math0001" num=""><img file="EP0241980A1_D0001.tif" /></maths> by a search unit arranged for selecting from the pairs of quantities a_q and ε(q, a_q) as generated by the arithmetic unit, that pair for which ε(q, a_q) is minimal and to determine the associated values for q and a_q > o as q_opt and a_opt; and a multiplying circuit arrangement for multiplying each of the sample values in the position q_opt+i wherein i = 0, ..., m-1 by a_opt and to substitute the product values as replacement values for the values of the invalid samples.

0012An arrangement illustrative of the invention is further characterized in that it is of such a structure that the flow charts of Figs. 4 and 5 can be performed.

0013The invention and how it can be put into effect will now be further described by way of example with reference to the accompanying drawing, wherein: <ul id="ul0002" list-style="none"><li>Fig. 1 shows a time diagram with reference to which the principle of the present invention will be described;</li><li>Fig. 2 is a general block circuit diagram to illustrate the basic function performed within the scope of the present in­vention;</li><li>Fig. 3 is a block circuit diagram of an embodiment of an arrangement according to the invention;</li><li>Fig. 4 is a flow chart to illustrate an input/output program performed within the scope of the invention; and Fig. 5 is a flowchart for further illustration of the restoration technique according to the invention.</li></ul>

0014The invention can advantageously be used for a sampled speech signal of which a number of samples is mutilated such by interferences that at the receiver side of an associated trans­mission system the samples are considered as invalid. Such a situation can, for example occur in a telephony system of the type known as MAT (mobile automatic telephony). Due to what is commonly referred to as selective fading, the signal received and conse­quently the speech signal received fails regularly and for a short period of time. It has been found that a speech signal modulated on a carrier of, for example, approximately 300 MHz is received undisturbed and disturbed at the receiving side with given inter­vals which depend on the speed at which a relevant vehicle function­ing as a mobile station of such a telephony system travels.

0015The restoration technique performed by the invention makes it possible to restore reliably invalid samples of a speech signal sampled at a frequency of 8000 Hz, if the interval con­taining unknown samples is less than approximately 12 msec, and if the relevant speech signal is available undisturbed on both sides of such an interval, for approximately 3 msec.

0016Hereinafter, the principle of the present invention will be further illustrated with reference to Fig. 1. The diagram shown in Fig. 1 is illustrative of a sequence of consecutive samples s_k, wherein k = 0, ..., N-1. Of the samples the samples s_t, ..., s_t+m-q, FIG. 1 interval B₁, form a sequence of samples which are considered invalid and must be restored. In other words, the replacement values for estimators <img file="EP0241980A1_D0002.tif" />_t+i, wherein i = 0, ..., m-1 must be found for these invalid samples. These replacement values must approach the original values of these invalid samples to the best possible extent, or in any case be such that in the ultimately restored signal substantially no observable errors occur.

0017The invention is based on the principle that such re­placement values can be derived from the values of that sequence of valid samples s_q+i whose environment approaches the environ­ment of the sequence s_t+i to the best possible extent, FIG. 1 interval B2. In the foregoing, q represents a sample position which can be chosen from a further specified range, while for i it holds that i = o, ..., m-1. As is shown in Fig. 1, the sequence s_t+i of invalid samples is bounded between the pre-environment defined by the samples s_t-p and s_t-1, and a post-environment of the sequence of the sequence of invalid samples. The sample position index q will now be chosen within the limitations of (1) p ≦ q ≦ t-p; and 2 p+m &lt; t. It is now possible to define for each sequence s_q+i wherein i = 0, ..., m-1, defined by a sample index position q also a pre-environ­ment, FIG 1 interval A2, and a post-environment FIG 1 interval C2, each comprising a number of p samples. The respective pre-environ­ment and post-environment can now be defined by s_q-p and s_q-1 and s_q+m and s_q+m+p-1, respectively.

0018The strategy in accordance with which the sequence of valid samples will ultimately be suitable to serve as a source for making available the replacement values of the samples which are considered invalid, implies that for a set of different q-values, the environment, more specifically the pre-environment and the post-environment of a sequence s_q+i, wherein i = 0, ..., m defined by the chosen q-value, is always compared with the environment, more specifically the pre-environment and the post-environment of the sequence s_t+i of unknown samples. That environment which, but for a scaling factor a_q, most closely approximates the environment of the sequence of invalid samples s_p+I then constitutes the boundary for the sequence s_q+i looked for. All this means in practice that the sampling values (2) <img file="EP0241980A1_D0003.tif" />_t+i = a_qs_q+i wherein i = 0, ..., p-1, are the desired sources for the estimation values which can be filled up in the position of the invalid samples, if the environ­ment, more particularly the pre-environment and post-environment, respectively, of these invalid samples is approached to the closest possible extent by the environment, more specifically the respect­ive pre-environment and post-environment which can be represented by a_qs_q-p+i, wherein i = 0, ..., p-1, and a_qs_q+m+i, wherein i = 0, ..., p-1.

