Abstract:
A computerized method for distribution among a multiple dictionaries of a target vocabulary. The vocabulary includes words for use in a speech recognition application installed in a computer system. Each word of the target vocabulary is found in only one of the dictionaries. The words are first categorized based on phonetic length, and distributed into multiple groups each of equal phonetic length. The first groups are secondly categorized based on combinations of vowel sounds. The words of the first groups are placed into second groups accordingly based on having identical vowel sounds. The second groups are thirdly categorized into third groups based on the consonants of the words of the second groups and placement of the consonants relative to the vowel sounds. The words within each of the third groups are compared in pairs for phonetic distance and the words of minimal pairwise phonetic distance between them are placed in fourth groups. The words of each of the fourth groups are distributed into the multiple dictionaries, preferably with no more than one member per fourth group distributed into each of the dictionaries. The multiple dictionaries are preferably mutually orthogonal, that is each of the dictionaries includes words of maximal phonetic distance from each other.

Description:
APPENDIX 
       [0001]    A listing of an orthogonal classification of a vocabulary into multiple dictionaries generated according to an embodiment of the present invention is attached to the present application as an Appendix. 
       FIELD AND BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to speech recognition and, more particularly, to the conversion of an audio speech signal to readable text data. Specifically, the present invention includes a method which improves speech recognition performance by distributing a large vocabulary of words into multiple dictionaries prior to parallel speech recognition processing using the multiple dictionaries. 
         [0003]    In speech recognition systems, a speech recognition engine typically incorporated into a digital signal processor (DSP), inputs a digitized speech signal, and processes the speech signal by comparing its output to a vocabulary found in a dictionary. Upon selecting the word which most closely matches a portion of the input speech signal, the speech recognition engine typically calculates a Confidence Level (CL) for the selected word match. Reference is now made to  FIG. 1  which illustrates representative behavior of Confidence Level for matching a single word, as a function of the number of words in the dictionary used in the processing. The confidence level (CL) is over 84% for less than ten words in the dictionary. When one hundred words are available in the dictionary, the confidence level (CL) is 67% and for 200 or more words in the dictionary, the CL is reduced to 63%. Furthermore, as the number of words in the dictionary increases, the CL calculated becomes unreliable to the extent that the true word recognition may have a lower CL than a false word recognition. For a large number, e.g. 200 or more words in the dictionary, the confidence level may vary dependent on the other words in the dictionary. As the dictionary increases in size, so does the sensitivity to different speakers, to speaker accent and/or to spoken variations by the same speaker. 
         [0004]    There is thus a need for, and it would be highly advantageous to have a method of improving speech recognition performance by distributing a large vocabulary of words into multiple dictionaries prior to parallel speech recognition processing using the multiple dictionaries and in such a way achieve higher confidence levels for a large vocabulary than is found for a single processing using the entire vocabulary. 
         [0005]    In human language, the term “phoneme” as used herein is the smallest unit of speech that distinguishes meaning or the basic unit of sound in a given language that distinguishes one word from another. An example of a phoneme would be the ‘t’ found in words like “tip”, “stand”, “writer”, and “cat”. 
         [0006]    A “phonemic transcription” of a word is a representation of the word comprising a series of phonemes. For example, the initial sound in “cat” and “kick” may be represented by the phonemic symbol ‘k’ while the one in “circus” may be represented by the symbol ‘s’. Further, ‘ ’ will be used to distinguish a symbol as a phonemic symbol, unless otherwise indicated. In contrast to a phonemic transcription of a word, the term “orthographic transcription” of the word refers to the typical spelling of the word. 
         [0007]    The term “phonetic distance” as used herein referring to two words Word1 and Word2 is a relative measure of how difficultly the two words are confused by a speech recognition engine. For a large “phonetic distance” there is a small probability of recognizing Word1 when Word2 is input to the speech recognition engine and similarly there is small probability of the recognizing Word2 when Word1 is input. For a small phonetic distance” there is a relatively large probability of recognizing Word1 when Word2 is input to the speech recognition engine and similarly there is relatively large probability of the recognizing Word2 when Word1 is input. The term “Levinstein distance” as used herein is the number of substitutions, insertions or deletions needed to transform one phonemic transcription, e.g. of Word1 into another, e.g. Word2. The “Levinstein distance” is a special case of “phonetic distance”. As will be described, a number of different algorithms may be used individually or in combination, according to different embodiments of the present invention for calculating phonetic distance. 
         [0008]    The term “phonetic length” as used herein referring to a single word is a measure of the number of syllables or vowel sounds in the word. 
         [0009]    U.S. Pat. No. 6,073,099 discloses a method including phonemically transcribing the first and second words into first and second transcriptions; (2) calculating a Levinstein distance between the first and second transcriptions as the number of edit operations required to transform the first transcription into the second transcription; (3) obtaining a phonemic transformation weight for each edit operation of the Levinstein distance; and (4) summing the weights to generate a value indicating the likelihood of confusion between the first and second words. U.S. Pat. No. 6,073,099 is included herein by reference for all purposes as if entirely set forth herein. 
         [0010]    The term “formant” as used herein is a peak in an acoustic frequency spectrum which results from the resonant frequencies of human speech. Vowels are distinguished quantitatively by the formants of the vowel sounds. Most formants are produced by tube and chamber resonance, but a few whistle tones derive from periodic collapse of Venturi effect low-pressure zones. The formant with the lowest frequency is called f1, the second f2, and the third f3. Most often the two first formants, f1 and f2, are enough to disambiguate the vowel. These two formants are primarily determined by the position of the tongue. f1 has a higher frequency when the tongue is lowered, and f2 has a higher frequency when the tongue is forward. Generally, formants move about in a range of approximately 1000 Hz for a male adult, with 1000 Hz per formant. Vowels will almost always have four or more distinguishable formants; sometimes there are more than six. Nasals usually have an additional formant around 2500 Hz. 
         [0011]    Plosives (and, to some degree, fricatives) modify the placement of formants in the surrounding vowels. Bilabial sounds (such as ‘b’ and ‘p’ as in “ball” or “sap”) cause a lowering of the formants; velar sounds (‘k’ and ‘g’ in English) almost always show f2 and f3 coming together in a ‘velar pinch’ before the velar and separating from the same ‘pinch’ as the velar is released; alveolar sounds (English ‘t’ and ‘d’) cause less systematic changes in neighboring vowel formants, depending partially on exactly which vowel is present. The time-course of these changes in vowel formant frequencies are referred to as ‘formant transitions’. {from http://en.wikipedia.org/wiki/Formant} 
         [0012]    The Speech Assessment Methods Phonetic Alphabet (SAMPA) is a computer-readable phonetic script using 7-bit printable ASCII characters, based on the International Phonetic Alphabet (IPA). Sampa was originally developed in the late 1980s for six European languages by the EEC ESPRIT information technology research and development program. As many symbols as possible have been taken over from the IPA; where this is not possible, other signs that are available are used, e.g. [@] for schwa (IPA          ), [2] for the vowel sound found in French deux (IPA [ø]), and [9] for the vowel sound found in French neuf (IPA [œ]). {from http://en.wikipedia.org/wiki/SAMPA} 
         [0013]    The following table is a list of SAMPA phonemic symbols for the Hebrew language. In the table, the first column includes the phonemic symbols, the second column includes transliterated keywords. The ″ symbol is used to denote the accented symbol. The ‘S’ symbol is similar to in English “sh” as in “Washington”. The ‘X’ sound is the voiceless velar fricative, “ch” as in the German composer Bach. The symbol “?” is the glottal stop. A glottal stop is a speech sound articulated by a momentary, complete closing of the glottis in the back of the throat. The symbol ?\ is the voiced pharyngeal approximant/fricative, a type of consonantal sound, approximant, or occasionally fricative, which means the sound is produced by constricting air flow through a channel at the place of articulation that is not usually narrow enough to cause turbulence. Its place of articulation is pharyngeal which means it is articulated with the root of the tongue against the pharynx. The voiced pharyngeal approximant/fricative is voiced, which means the vocal cords are vibrating during the articulation. It is an oral consonant, which means air is allowed to escape through the mouth rather than from the glottis or the mouth.
       {from http://en.wikipedia.org/wiki/Voiced_pharyngeal_fricative}       
 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
             
               
               
               
               
               
             
               
             
               
               
               
               
               
             
               
             
               
               
               
               
               
             
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 from Wells, J. C., 1997. ‘SAMPA computer readable phonetic 
               
               
                 alphabet’. In Gibbon, D., Moore, R. and Winski, R. (eds.), 1997. 
               