0019The scaling factor a_q depends on the value chosen for the sample position index q for a relevant equation. By intro­ducing such a scaling factor a_q it can be accomplished that environments of equal shapes, which are compared with each other can also be made equal to each other as regards their magnitude.

0020The above-described strategy is based on the assumption that one set of samples is "similar" to another set of samples, if the environments of these two sets of samples may be considered as "similar". For the case of voiced speech, in other words in the case of a signal considered as periodical, this assumption is certainly correct. Because of this periodical character of voiced speech it is, in addition, substantially certain that in a finite interval proceeding the sequence of samples to be restored, a sequence of valid samples is present whose values, after scaling, can serve as replacements for the samples considered as invalid. For the case of unvoiced speech, which has rather the character of noise, this is not always the case. Inspite thereof, it has been found that the restoration technique proposed leads to satis­factory results, also when a signal applied is unvoiced speech.

0021From the foregoing it will be obvious that looking for the sequence of valid samples s_q+i wherein i = 0, ..., m-1, which is suitable for supplying replacement values, which in practice means finding the correct values for the quantities p and q and a_q. As is also illustrated in Fig. 1, the samples suitable for use as the replacement values are looked for in a finite interval preceding the sequence of sample s_t+i, wherein i = 0, .., m-1 to be restored. As will also be obvious from Fig. 1, the sequence s_k, wherein k = 0, ..., t+m+p-1, wherein k ≠ t, ..., t+m-1, is available for that purpose. Taking account of the limitations imposed by formula (1), it holds that t-p-m-p - t-2.p-m defines a stock of valid samples which are suitable for making replacement values available for the invalid samples. As will further be clear from formula (1), p must in any case be less than (t-m)/2. It was found experimentally that P ≧ 25 provides satisfactory results. With a customary sampling rate of 8000 Hz of a speech signal, this implies that the subsequent sequence produces a delay of 3.125 msec.

0022For each chosen value of the sample position index q wherein q = p, ..., t-p a value of a = a_q can be determined, as a result of which the expression (3) ε(q,a) =<maths id="math0002" num=""><img file="EP0241980A1_D0004.tif" /></maths> [(s_t-p+i - a s_q-p+i)² + (s_t+m+i-a s_q+m+i)²] minimizes. Thus, each environment, more specifically the pre­environment and post-environment, respectively, as described by (4) [(s_q-p+-i)_i=0,....,p-1, + (s_q+m+i)_{i=0, ...,p-1}] q=p,...,t-p can be scaled such that the means square error as described by formula (3) relative to the environment, more specifically the pre-environment and post-environment, respectively, as defined by (5) [(s_t-p+i)_i=0,...,p-1, (s_t+m+i)_i=0,...,.p-1] is minimal. The scaling factor a_q suitable for use as a selected sample position index q is that value for a for which the first derivative of the expression defined by (3) becomes equal to zero. From the equation obtained thus a_q can be dissolved in accordance with<maths id="math0003" num=""><img file="EP0241980A1_D0005.tif" /></maths> The expression ε(q, a_q) can be written as <maths id="math0004" num=""><img file="EP0241980A1_D0006.tif" /></maths> The correlation procedure proposed by this equation (7) is schematically illustrated in Fig. 1 in that the environments (test invironments) in this Figure, defined by a given sample position index q are shown below the environment bounding the sequence of samples to be restored and with which the said first environments are compared.

0023The optimum value for the sample position index q, q = q_opr and the optimum value for the scaling factor a_q, a_q = a_opt is now that value of q and a_q for which the result of the equation (correlation) represented by equation (7) is minimal.

0024A method which is illustrative of the invention com­prises the following steps: For all the available values for the sample position index q, wherein q = p, ..., t-p, the value a_q as defined in equation (6) and the correlation result in accordance with equation (7) are each time determined. From all the pairs [a_q, ε(q, a_q)] _q=p,...,t-p thus determined that pair is determined for which ε(q, a_q) is minimal. The associated values for q and a_q are the values q_opt and a_opt. Thereafter, on the basis of the q_opt and a_opt thus determined, replacement values can be determined in accordance with<maths id="math0005" num=""><img file="EP0241980A1_D0007.tif" /></maths> It has been ascertained that a_opt must be positive. This implies that the pairs [a_q, ε(q, a_q)], wherein a_q &lt; 0 must be left out of consideration when looking for the minimum for ε(q, a_q).