               
                 Handbook of Standards and Resources for Spoken Language 
               
               
                 Systems. Berlin and New York: Mouton de Gruyter. Part IV, 
               
               
                 section B. (http://www.phon.ucl.ac.uk/home/sampa/hebrew.htm) 
               
             
          
           
               
                   
                 Symbol 
                 Keyword 
                 English gloss 
                 Orthography 
               
               
                   
                   
               
             
          
           
               
                 Consonants 
               
             
          
           
               
                   
                 Plosives 
                   
                   
                   
               
               
                   
                 p 
                 pil 
                 elephant 
                 
                           
                 
               
               
                   
                 b 
                 ″bajit 
                 house 
                 
                           
                 
               
               
                   
                 t 
                 tik 
                 bag 
                 
                           
                 
               
               
                   
                 d 
                 ″delet 
                 door 
                 
                           
                 
               
               
                   
                 k 
                 ″kelev 
                 dog 
                 
                           
                 
               
               
                   
                 g 
                 ga″mal 
                 camel 
                 
                           
                 
               
               
                   
                 ? 
                 Sa″?al 
                 asked 
                 
                           
                 
               
               
                   
                 Fricatives 
               
               
                   
                 f 
                 fa″lafel 
                 felafel 
                 
                           
                 
               
               
                   
                 v 
                 ″veRed 
                 rose 
                 
                           
                 
               
               
                   
                 s 
                 sof 
                 end 
                 
                           
                 
               
               
                   
                 z 
                 za″maR 
                 singer 
                 
                           
                 
               
               
                   
                 S 
                 SiR 
                 song 
                 
                           
                 
               
               
                   
                 X 
                 a″RoX 
                 long 
                 
                           
                 
               
               
                   
                 h 
                 haR 
                 mountain 
                 
                           
                 
               
               
                   
                 Affricate 
               
               
                   
                 ts 
                 tsa″laXat 
                 plate 
                 
                           
                 
               
               
                   
                 Nasals 
               
               
                   
                 m 
                 ma″Rak 
                 soup 
                 
                           
                 
               
               
                   
                 n 
                 na″fal 
                 fell 
                 
                           
                 
               
               
                   
                 Liquids 
               
               
                   
                 l 
                 la″van 
                 white 
                 
                           
                 
               
               
                   
                 R 
                 RoS 
                 head 
                 
                           
                 
               
               
                   
                 Semivowel 
               
               
                   
                 j 
                 jad 
                 hand 
                 
                           
                 
               
             
          
           
               
                 Vowels 
               
             
          
           
               
                   
                 i 
                 tik 
                 bag 
                 
                           
                 
               
               
                   
                 e 
                 ″even 
                 stone 
                 
                           
                 
               
               
                   
                 a 
                 a″maR 
                 said 
                 
                           
                 
               
               
                   
                 o 
                 Sa″lom 
                 peace 
                 
                           
                 
               
               
                   
                 u 
                 guR 
                 puppy 
                 
                           
                 
               
             
          
           
               
                 Rare, dialectal or marginal phonemes 
               
             
          
           
               
                   
                 Z 
                 ma″saZ 
                 massage 
                 
                           
                 
               
               
                   
                 X\ 
                 X\a″tul (Xa″tul) 
                 cat 
                 
                           
                 
               
               
                   
                 tS 
                 tSips 
                 chips 
                 
                           
                 
               
               
                   
                 dZ 
                 dZins 
                 jeans 
                 
                           
                 
               
               
                   
                 ?\ 
                 pa″?\al (pa″?al) 
                 acted 
                 
                           
                 
               
             
          
           
               
                 Stress mark 
               
             
          
           
               
                   
                 ” 
                 ″beReX 
                 knee 
                 
                           
                 
               
               
                   
                   
                 be″ReX 
                 he blessed 
                 
                           
                 
               
               
                   
                   
               
             
          
         
       
     
       SUMMARY OF THE INVENTION 
       [0015]    The term “orthogonal” is used herein in the context of the present invention, referring to a distribution of a vocabulary between multiple dictionaries or sets of words. The multiple dictionaries are substantially “orthogonal” when each dictionary includes words of maximal phonetic distance from each other. Similarly, when dictionaries are “orthogonal”, the vocabulary words of smallest phonetic distance between them appear in different dictionaries. The term “dictionary” hereinafter refers to one or more of the multiple dictionaries or sets of words after the vocabulary has been distributed between the sets, according to embodiments of the present invention. 
         [0016]    The term “channel” refers to speech recognition using one of the dictionaries into which the vocabulary has been distributed orthogonally. Whereas in the prior art, a single vocabulary of, for instance eight hundred words is used with a single speech recognition engine, an embodiment of the present invention includes the division of the vocabulary into eight orthogonal dictionaries each of one hundred words. Speech recognition using the present invention is “channelized” using eight channels with eight parallel speech recognition engines processing the same input audio signal each using a different dictionary. 
         [0017]    According to the present invention there is provided a computerized method for distribution among a multiple dictionaries of a target vocabulary. The vocabulary includes words for use in a speech recognition application installed in a computer system. Each word of the target vocabulary is found in only one of the dictionaries. The vocabulary and the dictionaries are stored in memory operatively attached to a computer system. The words are first categorized based on phonetic length, and distributed into multiple groups each of equal phonetic length. The first groups are secondly categorized based on combinations of vowel sounds. The words of the first groups are placed into second groups accordingly based on having identical vowel sounds. The second groups are thirdly categorized into third groups based on the consonants of the words of the second groups and placement of the consonants relative to the vowel sounds. The words within each of the third groups are compared in pairs for phonetic distance and the words of minimal pairwise phonetic distance between them are placed in fourth groups. At this point, there are many fourth groups with just a few words (preferably less than 8) within each of the fourth groups. The words of each of the fourth groups are distributed into the multiple dictionaries, preferably with no more than one member per fourth group distributed into each of the dictionaries. The multiple dictionaries are preferably mutually orthogonal, that is each of the dictionaries includes words of maximal phonetic distance from each other. The pairwise comparison is performed by one or more of the following steps: (i) comparing pairwise formants of the vowel sounds of the words, (ii) comparing an anatomical part most responsible for forming respective sounds; (iii) comparing empirically substitution of the words using a speech recognition engine, and (iv) calculating Levinstein distance between the words. The distribution into dictionaries is performed under the constraint of balancing the respective number of words in the dictionaries. While performing the distribution of the words into the dictionaries, weights are calculated for the words not yet distributed. The weights are a measure of phonetic distance for the words not yet distributed to the words already distributed into the dictionaries and distribution is preferably continued based on the weights. 
         [0018]    According to the present invention there is provided, a computerized method for distribution among multiple dictionaries of a target vocabulary including multiple words for use in a speech recognition application installed in a computer system. Each word of the target vocabulary is found in only one of the dictionaries. The vocabulary and the dictionaries are stored in memory attached to a computer system. Phonetic distance between the words are compared in pairs, and are placed into groups of minimal phonetic distance between them. The pairwise comparison is performed using one or more of (i) comparison of formants of the vowel sounds of the words, (ii) comparison of the anatomical part most responsible for forming respective sounds; and (iii) comparison based on empirical results of the likelihood of incorrectly substituting the words using a speech recognition engines. The words of the groups are distributed among into the multiple dictionaries and an audio signal is processed using multiple speech recognition engines, each engine referring to one of the dictionaries. Preferably, only one member of each group is distributed into each dictionary. The multiple dictionaries are preferably mutually orthogonal and each of the dictionaries includes words of maximal phonetic distance from each other. The distribution is performed under the constraint of substantially balancing the respective number of words in the dictionaries. 
         [0019]    According to the present invention there is provided a computer readable medium readable by a machine, tangibly embodying a program of instructions executable by the machine to perform a computerized method for distribution among a plurality of dictionaries of a target vocabulary. The target vocabulary includes words for use in a speech recognition application installed in a computer system. Each word of the target vocabulary is found in only one of the dictionaries, the method as disclosed herein. 
         [0020]    According to the present invention there is provided a computer readable medium readable by a machine, tangibly storing the multiple dictionaries produced by the methods as disclosed herein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
           [0022]      FIG. 1  illustrates representative behavior of Confidence Level for matching a single word, as a function of the number of words in the vocabulary used in the speech recognition processing; 