0025Fig. 2 shows a general circuit diagram of a configuration of components dissolved in performing basic functions of the method according to the invention.

0026An equidistantly sampled signal, for example a speech signal is applied in the shape of a sequence of samples s_k. A flag bit which indicates whether a relevant sample must be considered as valid or invalid, must be added to each of the applied samples. The samples are applied to a flat bit-recognizing means 2.1 which at an output generates a flag bit signal f_k which informs a control arrangement 2.2 of the fact whether an applied sample is be quali­fied as being valid or invalid. Via a different output of this flag bit-recognizing means the incoming signal samples s_k are applied to a randomly accessible register 2.3. This register has such as storage capacity that all the incoming samples required to complete a restoration cycle relating to a sequence of samples qualified as invalid, are available. The control arrangement 2.2 is <u style="single">inter alia</u> arranged for determining in accordance with a pre­determined routine, on the basis of flag bit signals applied and samples present in the register 2.3, whether a restoration proce­dure for restoring a number of samples which are considered invalid must be initiated and completed or not. As soon as this control arrangement has ascertained that of a sequence of samples stored in the register 2.3 a set must be restored, all the sampled positions q, wherein q = p, ..., t-p, illegible therefor are read from the register 2.3, all this in such a manner as defined with reference to Fig. 1, the sample values corresponding therewith being inputted in an arithmetic unit 2.4. Fundamentally, such an arithmetic unit performs the function whereby an environment of a relevant sequence of samples to be stored is compared to an environment of an equally large sequence of valid samples which appear in a chosen period of time earlier, such a comparison being made for a plurality of different periods of time chosen thus. More specifically, the arithmetic unit 2.4 is arranged to always calculate for a selected value q the quantities a_q and ε(q,a_q) in accordance with the formula (6) and (7), defined in the foregoing, from the sample values read from the register 2.3. The results of the operation performed by the arithmetic unit 2.4, more specifically the pairs a_q and ε(q,a_q) generated for the chosen q-values, are applied to a search unit 2.5. Fundamentally, this search unit performs the function whereby the environment which most closely approximates the environment of the sequence of samples to be restored is selected. More specifically, this search unit 2.5 is arranged for finding from the results applied from the arithmetic means 2.4 to this search unit, that combination for which the relevant quantity ε(q, a_q) wherein a_q &lt; 0 is minimal. The values associated there­with for q and a_q, namely q_opt and a_opt are supplied by this search unit 2.5 on the basis of the search result found, and applied to a multiplier 2.6. This multiplier is also arranged to read under the control of the control arrangement 2.2 the sample position q_opt, ..., q_opt+m-1 determined by the quantity q_opt supplied, and consequently to insert automatically the corres­ponding sample values. This multiplier is then operative for multiplying each of the sample values thus inserted, by the quantity a_opt supplied by the search unit 2.5, so that the ulti­mate replacement values can be filled up in the sample position t, ..., t+m-1. Thus, this multiplier performs basically the function whereby the valid samples which are bounded by the environment chosen by the search unit, are used to form the replacement values of the samples to be restored.

0027After such a restoration cycle the restored sequence of samples is supplied from the output of the register 2.3, also under the control of the control unit 2.2.

0028Generally the invention is suitable for use in those situations in which digitized signals, such as audio signals and more particularly speech signals, can be received in a disturbed state. The positions of the disturbed sample must be known, as also the position of the samples which determine environments. The invention is also suitable for use for disturbed analog speech signals, after the signals have been converted to the discrete form.

0029The invention is also suitable for use in restoring speech signals which during transmission were protected by an error-correcting and detection code. In that case those samples can be restored which can indeed be detected as an error, but can not be corrected anymore.