           [0023]      FIG. 2  illustrates a substitution matrix for phonemes in the Hebrew language; 
           [0024]      FIG. 3  illustrates schematically a method for distributing a target vocabulary into multiple orthogonal dictionaries; and 
           [0025]      FIG. 3A  is a flow diagram according to an embodiment of the present invention; and 
           [0026]      FIG. 4 . illustrates schematically a simplified computer system of the prior art. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0027]    The present invention is of a method which improves speech recognition performance by distributing a large vocabulary of words into multiple orthogonal dictionaries prior to parallel speech recognition processing using the multiple dictionaries. 
         [0028]    The principles and operation of a method of distributing a large vocabulary of words into multiple orthogonal dictionaries prior to parallel speech recognition processing using the multiple dictionaries, according to the present invention, may be better understood with reference to the drawings and the accompanying description. 
         [0029]    It should be noted, that although the discussion herein relates to distributing a large vocabulary into multiple orthogonal dictionaries in the Hebrew language, the present invention may, by non-limiting example, alternatively be configured by applying the teachings of the present invention to other languages as well. 
         [0030]    Before explaining embodiments of the invention in detail, it is to be understood that the invention is not limited in its application to the details of design and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting. 
         [0031]    The embodiments of the present invention may comprise a general-purpose or special-purpose computer system including various computer hardware components, which are discussed in greater detail below. Embodiments within the scope of the present invention also include computer-readable media for carrying or having computer-executable instructions, computer-readable instructions, or data structures stored thereon. Such computer-readable media may be any available media, which is accessible by a general-purpose or special-purpose computer system. By way of example, and not limitation, such computer-readable media can comprise physical storage media such as RAM, ROM, EPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other media which can be used to carry or store desired program code means in the form of computer-executable instructions, computer-readable instructions, or data structures and which may be accessed by a general-purpose or special-purpose computer system. 
         [0032]    In this description and in the following claims, a “computer system” is defined as one or more software modules, one or more hardware modules, or combinations thereof, which work together to perform operations on electronic data. For example, the definition of computer system includes the hardware components of a personal computer, as well as software modules, such as the operating system of the personal computer. The physical layout of the modules is not important. A computer system may include one or more computers coupled via a computer network. Likewise, a computer system may include a single physical device (such as a mobile phone or Personal Digital Assistant “PDA”) where internal modules (such as a memory and processor) work together to perform operations on electronic data. 
         [0033]    Reference is now made to  FIG. 4  which illustrates schematically a simplified computer system  40 . Computer system  40  includes a processor  401 , a storage mechanism including a memory bus  407  to store information in memory  409  and a network interface  405  operatively connected to processor  401  with a peripheral bus  403 . Computer system  40  further includes a data input mechanism  411 , e.g. disk drive for a computer readable medium  413 , e.g. optical disk. Data input mechanism  411  is operatively connected to processor  401  with peripheral bus  403 . 
         [0034]    Those skilled in the art will appreciate that the invention may be practiced with many types of computer system configurations, including mobile telephones, PDA&#39;s, pagers, hand-held devices, laptop computers, personal computers, multi-processor systems, microprocessor-based or programmable consumer electronics, network PCs, minicomputers, mainframe computers, and the like. The invention may also be practiced in distributed computing environments where local and remote computer systems, which are linked (either by hardwired links, wireless links, or by a combination of hardwired or wireless links) through a communication network, both perform tasks. In a distributed computing environment, program modules may be located in both local and remote memory storage devices. 
         [0035]    By way of introduction, a principal intention of the present invention is to provide a method for improving performance of a speech recognition engine by distributing the vocabulary used by the engine into multiple orthogonal dictionaries and subsequently processing an input audio signal in parallel using multiple instances of the speech recognition engine, each instance using one of the multiple dictionaries. Each dictionary preferably includes an equal number of words, i.e. the vocabulary is preferably distributed substantially equally among the dictionaries. The distribution of the vocabulary into orthogonal dictionaries improves speech recognition performance because each channel uses a smaller dictionary, thereby increasing the confidence level of the speech recognition. Furthermore, since the words in each channel have been selected, according to an embodiment of the present invention for orthogonality, that is to have a large phonetic distance from each other, an even higher confidence level may be achieved or in a different design a faster or simpler speech recognition algorithm may be used for the channels than would be required without distributing orthogonally into separate dictionaries according to the teachings of the present invention. 
         [0036]    Many speech recognition algorithms are known. One class of commonly used algorithms are based on hidden Markov models (HMM). The speech recognition algorithm for use with embodiments of the present invention may be of any such mechanisms known in the art. 
         [0037]    Implementation of the method and system of the present invention involves performing or completing selected tasks or steps manually, automatically, or a combination thereof. Moreover, according to actual instrumentation and equipment of preferred embodiments of the method and system of the present invention, several selected steps could be implemented by hardware or by software on any operating system of any firmware or a combination thereof. For example, as hardware, selected steps of the invention could be implemented as a chip or a circuit. As software, selected steps of the invention could be implemented as a plurality of software instructions being executed by a computer using any suitable operating system. In any case, selected steps of the method and system of the invention could be described as being performed by a data processor, such as a computing platform for executing a plurality of instructions. 
       A. Calculation of Phonetic Distance Between Phonemes and Words 
       [0038]    According to different embodiments of the present invention the speech recognition vocabulary is distributed into orthogonal dictionaries using one or more of the following techniques. For each pair of words selected from each of the dictionaries a relative long phonetic distance is achieved between the words of the dictionary: 
         [0039]    1) based on respective formants of corresponding vowel sounds of the two words. The formants f1 and f2 are typically in a known frequency range in Hertz of, relative frequencies, e.g. ratio of f2/f1 and/or relative amplitudes of f1 and f2. 
         [0040]    2) based on the anatomical part, e.g. lips, teeth, tongue, palate, throat, most responsible for forming the sound made by the letter. For example, in English sounds, from letters b, f, p, v and w are formed in the lips; sounds from letters l, j, r, sh, z, d, l, n, t are formed by the tip of the tongue and/or the front palate; sounds from letters g, k and y are produced by the base of the tongue on the rear palate; and sounds formed in the throat are from letters a, e, h, i, and o. 
         [0041]    3) based on empirical results of a speech recognition engine. In speech recognition there are three types of errors: insertion error, substitution error and deletion error. An insertion error occurs when the speech recognition engines inserts a syllable or word when a corresponding syllable or word was not in the audio signal. A substitution error occurs when the speech recognition engine substitutes a syllable or word when a corresponding syllable or word was not in the audio signal. A deletion error occurs when the speech recognition engine deletes a corresponding syllable or word. Reference is now made to  FIG. 2  which illustrates a substitution matrix for phonemes in the Hebrew language. Both the vertical and horizontal axis are the phonemes in the order as shown. The horizontal axis x indicates the phoneme input to the speech recognition engine and the vertical axis y (with numbering starting at the top and increasing downwords) indicates a particular phoneme recognized. The color at square {x,y} indicates the likelihood of recognizing the phoneme numbered on the y axis after inputting the phoneme numbered at the x axis. A key (0-18%) appears on the right which indicates a probability of a speech recognition engine to substitute one sound (at x) for another (at y). As an example, when the phoneme ‘t’ at the x=3 position is input there is a relatively high probability ˜16% to be substituted in error for a ‘ts’ phoneme at the y=15 position. Other phonemes with a probability of substitution error, above ˜10% include: {‘u’, ‘o’}, {‘t’, ‘p’}, {‘s’, ‘z’} {‘f’, ‘v’}, {‘f’, ‘s’}, {‘n’, ‘m’} Similar matrices may be constructed for insertion and deletion errors for different pairs of input sounds. 
         [0042]    4) based on Levinstein distance. The Levinstein distance is used, according to an embodiment of the present invention, to calculate a phonetic distance between words, for instance when a phonetic distance between phonemes of the words is determined from a substitution matrix ( FIG. 2 ) and/or from probabilities of insertion and/or deletion of phonemes based on empirical results from the speech recognition engine. 
       B. Distribution of Vocabulary into Multiple Dictionaries 
       [0043]    The first criterion for the construction of orthogonal dictionaries, according to an embodiment of the present invention, is to maximize the phonetic distance between all the pairs of words of the dictionary. Another preferable criterion is “balance” or distributing the number of words in the vocabulary substantially equally among the channels. Reference is now made to  FIG. 3  which illustrates schematically a method  30  for distributing a target vocabulary  301  into multiple orthogonal dictionaries. In order to distribute target vocabulary  301  into multiple orthogonal dictionaries, vocabulary are first categorized into groups of minimal phonetic distance between the members of each group, the categorization is performed, according to different embodiments of the present invention by applying the following steps, preferably in the order presented: 
         303  Categorization by Phonetic Length 
       [0044]    Step  303  includes categorization of target vocabulary  301  according the number of syllables in each word or phonetic length, or based on the number of vowel sounds in each word of target vocabulary  301 . The output of categorization (step  303 ) is shown in method  30  with F 1 , F 2 , F 3  . . . Fn which the integer following the F symbol indicates the number of vowel sounds, or syllables in the word. In Hebrew, there are twelve vowel sounds:
       {‘a’,‘i’,‘o’,‘e’,‘u’, ‘ai’,‘oi’,‘ei’,‘ui’,‘au’,‘ou’,‘eu’}       
 