0030Fig. 3 shows a block circuit diagram of an embodiment of an arrangement according to the invention. The block 3.1 is a receiver which at its output can develop from a received signal a sequence of samples s_k such as they were described with refer­ence to Fig. 2. The block 3.2 corresponds to the flag bit-recognizing circuit shown in Fig. 2. The values of the incoming sample appear at the output 3.3 in the form of 16-bit words. When at the output 3.3 a sample appears of a value which can be qualified as invalid, a flag bit signal formed by a 1-bit word and indicating that the relevant sample is invalid appears at the output 3.4. The values of the samples and the flag bit signals are applied to the respect­ive input arrangement 3.6 and 3.7 of a microprocessor configuration 3.5 arranged for implementating a restoration cycle for sample to be restored. In addition to an outputting member 3.8 via which the sample values are outputted, this processor 3.5 comprises a central processing unit 3.9 of which a control member, an arith­metic member and a logic unit form part. Furthermore, the processor 3.5 includes three memories 3.10, 3.11 and 3.12. The memories 3.10 and 3.11 are random access memories, the memory 3.10 acting as a cyclic buffer and the memory 3.11 acting as a working store for storing the intermediate results. The memory 3.12 is a static memory in which the programs for restoring invalid samples are stored. The memories 3.10, 3.11 and 3.12 are coupled to the central processing unit 3.9 <u style="single">via</u> a data bus 3.13, <u style="single">via</u> which bus data can be inputted and outputted. For conveying addresses, the memories 3.10, 3.11 and 3.12 and also the inputting and outputting arrangements 3.6, 3.7 and 3.8 are coupled to the central processing unit 3.9 <u style="single">via</u> an address bus 3.14. Furthermore, the processor 3.5 includes a clock 3.15 under whose control the operation of the central pro­cessing unit can be interrupted (interrupt) in order to have the inputting and outputting of data take place. Namely, digital audio signals require a constant through-put in order to accomplish that the frequency with which the samples appear at the output is the same as the rate at which the relevant signal is sampled. In­putting and outputting of data is controlled <u style="single">via</u> a program stored in the program store 3.12. Fig. 4 illustrates a flow-chart of this program.

0031This program can be described as follows: (for captions see Table 1). <ul id="ul0003" list-style="none"><li>Block 4.1, caption: "wait for interrupt"; description: data can be inputted and outputted during an interrupt which is initiated when a clock pulse from data clock 3.5 appears.</li><li>Block 4.2: caption "output/input"; description: upon the appearance of a clock pulse the value of a new sample is in­putted <u style="single">via</u> the input 3.6 and entered into the first free address in the memory 3.10, and the oldest present in memory 3.4 is read and outputted.</li><li>Block 4.3, caption: "input error?"; description: upon entering a new sample, the flag bit signal is at the same time entered <u style="single">via</u> input 3.7. When no invalid sample is detected, the subsequent clock pulse is waited for repeating the procedure.</li><li>Block 4.4, caption: "update error table"; description: if a sample is detected a being invalid, the address of this sample is entered into the memory 3.10, in a Table of invalid samples in the working memory 3.11.</li></ul>

0032Under the regime of the above-described program, the cyclic buffer store 3.10 operates as a delay line, whose delay is determined by the period of time necessary for a restoration cycle measured from the arrival of the first sample, which is qualified as invalid, increased by the period of time of the required post-­environment. The program stored in the memory 3.12, with which the replacement values for the samples to be corrected are calculated, is started when the input/output program detects sample as being invalid.

0033This restoration program will now be described in greater detail with reference to Fig. 5, which shows a flow chart with this program. (For captions see Table II).

0034Block 51: "wait for an error pattern that can be restored", description: as long as no pattern if invalid samples is detected the restoration program is inoperative.

0035Block 52: caption "compute q_opt and a_opt"; from a row of possible values of q = p, ..., t-p a value of a = a_q is determined for which the expression ε(q,a) =<maths id="math0006" num=""><img file="EP0241980A1_D0008.tif" /></maths> [(s_t-p+i-as_q-p+i)² + (s_t+m+i-as_q+m+i)²] is minimised. The scaling factor a_q which is suitable for use for a selected sample position index q is that value of a for which the first derivative of the expression defined by (3) becomes equal to zero. From the equation of a_q obtained therefrom there are dissolved in accord­ance with<maths id="math0007" num=""><img file="EP0241980A1_D0009.tif" /></maths>

0036The expression ε(q, a_q) can be written as<maths id="math0008" num=""><img file="EP0241980A1_D0010.tif" /></maths> The correlation procedure proposed by this equation (7) is schematical illustrated in Fig. 1 in that the environment defined by a given sample position index q (test environments) are shown in this Figure below the environments bounding the sequence of samples to be restored and with which said first environments are compared.

0037The optimum value for the sample position index q, q = q_opt and the optimum value for the scaling factor a_q, a_q = a_opt is now that value of q and a_q for which the result of the equation (correlation) represented by equation (7) is minimal. It was found that a_opt must be positive. This implies, that the pairs [a_q, ε(q, a_q)] wherein a_q &lt; 0 must be ignored in the search for the minimum for ε(q, a_q).

0038Block 53: caption "compute replacement samples <img file="EP0241980A1_D0011.tif" />_t+i, then the<maths id="math0009" num=""><img file="EP0241980A1_D0012.tif" /></maths>

0039Block 54: caption "replicate buffer", here the values of the in­valid samples are replaced by the estimated values s_t+i, I = 0, ..., M-1. Thereafter the program is again cycled through. <tables id="tabl0001" num="0001"><img file="EP0241980A1_D0013.tif" /></tables>