         [0046]    Monosyllabic words, denoted as F 1  include words with a consonant (consonant denoted with an asterisk *) after the vowel, such as a* with a vowel followed by a consonant, *a or with both a leading and a trailing consonant *a*. The Hebrew language unlike the English language does not have any words with a single vowel without at least one consonant. Monosyllabic words F 1  are categorized into 12 lists, one list for each vowel spoken in the Hebrew language. Examples of Hebrew words in transliteration in the group F 1  including monosyllabic words EL and AL. Disyllabic words F 2  include combinations of two vowel sounds, generally separated by an intervening consonant. For example, in transliteration words such as: “A-TEN”, “LE-XI, “DVA-RIM” are disyllabic F 2  words. The hyphen “-” is used to show the separation between the two syllables, The X is used in transliterated words represents the voiceless velar fricative ‘X’. Accordingly, trisyllabic F 3 , tetrasyllabic F 4  and pentasyllabic F 5  words are categorized (step  301 ) according to phonetic length. Words in transliteration from Hebrew in different F 3  groups include” “DA-A-GA”, “BE-NEI-NU”. 
         305  Categorization by Vowel Combinations 
       [0047]    Each of the groups of F 2 , F 3  , , , FN are preferably further categorized by vowel combinations denoted X 2 , X 3  . . . XN. For twelve vowels, there are 12×12 or 144 lists of words in X 2  and 12 3  or 1728 vowel combinations in X 3 . An Israeli Airline is “EL-AL” a word in group X 2 , “E-A”. Other examples of vowel combinations F 2  include “E-E” as in the Hebrew word transliterated LEXEM for the Hebrew word for “bread”; “E-I” as in the Hebrew word transliterated “LEXI”. Words in transliteration from Hebrew both in the same group X 4  “I-A-E-U” include “HISH-TAX-RE-RU” and “HIT-AR-E-RU”. 
         307  Categorization by Consonant Placement 
       [0048]    Each of the groups of X 1 , X 2 , X 3  . . . XN are preferably further sub-categorized according to consonant combinations into smaller groups or subcategories  31 . Consonants in the Hebrew language include:
       {‘b’,‘g’,‘d’,‘h’,‘v’,‘z’,‘x’,‘t’,‘y’,‘k’,‘l’,‘m’,‘n’,‘s’,‘ts’,‘tS’,‘dj’, ‘S’, ‘p’,‘f’,‘r’}
 
As an example of step  307 , the group X 2  with vowel order “E-E” is further subcategorized into subcategories  31  based on the placement of consonants around the two vowels of “E-E”. One sub-group  31  includes E*E*. (Asterisk * is in place of a consonant) Examples of E*E* words include in transliteration: EL-EX, E-TSEL, E-GED and examples of words in a different subcategory  31  *E*E, include in transliteration DE-REX, DE-LET, BE-GED.
       
 
         309  Group by Phonetic Distance 
       [0050]    According to embodiments of the present invention, each sub-group  31  is further analyzed to determine phonetic distance between any two words within each sub-group  31 . Words within each sub-group  31  which are close phonetically, i.e. have a short phonetic distance between them are placed in the same sub-group  33 . Phonetic distance between any two words within each sub-group  31  may be determined by any such techniques known in the art or by any of the techniques described in section A above, singly or in combination: (1) based on respective formants (2) based on based on the anatomical part, e.g. lips, teeth, tongue, palate, throat, most responsible for forming the sound made by the letter, (3) based on empirical results of a speech recognition engine. For instance, letters with sounds that are frequently confused ‘u’, ‘o’ are placed in same group  33  and/or  4 ) based on Levinstein distance between the words. 
         [0051]    As an example, Hebrew words in transliteration {NA-A-VOR, YA-A-VOR, LA-A-VOR} are trisyllabic (F 3 ) words, belonging to the same X 3  group with vowel sounds “A-A-O” and belonging to the same group  31 , “*A-A-*O*”. (Again * denotes a consonant) Given that the second consonant “V” and third consonant “R” (and corresponding consonant sounds) are identical in each of the three words, and the first consonants are in a group, {‘l’, ‘y’, ‘n’} which include sounds which are easily confused, known from anatomical part as pronounced in Hebrew (section A(2) above) and/or from empirical results (section A(3) above), the words {NA-A-VOR, YA-A-VOR, LA-A-VOR} are placed in a single sub-group  33 . Taking the words of each group  33  as pairs, there is a minimal phonetic distance between the words. Hence the words of each group  33  are easily recognized incorrectly or confused by a speech recognition engine. 
         [0052]    According to an embodiment of the present invention, the first sound of each of the words in groups  31  is selected and used to subdistribute each group  31  into even smaller groups  33  based for instance on the following eight letter groupings. LNY, FSRXK, BGD, TMV, the remaining consonants and the vowels. As discussed above, sounds {‘l’, ‘n’, ‘y’} are relatively easily confused, as are sounds {‘f’, ‘s’, ‘R’, ‘X’, ‘k’}, and sounds {‘b’, ‘g’, ‘d’}. 
         [0053]    Each group  33  containing a small number (e.g. 3 words) are sorted (step  311 ) into dictionaries  313 . Typically sorting (step  311 ) is performed in the order from smaller words (e.g. one syllable) to larger words so that similar words, (with minimal phonetic distance between them) in each group  33  are sorted into different dictionaries. Sorting (step  311 ) into dictionaries  313  is preferably performed so that the smallest dictionary during sorting  311  is incremented with another word, (if all other constraints are equal). 
         [0054]    Reference is now made to  FIG. 3A , a flow diagram according to an embodiment of the present invention. Groups  33  are subdistributed (step  315 ) are based (as above in method  30 ) on the eight letter groupings: LNY, FSRXK, BGD, TMV, the remaining consonants and the vowels. During sorting (step  311 ) groups  33  are selected first with sounds {‘l’, ‘n’, ‘y’}, {‘f’, ‘s’, ‘R’, ‘X’, ‘k’}, {‘b’, ‘g’, ‘d’}, the remaining consonants and the vowels, leaving initial sounds {‘t’}, {‘m’}, {‘v’} for processing last. While sorting (step  311 ) weights are incremented (step  317 ) for dictionaries  313  based on whether the {‘t’}, {‘m’}, or {‘v’} sounds appear in the word being sorted into each dictionary  313  and if so the weight, W t , W m  and/or W v  is increased by 1 for each dictionary  313 . Otherwise if the {‘t’}, {‘m’} or {‘v’} sound does not appear in the word being sorted the weights W t , W m  and/or W v  per dictionary  313  are not incremented. Subsequently when groups  33  with initial sounds {‘t’}), {‘m’}, {‘v’} are sorted into dictionaries  313 , the calculated weights may be used as a basis for selecting into which dictionary  313  to sort the words with initial sounds {‘t’}, {‘m’}, {‘v’}, the higher the weight, the more problematic the choice of dictionary  313 . If weights are substantially identical for adding a word into two dictionaries  313  then dictionary  313  of smaller number of words is selected for the word being sorted (step  311 ) 
         [0055]    After sorting (step  311 ), the dictionaries are preferably tested (step  321 ) for potential similarities between any two words. Examples of words, in Hebrew transliteration which may fall into the same dictionary are BANIM and ANI, or KIBALT and KIBALT. One method for testing includes calculating Levinstein distances between all the words within each dictionary  313 . 
         [0056]    Attached is an Appendix including a table in 24 pages. The columns are numbered from 1-8, include respectively 8 dictionaries  313  generated from target vocabulary  301  using method  30 . Target vocabulary  301  includes 3352 words in Hebrew transliteration distributed into 8 dictionaries of 419 words each in Hebrew transliteration. 
         [0057]    While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.